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MANAGING SOILORGANIC MATTERA PRACTICAL GUIDEA GRAINS RESEARCH & DEVELOPMENT CORPORATION PUBLICATION

Title: Managing Soil Organic Matter: A Practical GuideGRDC Project Code: KDI00023In submitting this report, the researchers have agreed to theGRDC publishing this material in its edited form.Report author: Dr Frances Hoyle, Department of Agricultureand Food Western AustraliaContent editor: Janet Paterson, Sci-ScribeCompilation and report contributions: Dr Meredith FairbanksThanks also to Professor Daniel Murphy and Dr Ram Dalalfor their valuable feedback.Department ofAgriculture and FoodISBN 978-1-921779-56-5Published July 2013© Grains Research and Development CorporationAll material published in this publication is copyright protectedand may not be reproduced in any form without writtenpermission from the GRDC.For free copies please contact: Ground Cover DirectFree phone: 1800 11 00 44Email: ground-cover-direct@canprint.com.auORMs Maureen CribbPublishing ManagerGRDCPO Box 5367KINGSTON ACT 2604Phone: (02) 6166 4500Email: maureen.cribb@grdc.com.auWeb: http://www.grdc.com.auNote: A postage and handling charge appliesDisclaimer:This publication has been prepared in good faith by the contributors on the basis of information available at the date of publication without any independentverification. The Grains Research and Development Corporation and the Department of Agriculture and Food Western Australia does not guarantee orwarrant the accuracy, reliability, completeness of currency of the information in this publication nor its usefulness in achieving any purpose. Readers areresponsible for assessing the relevance and accuracy of the content of this publication. The Grains Research and Development Corporation and theDepartment of Agriculture and Food Western Australia will not be liable for any loss, damage, cost or expense incurred or arising by reason of any personusing or relying on the information in this publication. Products may be identified by proprietary or trade names to help readers identify particular types ofproducts but this is not, and is not intended to be, an endorsement or recommendation of any product or manufacturer referred to. Other products mayperform as well or better than those specifically referred to.

FOREWORD1The one constant in agriculture is uncertainty and Australian grain growers are known globallyfor being ahead of the curve when it comes to adopting new farming practices to meet everychallenge.Climate change is already having a significant effect on how we farm today and how we adaptto future challenges will define what it means to farm sustainably this century. One of thebiggest influences on farm productivity and its resilience to climate variations is soil health.Soil organic matter contributes to a range of biological, chemical and physical properties ofsoil and is essential for soil health. This publication is a practical guide to understanding whatsoil organic matter is, why it’s important as well as how you can manage it on-farm to increasesoil functionality and enhance production benefits.When selecting farming practices to maximise the benefits of soil organic matter it is importantto consider the most important functions that soil organic matter provides to your crop andhow that will bring benefits to your soil health and future crops.MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEUnder the federal government’s commitment to the Kyoto protocol, farmers can potentiallyearn credits by storing carbon in their soil, or in trees by reducing greenhouse gas emissionson-farm. This allows the grains industry to play a vital role in contributing to positive change inAustralia’s environmental performance.I would like to make special mention and thank Dr Frances Hoyle, senior research scientistwith the Department of Agriculture and Food Western Australia, for authorship of the contentfor this publication.The Grains Research and Development Corporation promotes high quality science andresearch that raises awareness of current industry priorities and emerging issues.Access to this information will allow growers to adapt to a changing agricultural economy andadopt new farming practices to improve on-farm efficiencies, profitability and sustainability. Ihope you find this guide practical and informative.Tanya RobinsonProject Manager Natural ResourcesGrains Research and Development Corporation

CONTENTS2MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDETABLES, FIGURES AND PLATES 04INTRODUCTION 06- Organic matter status of Australian soilsCHAPTER 1 08Soil organic matter- What is soil organic matter?- Calculating soil organic matter- Soil organic matter function- Carbon to nitrogen (C:N) ratio and soil organicmatter- How much organic matter ends up in soil?CHAPTER 2 18Biological turnover of organic matter- Soil microorganisms• Soil fungi to bacteria ratio- Soil fauna• Earthworms• Soil-borne plant pathogens• Suppressive soils- Organic matter turnover times• Organic matter characteristics• Soil characteristics• Climate• Location in the soil profile- Ecosystem services• Soil bufferingCHAPTER 3 24The carbon cycle- Carbon balance in soils• What factors drive changes in the carbonbalance?• What is the rate of change?• How stable is organic matter?• Is organic matter worth building up?• Managing soil organic matter tooptimise agricultural productionsystemsCHAPTER 4 30Soil organic matter and nutrient availabilityin agriculture- Organic matter and cation exchange- Nitrogen, phosphorus, sulphur and organicmatter• Nitrogen supply• Nitrogen cycling• The influence of organic inputson soil nitrogen supply• Phosphorus• Phosphorus and organic matter• Managing organic phosphorous• Sulphur• Sulphur and organic matter• Sulphur deficiencies in the soil• PotassiumCHAPTER 5 38Soil organic matter and plant available water- What affects the water holding capacity of soil?- The influence of soil organic matter on soil water• Calculating how much water can be stored• How important is this ‘extra’ water toagricultural production?- Relationship between organic matter and waterrepellence• What causes water repellence?CHAPTER 6 46Organic matter losses from soil- Direct losses• Soil erosion- Indirect losses• Climate• Soil disturbance• Management of organic residues- Fate of captured carbon in soils

TABLES, FIGURES AND PLATES4MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEINTRODUCTIONFigure 1.1 Potential for soil organic matter gainresulting from a combination of effective rain(considering amount and availability) and residuepressure, which reflects plant and livestockremovals of organic matter.Figure 1.2 Soil organic carbon (tonnes perhectare) stocks in surface layers (0-10 cm) forsouth-west Western Australia. Areas of lowconfidence (e.g. subsystems that only have onesite) are masked out in white.CHAPTER 1Figure 1.3 Proportional make-up of organic matterin an agricultural soil.Table 1.1 Size, composition, turnover rate anddecomposition stage of the four soil organic matterfractions.Figure 1.4 Pattern of organic mattertransformation in soils.Table 1.2 Functional role of soil organic matter.Figure 1.5 A conceptual representation of therole of soluble, particulate, humus and resistant(inert) organic matter fractions for a range of soilfunctions.Figure 1.6 The influence of soil organic carbon oncation exchange capacity for Young River, WesternAustralia, in soils with variable clay content.Table 1.3 Indicative carbon to nitrogen (C:N) ratioof various organic residues.Figure 1.7 The influence of soil type, climate andmanagement factors on potential soil organicmatter content.Table 1.4 Rate-limiting influences on theaccumulation of soil organic matter.Plate 1.1 The organic horizon is often related to adarkening of soil colour and is particularly evidenton the soil surface.CHAPTER 2Table 2.1 Soil factors that can influence the rate oforganic matter turnover.CHAPTER 3Figure 3.1 Organic carbon cycling in soils.Figure 3.2 Theoretical changes in soil organiccarbon (%) representing an upper and lower limit,or a more typical state of flux.Figure 3.3 A model simulation showing theinfluence of cation exchange capacity (CEC) on thecapacity of soil to retain soil organic carbon.Table 3.1 The influence of soil organic matter onsoil attributes and functions.CHAPTER 4Table 4.1 Indicative cation exchange capacity ofdifferent clay minerals in soil.Table 4.2 Indicative cation exchange capacity fordifferent soil textures and organic matter.Figure 4.1 Nitrogen cycling in soil.Figure 4.2 Nitrogen release in soil resulting fromthe decomposition of plant residues with a range ofcarbon to nitrogen (C:N) ratios.Figure 4.3 Phosphorus cycle in agriculturalsystems.CHAPTER 5Figure 5.1 Change in water holding capacity forthe 0-10 cm soil layer of South Australian redbrownearths, with a one per cent increase in soilorganic carbon content.Table 5.1 Influence of soil characteristics on waterstorage capacity.Figure 5.2 Adjustment of organic carbon contentfor an equivalent soil mass associated withchanges in bulk density and sampling depth.Figure 5.3 A complex relationship exists betweensoil organic carbon, clay content and the severityof water repellence as measured at 400 sitesacross Western Australia.Plate 5.1 A sub-surface compaction layer shows adense impenetrable soil layer.Plate 5.2 Bulk density core.Plate 5.3 a) Water droplet sitting on the surfaceof a non-wetting soil and b) typical sub-surfacedryness observed after rain in water repellent sand.

5CHAPTER 6Figure 6.1 The effect of increasing temperatureon the amount of carbon lost from soil (kg carbonper tonne of soil per day) where stubble has beenretained.Plate 6.1 Canola roots contribute organic matterto soil.Plate 6.2 Soil erosion resulting from poor groundcover and compaction.Plate 6.3 Mouldboard plough in operation for thetreatment of non-wetting soil.Plate 6.4 The removal of products such as grainor hay can decrease organic matter inputs andcontribute to soil acidification.CHAPTER 8Figure 8.1 Conversion of soil analysis values forsoil organic carbon stock in a paddock to 10 cmdepth.Figure 8.2 Possible trial design comparing twotreatments with three replicates within a paddockCHAPTER 9Table 9.1 Sensory and soil indicators of organicmatter in the paddock.Plate 9.1 Assessing soil colour at a field site usinga Munsell colour chart.Plate 9.2 Pasture growth under retained stubbleprovides complete ground cover.Plate 9.3 Long-term experimental site with burntstubble (on left of image) and retained stubble (onright of image) demonstrating significant differencesin ground cover.Plate 9.4 Crop residues being a) green manuredand b) mulched in a continuous cropping system.Plate 9.5 Plant roots growing through soil.Plate 9.6 Soil showing earthworms present in anarable system in Australia.CHAPTER 10Figure 10.1 Summary of the relative effect ofdifferent management practices on soil carbonlevels.Table 10.1 Management options for improvinglong-term soil organic matter levels in agriculturalsoils.Table 10.2 Crop and pasture type suitable forusing as a green manure phase.Table 10.3 Type and application benefit of organicamendments in Australia.Plate 10.1 Proliferation of roots in a rip line on acompacted sand in Western Australia.Plate 10.2 Loss of organic matter and soilcondition associated with grazing damage (right)compared to un-grazed pastures (left).Plate 10.3 Green manuring by discing increasesorganic matter in soil.Plate 10.4 Bare fallow risks losing soil andassociated organic matter from wind or watererosion.Plate 10.5 Soil collapse and erosion resulting fromdispersion on a sodic soil.CHAPTER 11Figure 11.1 The trend in a) average temperature(°C) and b) annual total rainfall (mm) acrossAustralia from 1910 to 2011.Figure 11.2 Climate change projections foraverage annual a) temperature b) rainfall andc) potential evapotranspiration (source: http://climatechangeinaustralia.com.au) for 2050 under amoderate emissions scenario.CHAPTER 12Table 12.1 Change in the economic value of farmproduction in 2014-15; average per farm.MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDE

6INTRODUCTIONMANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEIncreasing soil organic matter is widely regardedas beneficial to soil function and fertility and inagricultural production systems is integral tosustainable farming. Storing the carbon componentof organic matter in agricultural, rangeland andforest soils is also seen as one way to decreaseatmospheric carbon dioxide levels and mitigate theimpact of climate change. Consequently, there isgreat interest in quantifying the capacity of varioussoil types and land management practices to supportincreases in soil organic matter and understandinghow these changes impact soil health, ecosystemservices and carbon sequestration in the mediumand long-term.ORGANIC MATTER STATUS OFAUSTRALIAN SOILSAustralian soils are ancient. They have inherentlypoor structure, fertility and low levels of organicmatter in their surface layers – a condition madeworse by historical land clearing and subsequentland management practices. Physical and chemicalsoil constraints such as salinity, acidity, disease,compaction and sodicity impact large areas ofAustralian soils. These factors limit their productivityand act as major constraints to increasing soilorganic matter.Australian soils are low in soil organic mattercontent by global standards, with the exceptionof soils that support high net primary productivitysuch as well-managed pastures and irrigatedsystems unconstrained by water availability. Recentestimates from the 2011 State of the Environmentreport suggest climate variability and the historicclearing of native vegetation for agriculture hasresulted in a 30-70 per cent decline in soil organicmatter content. However, while soil organic matterhas declined in many systems over the past 100-

200 years, recent assessments suggest there is ahigh to moderate potential to increase the carboncontent of soils across extensive areas of Australia(see Figure 1.1).Recent measures of soil organic matter (0-30 cm)in Australian soils suggest they can contain betweenthree tonnes of carbon per hectare (0.3 per centcarbon for desert loams) and 231 tonnes of carbonper hectare (14 per cent carbon for intensive dairysoils; see http://www.daff.gov.au/climatechange/australias-farming-future/climate-change-andproductivity-research/soil_carbonfor the report).For Australian dryland agricultural soils the organiccarbon content is more typically between 20-150tonnes carbon per hectare (or about 0.7-4.0 percent depending on soil bulk density).Quantifying existing soil organic carbon stocks inkey soil types under important farm managementregimes within some of Australia’s agriculturalregions was a key output of the Soil CarbonResearch Program (SCaRP), and has providedvaluable information to baseline soil organicmatter stocks across different environments(see Figure 1.2).Management and site factors interact to influencethe actual amount of soil organic matter in relationAustralian soils are low in soilorganic matter content by globalstandards, with the exception ofsoils that support high net primaryproductivity such as well-managedpastures and irrigated systemsunconstrained by water availability.to the attainable amount of soil organic matter (asdefined by climate and soil type). While there is astrong relationship between rainfall and the amountof soil organic carbon, regional data indicates thateven within a rainfall zone soil organic carbon canvary widely and the highest variability occurs in areasof high rainfall, which support increasing biomassproduction (Griffin et al. 2013). Such variabilityis likely due to a range of factors influencing netprimary productivity, including agronomic and soilmanagement as well as variations in soil type andmoisture – the influences of which are discussed inthis publication.7MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEFigure 1.1 Potential for soil organic matter gainresulting from a combination of effective rain(considering amount and availability) and residuepressure, which reflects plant and livestock removalsof organic matter (Baldock et al. 2009). Figure 1.2 Soil organic carbon (tonnes per hectare)stocks in surface layers (0-10 cm) for south westWestern Australia. Areas of low confidence (e.g.subsystems that only have one site) are maskedout in white. Lines constitute agricultural soil zones:grey is remnant vegetation (Griffin et al. 2013).

018SOIL ORGANIC MATTERMANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEAT A GLANCE•For all soils, organic matter is critical for biological processes and soil nutrient supply.•In coarse textured sandy soils, organic matter largely determines cation exchange capacityand can influence water holding capacity to a degree.•In finer textured clay soils, soil organic matter is critical in maintaining soil structure andstability.•Information on the amount and quality of organic matter returned to soil can and should beused to help inform fertiliser management strategies.•Soil organic matter content is determined by soil type, climate and on-farm management inthat order and is slow to build-up, particularly in more stable fractions.•Measuring soil organic matter over time helps inform growers of future changes insoil function.

WHAT IS SOIL ORGANIC MATTER?Soil organic matter makes up only 2-10 per cent ofthe soil mass, but is vital to its physical, chemicaland biological function (see Plate 1.1).All soil organic matter has its origin in plants. It canbe divided into both ‘living’ and ‘dead’ componentsin various stages of decomposition (see Figure 1.3)and ranges in age from very recent inputs (freshresidues) to those that are thousands of years old(resistant organic matter). Soil organic matter iscomposed of carbon and other organic particlessuch as hydrogen, oxygen and small amounts ofnitrogen, phosphorous, sulphur, potassium, calciumand magnesium.Between 5-10 per cent of below-ground soil organicmatter containing roots, fauna and microorganismsis living (see Figure 1.3). The microbial componentof this living pool is referred to as the microbialbiomass and is considered essential for organicmatter decomposition, nutrient cycling, degradationof chemicals and soil stabilisation. The remainingcomponents include non-living organic matter suchas dead and decaying plant and animal residues.Depending on soil type and farming system, theactual allocation of organic matter moving intodifferent pools can vary widely.Soil organic matter is a continuum of differentforms, with turnover times ranging from minutesthrough to hundreds and even thousands ofyears. It exists as four distinct fractions, with eachvarying in properties and decomposition rates (seeTable 1.1).Dissolved organic matter can originate from thehumus fraction (leaching of humic substances) asa soluble by-product from decomposition of theparticulate organic matter fraction, or from rootexudates. In agricultural systems, fresh residues andthe living component of organic matter contribute tothe particulate organic matter fraction (see Figure1.3). This pool is readily decomposable and cyclesrapidly (i.e. reproduces, dies and decomposeswithin years). In contrast, the stable humus fractionis nutrient rich and accumulates in the soil makingup more than half of the soil organic matter inmany soils (see Figure 1.3) and taking decades toturn over. The resistant organic matter fraction isrelatively inert and can take thousands of years toturn over. The relative contribution of each organicmatter fraction (as depicted in Figure 1.3) to the totalorganic matter pool varies widely depending on soiltype and farming system.Soil organic matter cycles continuously betweenits living, actively decomposing and stable fractions(see Figure 1.4). When conditions result in lessorganic matter (e.g. animals/plants) entering thesoil organic pool, the particulate organic fractiondeclines. If this pattern continues, the total amountof soil organic matter will decline over time.9MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEFigure 1.3 Proportional make-up of organic matter in an agricultural soil.

Table 1.1 Size, composition, turnover rate and decomposition stage of the four soil organic matter fractions.Fraction Size Turnover time CompositionDissolved organicmatter< 45 µm(in solution)Dissolved organic matter generallyturns over very rapidly (minutes todays)Made up of soluble root exudates, simple sugars anddecomposition by-products. It generally constitutes lessthan one per cent of total soil organic matter.Particulate organicmatter53 µm – 2 mm From months to decades Composed of fresh and decomposing plant and animalmatter with an identifiable cell structure. Makes upbetween 2-25 per cent of total soil organic matter.Humus < 53 µm Decadal(from tens of years up to hundredsof years)Made up of older, decayed organic compounds that haveresisted decomposition. Includes both structural (e.g.proteins, cellulose) and non-structural (e.g. humin, fulvicacid) organic molecules. Often makes up more than 50per cent of total soil organic matter.Resistant organicmatter< 53 µm and insome soils < 2mmRanges from hundreds to thousandsof yearsResistant organic matter is relatively inert material madeup primarily of chemically resistant materials or remnantorganic materials such as charcoal (burnt organicmaterial). This pool can constitute up to 30 per cent ofsoil organic matter.10MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDE1. Additions: When plants and animals die theybecome part of the soil organic matter.2. Transformations: When soil organisms breakupand consume organic residues to grow andreproduce, the organic residues are transformedfrom one form into another. For example, freshresidues are broken down into smaller pieces(< 2 mm) and become part of the particulateorganic matter fraction. As the material is furtherdecomposed (< 53 um) a smaller proportionof more biologically stable material enters thehumus pool.3. Nutrient release: Nutrients and othercompounds not required by microbes arereleased as a result of this transformation andcan then be used by plants.4. Stabilising organic matter: As the organicresidues decompose, a proportion becomeschemically stabilised enabling it to resist furtherchange. Protection can also be affordedthrough occlusion within aggregates and throughthe formation of organo-mineral complexes.These materials contribute to the resistantorganic fraction.Figure 1.4 Pattern of organic mattertransformation in soils (figure adapted fromUniversity of Minnesota extension publicationWW-07402).CALCULATING SOIL ORGANIC MATTEROrganic matter is different to organic carbon in thatit includes all the elements that are componentsof organic compounds. Soil organic matter isdifficult to measure directly, so laboratories tend tomeasure soil organic carbon and use a conversionfactor to estimate how much organic matter is heldwithin a soil.

11MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDESoil organic carbon is a component of soilorganic matter, with about 58 per cent of themass of organic matter existing as carbon.Therefore, if we determine the amount ofsoil organic carbon in a sample and multiplyit by 100/58 (or 1.72) we can estimate theproportion of organic matter in the soil sample:Organic matter (%) = total organiccarbon (%) × 1.72While the ratio of soil organic matter to soil organiccarbon can vary with the type of organic matter, soiltype and depth, using a conversion factor of 1.72generally provides a reasonable estimate of soilorganic matter suitable for most purposes.Calculating soil organic mattercontent in soilThe amount of soil organic matter in your soilcan be calculated as follows for a soil with 1.2per cent soil organic carbon:(Total organic carbon per cent x 1.72) x soilmass (tonnes per hectare)= [(1.2 x 1.72)/100] x 1200 (for a soil with abulk density of 1.2 to 10 cm depth)= 24.8 tonnes organic matter per hectareSOIL ORGANIC MATTER FUNCTIONOrganic matter renders soils more resilientto environmental change and also influencescharacteristics such as colour and workability.It is central to the functioning of many physical,chemical and biological processes in the soil. Theseinclude nutrient turnover and exchange capacity,soil structural stability and aeration, moistureretention and availability, degradation of pollutants,greenhouse gas emissions and soil buffering (seeTable 1.2).An optimal level of soil organic matter is difficultto quantify because the quality and quantity ofdifferent organic matter fractions needed to supportvarious functions varies with soil type, climate andmanagement. However, it is generally consideredthat soils with an organic carbon content of lessthan one per cent are functionally impaired.Soil function is influenced by the size, qualityand relative stability of the four soil organic matterfractions (see Figure 1.5). In this figure, the widthof the patterned or shaded areas within the shapeindicates the relative importance of the soil organicmatter fractions to a particular function or process.For example, on the left the decreasing width ofthe striped area indicates the importance of thehumus fraction declines as the clay content of asoil increases. This is because the clay particlesprovide a large surface area for cation exchange,which renders soil organic matter increasingly lessimportant as clay content increases. By comparison,

Table 1.2 Functional role of soil organic matter.Physical functions Chemical functions Biological functionsImproves soil structural stabilityIncreases capacity tohold nutrients (i.e. cationexchange capacity)Influences water retention Buffers pH Major store of plant nutrients(N, P, S)Energy (food source) for biological processes such as:Microbial decomposition; Nutrient transformation; Degradation of pollutantsBinding soil particles and organic matter in stable aggregatesBuffers changes in temperatureImmobilises heavy metalsand pesticidesImproves soil resiliencehumus can be seen to be equally important forthe provision of nutrients across all soil types andin particular it is critical to the supply of potentiallymineralisable nitrogen (see Figure 1.5).is not known how this relationship would differ whenconsidering clay applied to soils for the treatment ofnon-wetting. 12MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEFigure 1.5 A conceptual representation of therole of soluble, particulate, humus and resistant(inert) organic matter fractions for a range of soilfunctions (from Hoyle et al. 2011).While not all of the relationships depicted inFigure 1.5 have been quantified for Australiansoils, the contribution of the soil organic carbonfraction to the ability of a soil to hold nutrientshas been demonstrated at Young River, WesternAustralia (see Figure 1.6). This study suggests astronger relationship between soil organic carbonand nutrient exchange in soils, with less than 10per cent clay and organic carbon explaining nearly40 per cent of the variation in cation exchange. Insoils with greater than 10 per cent clay, a one percent increase in organic carbon increased nutrientexchange by about 3 meq/100 g soil, but explainedjust six per cent of the variation in higher clay soils. ItFigure 1.6 The influence of soil organic carbon oncation exchange capacity for Young River, WesternAustralia in soils with clay content between 0-10%(o), 10-20% (x) and 20-30% ( ). Sourced fromwww.soilquality.org.au.While the relative importance of any given fractionof organic matter will vary from one soil to anotherand depend on factors such as climate andcropping history, we do know that organic matterinfluences plant growth primarily through its effecton the physical, chemical and biological propertiesof the soil.For example, fresh crop residues which are readilybroken down provide energy for key soil biologicalprocesses such as nutrient cycling. The particulateorganic matter fraction decomposes at a slowerrate than crop residues and is important for soilstructure, energy for biological processes andprovision of nutrients. Humus generally dominates

Table 1.3 Indicative carbon to nitrogen (C:N) ratio ofvarious organic residues.Organic materialUnits of carbon per unit of nitrogen(C:N ratio)Poultry manure 5:1Humus 10:1Cow manure 17:1Legume hay 17:1Green compost 17:1Lucerne 18:1Field pea 19:1Lupins 22:1Grass clippings 15 – 25:1Medic 30:1Oat hay 30:1Faba bean 40:1Canola 51:1Wheat stubble 80 – 120:1Newspaper 170 – 800:1Sawdust 200 – 700:1soil organic matter and is particularly important inthe provision of nutrients, cation exchange, soilstructure, water-holding capacity and in supportingbiological processes. Indirectly, the humus pool alsoinfluences micronutrient uptake and the performanceof herbicides and other agricultural chemicals. Theresistant organic matter fraction is dominated byold recalcitrant residues and char — a product ofburning carbon-rich materials (e.g. grasslands). Thisfraction decomposes over millennia and althoughbiologically inert contributes to cation exchangecapacity, water holding capacity and the stability(persistence) of organic carbon in soils.CARBON TO NITROGEN (C:N) RATIOAND SOIL ORGANIC MATTERPlant material contains about 45 per cent carbonand depending on residue type between 0.5-10 percent nitrogen. The ratio of organic carbon to totalWhile plant residues and otherorganic inputs vary widely intheir C:N ratios, the C:N ratio ofsoil organic matter is generallyconstant for a given environment(ranging from 10:1 to 15:1).nitrogen is referred to as the carbon to nitrogen(C:N) ratio. This ratio indicates the proportion ofnitrogen and other nutrients relative to carbon inthat material. Organic matter varies widely in its C:Nratio (see Table 1.3) and reflects how readily organicmatter decomposes, providing an indication of boththe amount and rate of nitrogen release that mightbe expected to result from decomposition.Carbon to nitrogen (C:N) ratio of organicresiduesThe C:N ratio of organic inputs influence theamount of soil nitrogen made available toplants. Organic residues with a C:N ratio ofbetween 25:1 and 30:1 have sufficient nitrogenavailable for microbes to decompose themwithout needing to use soil nitrogen stores.Residues with a lower C:N ratio (< 25:1) suchas pulses and legume pastures will generallyresult in more rapid decomposition of organicresidues and tend to release plant-availablenitrogen. Residues with a higher ratio (> 30:1)such as cereal crops will decompose moreslowly and result in less plant-available nitrogenbeing released.To grow and reproduce soil microbes requiresa balanced amount of carbon and nitrogen thatreflects a relatively low C:N ratio (generally less than15:1). In plant residues such as wheat stubble, whichhave a high C:N ratio (120:1) and contain relativelymore carbon than nitrogen, soil microbes must findanother source of nitrogen to fully digest wheatstubble and this often results in soil nutrient reservesbeing immobilised and soil becoming nitrogendeficient. Such nitrogen deficiency is often seenin the field where stubble has been incorporatedduring sowing operations and nitrogen availability13MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDE

Figure 1.7 The influence of soil type, climate and management factors on potential soilorganic matter content (Ingram and Fernandes 2001).14MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEdecreases in the soil because it becomes tied-up,or immobilised in the microbial biomass. Therefore,residues with a high C:N ratio are considerednutrient poor and can take years to decompose. Incontrast, fresh legume residues with a low C:N ratio(less than 25:1) have proportionally less carbon perunit of nitrogen enabling them to decompose fasterand release surplus nitrogen for plant use.While plant residues and other organic inputsvary widely in their C:N ratios, the C:N ratio of soilorganic matter is generally constant for a givenenvironment (ranging from 10:1 to 15:1). This resultsfrom the dominance of the humus and resistantorganic matter fractions in soil and reflects thesignificant loss of carbon associated with organicmatter decomposition. In Australia, agricultural soilsgenerally store between 10-12 units of carbon forevery unit of nitrogen (i.e. 10 tonnes carbon perhectare for every tonne of nitrogen). Therefore,increasing organic matter (and hence organiccarbon) in soil requires a balance of carbon andnitrogen as well as other nutrients.To maintain or increase a soil’sstock of organic carbon long-term,increased amounts of organicmatter must be continually added.Any decline in the amount oforganic material being returned tosoils will result in a decrease in thesoil’s organic carbon content.When soil organisms digest organic residues partof the carbon originally in these residues is used fornew growth and cell division, with the remainderbeing emitted as carbon dioxide. As a general rule,less than one-third of the applied carbon in freshresidues remains in the soil after the first few monthsof decomposition. As the material is decomposedthe C:N ratio decreases and the remaining organicmaterial becomes more resistant to further decay.

15MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEHow do I calculate the impact of incorporating organic residues on my soil nitrogen supply?ScenarioA grain grower retains three tonnes per hectarewheat stubble, with a C:N ratio of 120:1 andwants to estimate the likely impact of the stubbleon soil nitrogen levels in the paddock.Step IThe amount of carbon present in the plantresidues added to the soil.3000 kg of stubble x 0.45 carbon content =1350 kg of carbon in plant residuesStep IIThe amount of nitrogen present in the organicmatter added to the soil. The stubble contains1350 kg of carbon and has a C:N ratio of 120:1.1350 kg carbon ÷ 120 = 11.25 kg nitrogen inorganic matterStep IIIAllow for 30 per cent of the carbon being usedby microbes to grow, with the remaining 70 percent respired as carbon dioxide.The amount of carbon used by microbes is 1350kg carbon x 0.3 = 405 kg carbonGiven that microbes have a C:N ratio of 12:1they therefore require 1 kg of nitrogen for every12 kg of carbon to grow.405 kg carbon ÷ 12 = 34 kg NStep IVCompare the two nitrogen values; the freshorganic matter contained 11 kg of nitrogen andthe microbes require 34 kg of nitrogen to grow.The nitrogen balance = 11 – 34= - 23 kg N haA NEGATIVE balance indicates that nitrogenin the organic matter was LESS than thenitrogen required by the microbes. This nitrogendeficit will be sourced from the soil, making itunavailable for plants. In this case, a growershould consider fertiliser strategies which willensure sufficient nitrogen is available to plantsearly in their growth.If the nitrogen balance had been POSITIVEa surplus of nitrogen would then becomeavailable to plants because there would beMORE nitrogen than required for microbial use.If this was the case, growers may consider splitapplications of nitrogen to save input costs andminimise losses of nitrogen via leaching.Knowing the contribution of organic matterto your soils nutrition can help inform moreprofitable fertiliser management strategies.

16HOW MUCH ORGANIC CARBONREMAINS IN SOIL?Microbes digest up to 90 per cent of organic carbonthat enters a soil in organic residues. In doing so,they respire the carbon back into the atmosphereas carbon dioxide (CO 2 ). Microbes continually breakdown organic residues eventually converting a smallproportion of them to humus, which gives the topsoilits dark colour. While up to 30 per cent of organicinputs can eventually be converted to humusdepending on soil type and climate, in Australianagricultural soils this value is often significantly less.Examples of the influence of soil type, climateand management on the amount of soil organicmatter (and carbon) accumulated in and lost fromagricultural soils is presented in Table 1.4. To maintainor increase a soil’s stock of organic carbon longterm,increased amounts of organic matter mustbe continually added. Any decline in the amount oforganic material being returned to soils will result ina decrease in the soil’s organic carbon content.MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDESoils naturally higher in clay content generallyretain more organic matter than sandy soils.Soil type, rainfall and temperature limit the amountof soil organic matter generated via plant biomassand subsequently stored as humus for the longterm.As a result, soils rarely reach their theoreticalpotential for organic matter storage (see Figure1.7). Management practices also have a significantinfluence on whether actual soil organic matter (andcarbon) reaches its attainable level as determinedby climate (see Figure 1.7). Beyond this threshold,continuous inputs of external organic carbon sourcesare required, which can be logistically difficult andexpensive, and risk the depletion of organic carbonin another location.Plate 1.1 The organic horizon is oftenrelated to a darkening of soil colour and isparticularly evident on the soil surface.Source: Department of Primary Industries, Victoria

Table 1.4 Rate-limiting influences on the accumulation of soil organic matter.InfluenceCause17Soil typeNaturally occurring clay in soil binds to organic matter, which helps to protect it from being broken down or limitsaccess to it by microbes and other organisms.ClimateLand and soilmanagementIn contrast, organic matter in coarse textured sandy soils is not protected from microbial attack and is rapidlydecomposed.In comparable farming systems with similar soil type and management, soil organic matter increases withrainfall. This is because increasing rainfall supports greater plant growth, which results in more organic matteraccumulating in the soil.Organic matter decomposes more slowly as temperatures decline. Under moist conditions each 10°C increasein temperature doubles the rate of organic matter decomposition (Hoyle et al. 2006). This means moist, warmconditions will often result in the most rapid decomposition of organic inputs.Maximising crop and pasture biomass via improved water-use efficiency and agronomic management willincrease organic matter inputs.MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEAs a large proportion of organic matter is present in the top 0-10 cm of soils, protecting the soil surface fromerosion is central to retaining soil organic matter.Tillage of structured soils decreases soil organic matter stocks by exposing previously protected organic matter tomicrobial decomposition.Adding off-farm organic residues such as manures, straw and char can increase soil organic matter dependingon the quality of the added residues.Landscape can influence water availability. Transfer of soil and organic matter down slope via erosion canincrease the amount of soil organic matter in lower parts of the landscape.Soil constraints can influence humus formation by constraining plant growth and decomposition rates. This couldslow both the amount and transformation rate of organic matter moving into more stable fractions.Microbes and particularly bacteria grow poorly in strongly acidic or alkaline soils and consequently organic matterbreaks down slowly. Soil acidity also influences the availability of plant nutrients and in turn the amount of organicmatter available for soil biota growth.

0218BIOLOGICAL TURNOVEROF ORGANIC MATTERMANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEAT A GLANCE•Soil biota is primarily responsible for decomposing organic matter and nutrient cycling insoil.•Less than five per cent of soil biota is active at any one time, so estimates of the numberof organisms as a meaningful indicator of soil quality without a measure of their specificactivity or function are questionable.•Soil microorganisms are active only when soil is moist and are often constrained by a lackof food.•Greater diversity in soil biota is linked to the suppression of soil pathogens and maintainingsoil function under variable conditions.•Organic matter breakdown is influenced by its composition, physical location and climate.

A diverse range of organisms both beneficial andharmful to plants is active in soil. These organismscontribute significantly to the living component of soilorganic matter (see Chapter 1) and can be classifiedaccording to their size or function in the soil:- Microorganisms (e.g. bacteria, fungi,actinomycetes, viruses, protozoa and algae) lessthan 0.2 mm in size.- Soil fauna (e.g. earthworms, ants, nematodes,beetles, mites, termites, centipedes andmillipedes), includes both macro-sized organisms(larger than 2 mm) and meso-sized organisms(between 0.2 and 2 mm).SOIL MICROORGANISMSMicroorganisms play a significant and criticalrole in nutrient and carbon cycling within soil.Soil microbes decompose fresh animal, crop andpasture residues, using the carbon and nutrientsfor food and growth. In the process they producenew compounds, which can be used by a largevariety of organisms, or they incorporate some ofthe carbon and nutrients that were in the organicmatter into their own microbial biomass. As a result,in many soils the microbial biomass is often directlyproportional to the size of the actively decomposingorganic matter fraction (Hoyle et al. 2011). While alarge proportion of the organic matter that enters soilis available for mineralisation, nutrients can remaintrapped in tiny (< 53 µm) particles of organic residuethat are either chemically or physically protectedfrom decomposition and remain unavailable toplants.By breaking down carbon structures, soilmicrobes play a significant role in nutrient cyclingprocesses. In addition, the microbial biomassitself is also a significant contributor to carbon andnutrient cycling because they reproduce and diequickly, contributing significantly to fluctuations innutrients available for plant or microbial uptake.Free living nitrogen fixation can also occur inspecialised bacteria and has been estimated atthe rate of 0-15 kg of nitrogen each year (Peoples2002). In low fertility soils, much of the carbon andother nutrients originally contained within the cropand pasture residues remain unavailable to plantsdue to its uptake (immobilisation) in the microbialmass. However, any nitrogen, phosphorous andsulphur that are in excess of microbial requirementsis released into the soil and becomes available forplant use.If photosynthetic carbon inputs such as cropresidues or root exudates become totally absent,decomposers come to dominate and increasingly thesoil biota consume stored carbon sources resultingin declining soil organic carbon levels. The use ofherbicides, pesticides and fungicides that cause atemporary decline in beneficial microorganisms thatbuild humus, suppress diseases and make nutrientsavailable to plants can influence the turnover ofcarbon in soil.Soil fungal to bacterial ratioAn estimate of the biomass (or a count) of fungi andbacteria is sometimes used to determine the ratioof fungi to bacteria, with a ratio between 0.5-1.5reportedly associated with enhanced soil health,nutrient cycling and residue breakdown. However,there is little evidence or quantification to support thisrelationship in the context of Australian agriculturalsystems, which are often dominated by bacteria. Inaddition, the methods sometimes used to determinethe fungi to bacteria ratio may only capture a smallproportion of the total microbial biomass or are notspecific enough to target specific organisms.SOIL FAUNASoil fauna influence organic matter transformations(e.g. loss of organic matter, nitrogen pathways)in concert with changes in soil moisture andtemperature, which influence the abundanceand diversity of different functional groups (Osler2007). The impact of soil fauna on organic matterdecomposition rates suggests their contribution isgreatest on the poorest quality litter (Osler 2007).Larger soil fauna such as earthworms and insectsare primarily associated with fragmentation andredistribution of organic matter (see Plate 2.1),breaking down larger pieces through ingestion ortransporting and mixing them through the soil. In theprocess they recycle energy and plant nutrients andcreate biopores.Biopores are channels or pores formed byliving organisms that help water drain morefreely and can improve the ability of roots topenetrate hard soil layers.In low fertility, sandy textured soils typical ofextensive areas of Australian agriculture, anincreasing diversity of soil fauna has been linkedto higher carbon dioxide respiration, soil organiccarbon and mineral nitrogen production above19MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDE

20MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEthat of soils containing only bacteria and fungi(Kautz and Topp 2000).EarthwormsEarthworms are generally considered positive for thehealth of broadacre agricultural soils, but quantifyingtheir impact on soil function has not been extensivelystudied in Australia. Since earthworms do not haveteeth they ingest both organic matter and soil, usingthe soil to help grind up organic residues internally.Their waste (worm casts) is a resultant mix ofstrongly aggregated soil and organic residues thatare rich in plant available nitrogen (0.6 per cent) andphosphorus (2.8 mg/100 g). The carbon content ofthese casts is on average 1.5 times that of the bulksoil (Bhadauria and Saxena 2010).Significant numbers of earthworms are required tostimulate an improved soil structure. For example,Fonte et al. (2012) determined the equivalent of 144worms per m 2 in the top 10 cm of a soil that alsohad high fungi and bacteria numbers and activelygrowing roots, was required before a six per centincrease in aggregate stability was measured. Thishas led to estimates of drainage rates up to 10times faster and infiltration rates six times faster insoil with earthworms compared to those withoutearthworms.Significant numbers ofearthworms are required tostimulate an improved soilstructure.In Australian cropping soils, only a few speciesof earthworms are associated with improved plantgrowth (Blackmore 1997). Current estimates ofearthworm populations in Australian agricultureare between 4/m 2 and 430/m 2 , although higherpopulations have been observed in some pasturesystems (Buckerfield 1992; Mele and Carter 1999;Chan and Heenan 2005). In north-eastern Victoriaand southern New South Wales, the density ofearthworms across 84 crop and pasture sitesaveraged 89 earthworms/m 2 , with a low speciesrichness of 1-2 per site (Mele and Carter 1999). Thisis similar to the average abundance (75 earthworms/m²) reported by Chan and Heenan (2006).Earthworm populations generally increased inwetter environments above 600 mm annual rainfalland were three times higher in pasture sites thancropping paddocks (Mele and Carter 1999). Zerotillagecombined with stubble retention was alsoshown to support earthworm populations up toseven times higher than in disturbed systems whereresidues were burnt (Chan and Heenan 2006).Variability in earthworm numbers has also beennoted within seasons, increasing during the wintermonths to between 160/m 2 and 501/m 2 for cropand pasture systems respectively and betweenseasons (Chan and Heenan 2006).Earthworms are not readily supported in somesoils, including very coarse sands or acidic soils(pH Ca less than 4.5), and are often absent regardlessof management. In part this is due to low levels ofcalcium in these soils, which is required continuouslyby earthworms.Soil-borne plant pathogensOrganisms that attack living plant tissue andcause plant diseases are called pathogens. In soil,undesirable organisms include a range of insects,parasitic nematodes, protozoa, viruses, bacteriaand fungi, which may be present even where thereare no visible symptoms. For a disease to developseveral criteria must be met. There must be asuitable host plant, a pathogen and an environmentsuited to its growth. Climatic patterns also affectthe types of fungal pathogens that are dominant ina region. Disease outbreaks can be caused by anincrease in the population of the pathogen, or byan increase in susceptibility of the plant, which isaffected by factors such as its age and nutritionalstatus, environmental stress, crop type or variety.The degree of root damage will generally relate tothe number and type of disease pathogens present.Undesirable organisms influence soil health andproduction through their influence on root and plantvigour and changes in the soil food web. A highincidence of pathogens can slow root growth anddecrease the ability of roots to acquire water andnutrients, decreasing grain and pasture yields, andconstraining organic matter inputs into soil. In doingthis, the addition of fresh organic inputs that favoursthe growth of a diverse range of beneficial organisms(as compared to pathogens) is constrained.Soils with high levels of soil organic matter andbiological activity seem to prevent pathogensfrom taking hold due to increased competition forresources, which constrains pathogen activity, or

21MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEby sheltering antagonistic or predatory microbes.For example, a decline in plant-parasitic nematodesand an increase in saprophytic nematodeswere observed with the use of diverse rotationalsequences, addition of organic matter, covercrops, green manures, composts and other soilamendments (Widmer et al. 2002).Suppressive soilsSuppressive soils are those that naturally suppressthe incidence or impact of soil-borne pathogens.Disease suppression can develop over 5-10 yearsand is a function of the population, activity anddiversity of the microbial biomass (Roget 1995,2006). In southern Australia, sites with high levelsof disease suppression to rhizoctonia (Rhizoctoniasolani) and take-all (Gaeumannomyces graminis var.tritici) have been associated with higher inputs ofbiologically available carbon and greater competitionfor food resources (Roget 1995, 2006).The increase in available carbon at these siteswas associated with intensive cropping, full stubbleretention, limited grazing, no cultivation and highyielding crops. However, the adoption of thesepractices does not always result in suppression ofsoil pathogens as evidenced by the many instancesin which disease continues to be prevalent. To datethere are no indicator species associated with thedevelopment of suppression in agricultural soils,though research in this area is progressing.ORGANIC MATTER TURNOVER TIMESThe ‘turnover’ or decomposition rate of organicmatter refers to the time taken for organic matter tomove into and through the various organic matterpools, including living, actively decomposing andstable. This movement is a continual processand is vital to the functioning of all ecosystems.As new organic matter enters the soil it supportsbiological processes, releases nutrients throughdecomposition and contributes to soil resilience.The difference between organic matter inputs andoutputs and the rate at which they are transformeddetermines the size and stability of the organic matterpools (see Chapter 1). Soil biological function is lesssensitive to the total amount of organic matter thanthe rate at which organic matter turns over, whichis related to the size and nature of the soil organicmatter pools and soil depth (Dalal et al. 2011).Understanding and quantifying the mechanismsdriving turnover of organic matter between the poolsis critical to the capacity to increase and maintainsoil organic carbon in different soils and climates.The resistant soil organic matter fraction can takeseveral thousand years to turn over and is relativelyinert, while the stable humus pool generally takesdecades or centuries. In contrast, the activelydecomposing pool, which includes the particulateand dissolved organic fractions, has a turnover timeof less than a few hours through to a few decades.As organic matter is decomposed and moves fromthe rapidly decomposing fraction through to the

22MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEstable humus or resistant organic matter fractions, itbecomes both more resistant to decomposition andincreasingly nutrient rich (i.e. the carbon to nitrogenratio declines).Organic matter characteristicsThe carbon to nitrogen ratio of the organic matterbeing decomposed has a significant influenceon the rate and amount of nutrient release (seeChapter 1). As the ratio decreases, organic matteris generally decomposed more rapidly and there isgreater potential for a net release of plant-availablenutrients.The chemical composition of plant residuesstrongly influences the rate at which organicmatter is decomposed. Soluble sugars, metaboliccarbohydrates and amino acids are rapidlydecomposed, while more resistant plant materialsuch as lignin, cellulose and polyphenols takesignificantly longer to break down. This differentialin decomposition rate, results in residues with asimilar carbon to nitrogen ratio, but different chemicalcomposition having widely variable decompositionrates. For example, Lefroy et al. (1994) found thedecomposition rate of Asian pea leaf residues wasmore than double that of medic hay due to differencesin the chemical composition of the residues.Suppressive soils are thosethat naturally suppress theincidence or impact of soil-bornepathogens.The physical location of organic residues within thesoil and the level of soil disturbance also influencethe rate at which organic matter is broken down ormineralised. For example, Hoyle and Murphy (2011)found nitrogen was mineralised more rapidly asresidues were incorporated with increasing intensityinto a red-brown earth compared to being left onthe soil surface.Soil characteristicsThe way in which a soil is constructed (i.e. itsarchitecture) can influence soil organic matteraccumulation or loss. Protection of organic matterin soil is most often associated with soil texture,specific surface area and mineralogy, which influencehow organic carbon may be adsorbed. Otherchemical and physical attributes of the soil such aspH, soil water and porosity can also modify the rateof organic matter decomposition and therefore therate at which nutrients and carbon are cycled withinthe soil (see Table 2.1).Table 2.1 Soil factors that can influence the rate oforganic matter turnover.Decrease soil organic matterturnoverIncreasing soil depthNutrient deficiencyHigh lignin and wax contentWaterlogged (anaerobic) soilLow temperaturesClay texturesAggregationVariable charge surfacesIncrease soil organic matterturnoverSurface litterNutrient richHigh carbohydrate contentFree draining soilsHigh temperaturesSandy texturesUnstructured soilsLow charge surfacesSoils with increasing clay content can often retainorganic matter for longer than coarse textured sandysoils by restricting access to organic residues. Thisis associated with a greater proportion of smallpore sizes, which house smaller organic particlesthat microbes can’t get to, and increasing soilaggregation which can encase organic particlesand prevent access. While this effectively increasesthe retention of soil organic matter for longer it doesnot necessarily remove it from the decomposingpool. For example, management practices orevents that expose this protected material thenleave it vulnerable to decomposition. Thereforemanagement practices that change the structure ofa soil can influence the breakdown of organic matterfractions.ClimateIn moist soils, higher temperatures increase theturnover rate of organic matter. For example, Hoyleet al. (2006) showed the mineralisation rate oforganic matter doubled with each 10°C increasein average soil temperature between 5-40°C in soilheld at 45 per cent water holding capacity. Adequateamounts of water and oxygen are also required fordecomposition to occur.Rainfall and soil porosity determine availablesoil water. Changes in the water filled pore space

23influence carbon mineralisation, with higherdecomposition rates associated with an increasingproportion of soil pores larger than 3 mm in size.Therefore, sandy soils with a high porosity have afaster rate of decomposition than clay soils. Whilethe amount of carbon mineralised at optimal watercontent is similar, the latter can vary between soiltypes. For example, the optimum water filled porespace for carbon mineralisation was determined at45 per cent on sand with 10 per cent clay, comparedto a water filled pore space of 60 per cent on aheavier soil with 28 per cent (Franzleubbers 1999).Location in the soil profileIn Australian soils, about 60 per cent of themicrobial biomass in the top 30 cm is located justbelow the surface (0-10 cm). Large soil biota suchas earthworms, mites, termites and ants, whichactively break-up large organic residues are alsopresent in higher concentration near the soil surface.As a result, the turnover rate of organic material insurface soil (0-10 cm) is almost double that belowthis depth.ECOSYSTEM SERVICESGlobal carbon balanceBiological processes in soil contribute to fluxes ingreenhouse gas emissions (see Chapter 7). Theinfluence of these processes on soil organic matteris therefore critical to the global carbon balance.As soil organisms break down soil organic matterthey release carbon dioxide, which when lost to theatmosphere is a potential source of greenhousegases. In Australia, up to 75 per cent of the carbonin fresh organic residues may be released as carbondioxide during the first year of decompositiondepending on climate and as much as 90 per centover the long-term. Soils also represent the largest‘sink’ or store for organic carbon. Therefore, thebalance between accumulation of carbon resultingfrom organic inputs and losses of carbon resultingfrom the decomposition of organic matter is criticalto mitigating greenhouse gas emissions.Soil bufferingOverall, adequate amounts of soil organic mattermaintain soil quality, preserve sustainabilityof cropping systems and help to decreaseenvironmental pollution (Fageria 2012). Addingorganic matter to soils can decrease the toxicityof heavy metals to plants through absorption anddecrease the contamination of waterways andground water by pesticides through adsorption(Widmer et al. 2002).Soil acidification which occurs widely across arange of agricultural soils is primarily associatedwith the removal of agricultural produce and highrates of nitrification, which contribute to leaching.Soil organic matter can contribute to in-situ soilacidification as humic and fulvic acids accumulatein soil, but can also have a buffering effect againstthe process depending on the quality of the organicmatter. While the incorporation of high inputs oforganic matter is thought to help neutralise soil aciditythis effect is most pronounced when residues havebeen burnt. The application of lime to low pH soilsto meet minimum targets in surface (> pH Ca 5.5) andsub-surface soils (> pH Ca 4.8) as recommended forWestern Australian soils remains the most effectiveamelioration strategy (targets may vary by region).MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDE

2403MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDETHE CARBON CYCLEAT A GLANCE• Soil organic matter content is determined first by soil type then by climate and then bymanagement.• Increasing stable soil organic matter pools is a long-term focus in agricultural systems. Itisn’t going to happen overnight.• In many instances less than 15 per cent of carbon inputs eventually contribute to the soilorganic carbon pool.• Soil function can be constrained when soil has less than one per cent organic carbon.

Carbon cycling between the soil, plants andatmosphere involves the continuous transformationof organic and inorganic carbon compoundsby plants and organisms (see Figure 3.1). Soilrepresents a reservoir able to both store and releasecarbon within the global carbon cycle and as such isconsidered both a sink and source for carbon.Soils contain carbon in both organic and inorganicforms, with the exception of calcareous soils, whichis largely held as soil organic carbon. This organiccarbon continually enters and leaves the soilresulting in both carbon accumulation and loss. Atany one time the amount of organic carbon in soilrepresents the balance between inputs and losses.Since carbon turnover can beconstrained by available nutrientsas suggested by the carbon tonutrient ratio, it is likely that morefertile soils will lose organic matterat a faster rate than lower nutrientcontent soils.A significant amount of the organic carbonaccumulated in soils has resulted fromphotosynthesis where plants convert atmosphericcarbon dioxide into above-ground shoot growth andbelow-ground root growth. As primary productivityincreases, organic inputs resulting from shoots,roots and micro-organisms grow and contribute toa build-up in soil organic carbon.Carbon emissions from soil back to theatmosphere occur in the form of carbon dioxide,largely as a result of agricultural practices drivingchanges in microbial processes. These emissionshave resulted primarily from the decomposition oforganic matter, reflecting the historical declines thathave been measured in soil organic matter for manyagricultural soils and contributed to the measuredincreases in atmospheric carbon dioxide resultingfrom human activities.CARBON BALANCE IN SOILSSoil organic carbon is in a constant state of flux,slowly responding to environmental or managementchanges and moving to reach a new equilibriumlevel after changes occur (see Figure 3.2). Forexample, in systems where plant production isconstrained, organic matter inputs decline and soilbiota increasingly deplete stored soil organic carbonfor energy. This results in declining soil organiccarbon levels until a lower limit, determined by soiltexture and a decline in biological activity, whichis essentially starved of decomposable carbon, isreached. In contrast, systems with increasing organicinputs to soil can attain a higher level of soil organic25MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDE Figure 3.1 Organic carbon cycling in soils.

less than 15 per cent of carbon inputs eventuallycontribute to the soil organic carbon pool (Chan etal. 2010). Figure 3.3 A model simulation showing theinfluence of cation exchange capacity (CEC) on thecapacity of soil to retain soil organic carbon(Grace et al. 2006).For example, a soil with a cation exchange capacityof 5 meq per 100 grams soil (e.g. typical of a sandysoil) might be expected to retain 30 per cent of thecarbon added, so one tonne of dry plant materialcontains 450 kg carbon, but will only retain about135 kg of organic carbon in the soil. If the soil hada background organic carbon content of 10 tonnescarbon per hectare in the top 10 cm this wouldreflect a change in organic carbon of just over 0.01per cent. Therefore, increasing soil organic carbonis normally a slow, incremental process and inputsmust be sustained to maintain these changes. Itis also why it is difficult to detect short-term (lessthan 10 years) measureable changes in soil organiccarbon.O’Halloran et al. (2010) suggest a rule for irrigatedmixed farming systems where the retention of anextra tonne of organic matter every year for 10years will increase soil organic carbon by nearly 0.3per cent — as occurred in northern Victoria andsouthern New South Wales. In their study, the extraorganic matter includes the contribution from abovegroundshoot and below-ground root biomass.In Western Australia, the potential of a sandy soilto store organic carbon was evaluated in the lowrainfall zone (less than 350 mm annual rainfall). Inthis study, 60 tonnes of organic matter added aschaff over a period of eight years to a continuouscropping system resulted in an additional eighttonnes of carbon per hectare measured to 30 cm(Liebe Group unpublished). This reflects a retentionfactor of approximately 30 per cent. While we do notpropose that this is an economic or logistical reality,it does demonstrate the potential to increase soilorganic matter to an attainable potential determinedby soil type and climate (see Chapter 1).HOW STABLE IS ORGANIC MATTER?The chemical compounds in organic matter breakdown at different rates. The first organic compoundsto be broken down are those that have simplecellular structures such as amino acids and sugars.Cellulose breaks down more slowly and phenols,waxes and lignin remain in the soil for long periods oftime due to the complex structure of the molecules.Plant stems and leaves break down at differentrates due to the differences in their molecularstructure and the strength of their chemical bonds.Leaves generally have more cellulose, which is asimple molecule that decomposes rapidly, and lesslignin which has a complex structure and breaksdown more slowly. Specialised enzymes whichincrease the decomposition rate are required fortheir degradation. When lignin is associated withcellulose within plant cells it becomes more difficultto degrade. Under most conditions it might takethree times as long to degrade stem material as itdoes to degrade leaves.The chemical compounds inorganic matter break down atdifferent rates.The stability of organic carbon is also related to thesoil organic matter fraction in which it resides. In thisrespect, soil organic carbon can be partitioned intofractions based on the size and breakdown ratesof the soil organic matter in which it is contained(see Chapter 1). Carbon sources in the active pool,including fresh plant residues, particulate organic27MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDE

Table 3.1 The influence of soil organic matter on soil attributes and functions.Soil attribute or function Mode Impact Quantification29Soil architectureSoil fertilityWater infiltrationMicrobial by-products such asbacterial glues and fungal hyphaecan increase soil aggregation andstability of soil structure.Decomposition of organic matterdetermines nutrient supply.Microbial by-products suchas bacterial glues and fungalhyphae increase potential for soilaggregation.Increasing organic matter can beassociated with development ofwater repellence on soil with lessthan 15 per cent clay.Can improve water infiltration,soil porosity and the exchange ofwater and oxygen.Increased plant productivity.Decreased input costs.Decreases moisture loss fromrun-off, evaporation.More water enters the soil profile.Increased soil porosity helps theexchange of water and oxygen.Minimal impact on coarsetextured sands.Up to 80 per cent of nitrogenuptake results from the turnoverof organic matter.Soils can supply between 50-100kg nitrogen per hectare eachyear.Organic matter also containsphosphorous and sulphur.Increasing the rate of entry willlead to a greater potential formore available water.MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEWater holding capacity (or‘bucket size’)Soil organic matter can holdseveral times its own weight inwater.Increased plant available waterand decreased deep drainagebelow the root zone.Supports higher productivity.A one per cent increase insoil organic carbon stores theequivalent of a maximum 5.6 mmof soil water in the top 10 cm.Depending on soil type only aproportion of this will be plantavailable.Soil biological processesOrganic matter is the primarysource of energy required by soilmicroorganisms for growth andreproduction.Influences nutrient cyclingand availability, soil diversity,resilience and impact of stressevents(i.e. pathogen impact, recovery).The microbial efficiencyinfluences what percentage oforganic matter is retained in soils.A greater proportion of carbon isretained in soils with increasingclay content.In Australia, typically less than30 per cent of carbon is retained(can be up to 50 per cent).Buffers soil pH(helps maintain acidity at aconstant level)Organic matter is alkaline in itsnature.Large amounts of residues or aconcentration of burnt residuescan increase soil pH.Burnt residues can have a limingeffect (e.g. often observed onburnt windrows) resulting inhigher yielding areas.Burning residues will result inless labile carbon entering thesoil.Soil resilience — defined asthe ability of a soil to recoverto its initial state after astress eventIncreasing organic mattersupports an increasing diversityof microorganisms, many ofwhich can undertake similar rolesallowing for some redundancy.Increased ability of soil to recoversoil function after a disturbance.Less than 10 per cent of thepopulation is active at any time.It is estimated that losses of upto 20 per cent of the microbialbiomass would have little impacton soil function.

0430MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDESOIL ORGANIC MATTER ANDNUTRIENT AVAILABILITY INAGRICULTUREAT A GLANCE• Organic matter contains a large store of nutrients.• As a general rule, for every tonne of carbon in soil organic matter about 100 kg of nitrogen,15 kg of phosphorus and 15 kg of sulphur becomes available to plants as the organicmatter is broken down.• Between 2-4 per cent of soil organic matter is decomposed each year.• Organic matter contributes significantly to cation exchange capacity in sandy soils.

Soil organic matter has a reservoir of nutrients boundwithin its organic structure, which are released intothe soil solution as soil microorganisms mineralise(break down) the organic matter for their ownmetabolism and growth.The amount of nutrients provided to plants viamicrobial breakdown of organic matter depends onthe type of material that is being mineralised and itsratio of carbon and other nutrients such as nitrogen,phosphorus and sulphur.As a rule, for every tonne of carbon in soil organicmatter about 100 kg of nitrogen, 15 kg of phosphorusand 15 kg of sulphur becomes available to plants asorganic matter is broken down. As well as releasingplant nutrients, the microbes also release between50-90 per cent of the carbon in organic matteras carbon dioxide.The rate at which organic matter is broken downdetermines how rapidly the nutrients within theorganic structure become available to plants. Theparticulate organic matter fraction breaks downrapidly, so its nutrients are made readily availableto plants and microbes. In contrast, the humusfraction breaks down over decades and providesa large but slow-release supply of plant-availablenutrients. Other more recalcitrant forms of carbonare relatively inert, taking hundreds to thousandsof years to break down and having little influenceon the amount of nutrients released into thesoil solution.31MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDE

32MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEOrganic matter and cation exchangeThe ability of a soil to hold positively charged cationssuch as calcium, magnesium, potassium, sodium,hydrogen and aluminium at a given pH determinesits cation exchange capacity. Cation exchangemeasures the ability of a soil to hold on to and supplynutrients to plants. The cation exchange capacity ofa soil provides information on its structural stability,resilience, nutrient status and pH buffering capacity.Sodium and aluminium are negatively correlatedwith plant growth. Soil test results are expressedeither in milliequivalents per 100 grams soil(meq/100 g) or centimoles of charge per kilogram(cmol/kg).Soils have variable cation exchange capacityranging from sands, with a very low cation exchangecapacity often less than 3 meq/100 g, to vermiculite,which may hold up to 200 meq/100 g. Kaoliniticclays have a moderate cation exchange capacityof about 10 meq/100 g, while other clays such asillite and smectite have a higher exchange capacity(Purdie 1998).Table 4.1 contains information on the cationexchange of clay minerals.Table 4.1 Indicative cation exchange capacity ofdifferent clay minerals in soil (Moore et al. 1998).Clay mineralCation exchange capacity (meq/100g)Kaolinite 3-15Illite 10-40Montmorillonite 70-100Smectite 80-150Vermiculite 100-150Humified organic matter has a very high cationexchange capacity from 250-400 meq/100g. Therefore, in soils with low clay content theamount of humus and resistant soil organic matteris increasingly important to nutrient exchangebecause its large surface area gathers (adsorbs)cations from the soil solution, holding nutrientsthat would otherwise leach. Williams and Donald(1957) estimate that each percentage increase insoil organic carbon is the equivalent of 2.2 meq/100g cation exchange and in some soils contributesas much as 85 per cent of the cation exchangecapacity (Helling et al. 1964; Turpault et al. 2005;Hoyle et al. 2011).The contribution of organic matter to soil cationexchange capacity declines with soil depth, decreasingsoil pH (i.e. increasing soil acidity) and with increasingclay content (see Table 4.2).Table 4.2 Indicative cation exchange capacity fordifferent soil textures and organic matter.Soil textureCation exchange capacity (meq/100g)Sand 1-5Sandy loam 2-15Silt loam 10-25Clay loam/silty clay loam 15-35Clay 25-150Organic matter 40-200Humified organic matter 250-400NITROGEN, PHOSPHORUS, SULPHURAND ORGANIC MATTEROrganic matter contains a large store of nutrients— the majority of which are unavailable for plantuptake. It estimated that 2-4 per cent of soil organicmatter is decomposed each year (Rice 2002). Usingan average three per cent turnover and based on acarbon to nutrient ratio of 1000 (C):100 (N):15 (P):15(S), this suggests for a soil which has 1400 tonnesof soil per hectare and a soil organic carbon contentof 2.1 per cent, there would be a release of about 88kg nitrogen, 13 kg phosphorous and 13 kg sulphureach year from organic matter.Nitrogen supplyIn most soils, while nearly all nitrogen is presentin organic form, plants are generally better able totake up inorganic (mineral) nitrogen forms such asammonium (NH 4+) and nitrate (NO 3-). Nitrate is thedominant form of nitrogen taken up by agriculturalplants.The conversion of organic nitrogen to inorganicnitrogen is a biological process associated withthe mineralisation (decomposition) of organicmatter. Mineralisation results in the productionof ammonium, which is predominantly takenupby and immobilised within soil microbes andthen transformed via nitrification to nitrate. Theseprocesses can be limited by soil pH Caless than 5.5,poor soil permeability resulting in water-logged soils,carbon availability, drying soils and temperaturesbelow 20°C (Mengel and Kirkby 1987).Soil biological processes are also integral to the

33MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEavailability of the majority of inorganic fertilisersapplied to soil, which are transformed into nitrateby soil microbes before being taken up by plants.This includes urea which is either decomposedby enzymes or chemically hydrolyzed to produceammonia and carbon dioxide. The ammonia isthen converted by microbes into ammonium andsubsequently converted into nitrate by specialistmicroorganisms through a process known asnitrification.In Australia, inorganic mineral fertilisers oftenmake up as little as 20 per cent of crop uptakedue to relatively low fertiliser applications and poornitrogen use efficiency. Biological processes supplythe remainder and in some cases contribute up to80 per cent of crop nitrogen uptake (Angus 2001).Although direct uptake of ammonium fertilisersby plants can occur most nitrogen fertilisersapplied in an ammonium (NH 4 +) form areconverted to nitrate (NO 3 -) by the soil microbesand are then taken-up by plants in this form.Inorganic nitrogen moves readily in soil and isrequired in relatively large amounts at critical stages incrop growth such as terminal spikelet, which occursabout eight weeks after sowing, and during grain fill.In wheat, nitrogen deficiency early in the season limitstiller formation and spikelet and floret number, whichin turn reduces yield potential. Later in the seasonnitrogen deficiency can result in smaller or fewer grainand where sufficient moisture during grain filling inlower grain protein.Nitrogen cyclingSoil nitrogen is primarily determined via biologicalprocesses, which are influenced by rate limitingfactors such as soil pH, tillage, soil moisture andtemperature. Ammonium released from organicmatter mineralised by soil microbes determinesthe supply (rate and amount) of inorganic nitrogen.The rate at which nitrogen is immobilised withinsoil microbes and converted to nitrate is directlyproportional to microbial demands for nitrogen(Murphy et al. 2003) and determines the net amount(or surplus) of soil nitrogen that becomes available forplant uptake. While both plants and microorganismscan use ammonium a large proportion of it isconverted into nitrate. Once dissolved in solution,nitrate is more readily taken-up by plants, but is alsoeasily leached (see Figure 4.1).Plant-available nitrogen originates from fertiliserinput, nitrogen fixation and mineralisation of organicmatter. The fate of mineral nitrogen within the profileis the result of immobilisation, plant uptake, leachingand gaseous losses.

Figure 4.1 Nitrogen cycling in soil.34MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEThe influence of organic inputs onsoil nitrogen supplyThe ratio of carbon to nitrogen in organic matterinfluences how much nitrogen will eventuallybecome available to plants through microbialdecomposition (Hoyle and Murphy 2011). Forexample, when organic matter with a carbon tonitrogen ratio greater than 25:1 is broken downby microbes much of the nitrogen contained inthe organic matter is taken-up and immobilised bythe microbial population resulting in relatively littlenitrogen being made available to plants (see Chapter1). Increasingly poor quality residues such as wheatstraw, with an even wider carbon to nitrogen ratio(more than 50:1), result in microbial uptake andimmobilisation of existing plant-available nitrogen.This can lead to nitrogen deficiency during periods ofhigh crop demand, which contrasts with high qualityresidues with a carbon to nitrogen ratio less than25:1 where surplus nitrogen is released to the soil(see Figure 4.2). Figure 4.2 Nitrogen release in soil resulting fromthe decomposition of plant residues with a rangeof carbon to nitrogen (C:N) ratios. Adapted fromHoyle et al. (2011).

35 MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDE Figure 4.3 Phosphorus cycle in agricultural systems.While the sustained release of nitrogen associatedwith organic sources might be expected to moreeffectively raise grain protein than inorganicnitrogen fertiliser applied at the start of the season,environmental and management factors make itdifficult to predict when the organic nitrogen willbecome available and matching organic nitrogensupply to crop demand is not straightforward.The polyphenol and lignin content of organic matteralso influence the amount of nitrogen released fromorganic residues, with increasing amounts of thesesubstances limiting microbial access and nitrogenrelease (Ha et al. 2008).PHOSPHORUSPhosphorus is required for cell growth duringearly plant development. Demand for phosphorusincreases once phosphorus reserves in the seedhave been exhausted following plant establishment.Australian soils are inherently low in phosphorusand significant amounts of inorganic phosphorusfertiliser have been historically added to agriculturalsoils to support profitable crop and pastureproduction. Very soon after application, inorganicphosphorus reacts with clays and iron or aluminiumoxides rendering it ‘fixed’ and unavailable to plants(see Figure 4.3).Precipitation also results in a range of insolublesecondary phosphorous compounds forming inthe soil and as a result as little as five per cent ofthe phosphorus applied is available to crops inthe year of application due to these processes.These are distinguishable from direct losses suchas product removal, run-off and leaching becausethe phosphorous though relatively unavailableremains in the soil (see Figure 4.3). Plant-availablephosphorus in the soil solution is dominated bynegatively charged orthophosphate ions (H 2PO 4-and HPO 42-) though small quantities of solubleorganic phosphorus compounds might also bepresent. As a consequence, plant growth responsesto phosphorus fertiliser are common despite soil

mycorrhizal associations have been known to causeyield penalties (Ryan et al. 2002, 2005).Arbuscular mycorrhizal fungi have a symbiotic(defined in the broadest terms as two ormore organisms living together) associationwith the root of a living plant and are primarilyresponsible for nutrient transfer.Organic phosphorus sources are relatively slowrelease,but it is not easy to predict exactly whensoluble phosphorus will become available. Toensure a more reliable supply of phosphorus it isbest to apply a combination of mineral and organicsources. Soils with a high tendency to adsorb orfix phosphorus are likely to require phosphorusfertiliser application to meet crop requirementseven when soil tests suggest sufficient levels ofphosphorus exist.Diffusive Gradient in Thin-Films (DGT) is a newmethod of measuring soil phosphorus (Mason et al.2010) and is shown to be more accurate than otherconventional methods in estimating the phosphorusrequirement of crops. It provides an improvedmeasure of the phosphorus available for plantuptake and determines the likely yield responsefrom additional fertiliser (see http://soilquality.org.au/factsheets/dgt-phosphorus for further information).SULPHURSulphur is essential for plant protein production andsulphur deficiency can lower grain quality. Sulphur isalso critical for effective nitrogen fixation in legumes.Cereals typically require twice the amount of sulphuras phosphorus. As sulphur is relatively immobile inplants, a sustained supply of the mineral is requiredfrom the soil.Sulphur and organic matterMost sulphur in soil is bound in soil organic matterfor surface soils (0-10 cm). The ratio of carbon tonitrogen to phosphorous to sulphur in soil organicmatter is usually about 108:8:1:1 (108 units ofcarbon, eight units of nitrogen and one unit ofphosphorous and sulphur). Many soils also containgypsum in their subsoil layers.Sulphate (SO 4-2) is mineralised when soil organicmatter is broken down and is the most plant-availableform of sulphur in well aerated soils. Sulphate ismade available to plants from organic matter with acarbon to sulphur ratio of less than 200:1. Residueswith a carbon to sulphur ratio of more than 400:1usually result in sulphur immobilisation (Delgadoand Follet 2002). Sulphate not taken up by plants isvulnerable to leaching and in coarse textured soilsunder high rainfall sulphur is often deficient.Sulphur deficiencies in the soilHistorically, sulphur deficiency has been rarein Australia because of the widespread use ofsuperphosphate fertilisers containing sulphur.However, in recent years sulphur deficiency hasbecome more evident with the switch to low-sulphatephosphate fertilisers and the increasing adoptionof canola, which has a high sulphur requirement.Sulphur deficiency is particularly apparent in soilswith high nitrogen availability, which increases yieldpotential and therefore sulphur demand. Removalof sulphur in organic matter via the harvest of cropsand pastures can be rectified with a replacementstrategy of between 2-5 kg of sulphur per tonne ofgrain or biomass removed (or in the case of canola10 kg of sulphur per tonne of removed grain orbiomass).POTASSIUMBetween 95-98 per cent of the potassium in soilis unavailable to plants and exists as a structuralcomponent of soil minerals until broken downby weathering processes. Instead, plants largelyacquire potassium in the form of exchangeablepotassium, or dissolved potassium available in soilsolution. Available soil potassium results from thenet effects of supply processes, including mineralweathering, addition of fertilisers and mineralisationof organic inputs against losses associated withleaching, erosion, plant uptake and fixation.Soil organic matter increases the soil’s cationexchange capacity and in doing so increasesthe amount of soluble potassium, calcium andmagnesium available for release during mineralisation(Delgado and Follet 2002). Potassium is alsoreleased relatively quickly from crop residues tocontribute to the non-exchangeable, exchangeableand soil solution potassium pools. However, unlikenitrogen and phosphorous, available potassium inmany situations appears more closely linked to soiltype (clay complex) than soil organic matter.37MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDE

Soil organic matter and in particular the humusfraction can hold several times its own weight inwater. It seems logical, then, that increasing theorganic matter content of soil would have a positiveimpact on the water holding capacity of a soil.However, while there is indeed an establishedlink between soil organic matter and water holdingcapacity, its importance declines with soil depth andincreasing clay content (Hoyle et al. 2011). About60 per cent of the organic matter in the top 30 cmof soil occurs in the surface layer (0-10 cm), so theinfluence of soil organic matter on soil water is mostevident in the topsoil.There is little influence of organic matter on plantavailable water late in the season when soil moistureis usually below 30 cm. Clay also functions toabsorb soil water, decreasing the relative influenceof soil organic matter on water holding capacity asthe clay content of a soil increases (see Figure 5.1). Figure 5.1 Change in water holding capacity forthe 0-10 cm soil layer of South Australian redbrownearths, with a one per cent increase in soilorganic carbon content (Hoyle et al. 2011).The relative value of available soil water dependson the amount and frequency of rainfall in any oneseason. For example, any additional water holdingcapacity provided by soil organic matter is only likelyto be beneficial when intermittent rainfall results inlow, infrequent or variable soil water. In contrast,frequent or high in-crop rainfall is likely to diminish theimpact of soil organic matter on soil water holdingcapacity. In addition, and depending on soil type,the amount of extra water held does not necessarilymean it will be available to plants due to its locationin the profile relative to roots, or because it is heldso tightly by the soil that it cannot be extracted bythe plant.Maximum amount of extra water stored froma 1% increase in soil organic carbon (SOC)1% SOC = 14 t C ha* = 5.6 mm*** Calculated to a depth of 10 cm in soil with abulk density of 1.4** Actual plant available water will be influencedby soil texture** Assumes soil organic matter holds four timesits weight in waterWHAT AFFECTS THE WATER HOLDINGCAPACITY OF A SOIL?The texture and structure of a soil determine its abilityto form soil aggregates, which help determine waterand nutrient storage (see Table 5.1). A soil organiccarbon content of two per cent is considered optimalfor aggregate stability (Kay and Angers 1999).The spaces or pores that form between soilaggregates are capable of storing water andprovide a home for soil biota. Soils with a widerange of particle sizes are unstable and are easilycompacted to form dense, often impenetrablelayers that constrain root growth. Organic matterhelps to create and stabilise soil pores, promotesthe formation of soil aggregates and contributes tosoil water storage via its capacity to absorb water.BARRIERS TO ROOT GROWTHPhysical, chemical and biological subsoil constraintscan prevent plant roots from growing to depth.Pathogens, physical impediments or chemicalbarriers such as subsoil acidity and salinity can slowplant root growth and prevent access to subsoilmoisture. Improving soil structure and removingthese barriers to plant growth can improve thewater storage capacity of the soil and increase thearea and depth of soil available to plant roots forexploration.Water and nutrients can move through compactedsoil (see Plate 5.1), but often remain unavailable toplants because of restricted root growth. When soilstrength measures about 2000 kilopascal (kPa), orhas a bulk density higher than 1.7 g/cm 3 root growthtypically stops, continues horizontally or occurs at aslower rate. Bulk density is influenced by soil organic39MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDE

Table 5.1 Influence of soil characteristics on water storage capacity.Soil characteristic Effect on soil structure and water storage Impact on cropsIncreasing clay contentLow clay or silt content (e.g. uniform, coarsetextured soils such as deep sands, sandyearths)Poor soil structure (e.g. dispersive, hardsetting, naturally compacting soils)Texture contrast soils(e.g. sand over clay duplex)Cracking clays(light clay texture throughout soil profile, withcoarser material on the surface)Helps form soil aggregates with small poresize, which increases water holding capacity.Clay soils with a high level of exchangeablesodium are likely to disperse and have pooraggregate stability.Results in poor aggregation.Water drains freely through profile.Low water and nutrient storage capacitywithin the root zone.Poor water infiltration.Increased risk of erosion.Low water storage capacity.Plant available water depends on:• surface soil texture• depth to subsoil• nature and texture of subsoil• interface between surface and subsoilWater preferentially flows into cracks, whileareas between cracks remain dry due to thedense soil structure and rapid water flow.Increased yield potential.Small pores can make it more difficult forplants to extract soil water under dryingconditions.Poor aggregate stability can result in surfacecrusting, hard-setting, waterlogging and lessavailable water.Crops and pastures can run out of water in atight (dry) finish.More prone to developing water repellence,restricting water entry.Crop yields often below potential due tolower water storage and restricted accessto water.Perched water table above dense claysubsoil can result in waterlogging.Water availability and yield potentialdetermined by infiltration pattern and rootingdepth.40Increased but spatially variable waterstorage capacity.MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEHigh organic content soilsFresh organic matter stimulates biologicalactivity including soil fauna such asearthworms and termites, which in turnhelp form the soil pores that increase soilporosity, water infiltration and soil watercapacity.matter content, porosity and structure. Less dense,well-structured soils have a lower bulk density thanpoorly structured, low-organic or ‘massive’ soils(cemented in a large mass). Light textured sandysoils are prone to compaction and higher bulkdensity, both of which tend to increase with depth.Water movement is constrained at bulk densitieshigher than 1.6 g/cm 3 .Biological secretions, fungal hyphae andworm casts help to stabilise soil structure bybonding organic materials to soil minerals.Higher yields when associated withincreased soil water storage and nutrientcycling.A measure of bulk density is also needed to calculate‘stock’ values of soil properties per unit area such asthe amount of soil organic carbon per hectare and canbe done as outlined on page 41.Such calculations allow soil properties to bemonitored accurately over time and ensure thatwhen changes in bulk density occur the soilresource condition (e.g. soil organic carbon content)

Bulk density (BD) = soil mass per knownvolume of soil = g/cm 3Measuring bulk densityThis method works best for moist soils withoutgravel.1. Prepare a flat, undisturbed surface at thedepth you wish to sample.2. Collect a known volume of soil (steel coreor tube or PVC pipe; minimum 40 mmdiameter and 100 mm depth) by pushing orgently hammering core into soil taking carenot to compact it (see Plate 5.2).3. Remove core, brush away any soil on theoutside of tube, check soil is flush with coreends and place the soil from the core into alabelled plastic bag.4. Record date, sample code and location ofsample (if possible using GPS).5. Weigh sample after drying to a constantweight and record soil weight.CalculationsSoil volumeSoil volume (cm 3 ) = 3.14 x r 2 x hh = height of the ring measured with the rulerin cm (e.g. 10 cm)r = radius = half the diameter of the ring in cm(e.g. 7 cm ÷ 2 = 3.5 cm)Soil volume = 3.14 x 3.5 x 3.5 x 10 = 385cm 3Dry soil weightTo calculate the dry weight of the soil:1. Weigh an ovenproof container such as a pietin (record weight in grams = W1).2. Carefully remove all soil from the bag intothe container.3. Dry the soil at 105ºC until a constant weight(usually 24-48 hours). Depending on sizeof the core and soil moisture this may takelonger.4. When dry, weigh the sample (record weightin grams = W2).5. Record the dry soil weight (g) = W2 - W1Bulk density calculationBulk density (g/cm 3 ) = dry soil weight (g)/soilvolume (cm 3 )can be adjusted accurately (see Figure 5.2). In thisexample, a soil with an initial bulk density of 1.2 g/cm 3 and an organic carbon concentration of 1.2per cent (14.4 tonnes organic carbon per hectareto 10 cm) was exposed to farming practices, whichused over a period of 10 years resulted in topsoilcompaction (see Figure 5.2). This increased thebulk density of the topsoil to 1.4 g/cm 3 , but did notchange the percentage of organic carbon in the soil.Without adjusting for the change in bulk density toan equivalent soil mass, a change in organic carbonstock of 2.4 tonnes per hectare would result. If thestocks are adjusted to an equivalent soil weight,then results show no change in organic carbonstocks (see Figure 5.2).THE INFLUENCE OF SOIL ORGANICMATTER ON SOIL WATERPlant residues that cover the soil surface preventthe soil from sealing and crusting. This can resultin better water infiltration and decreased waterlosses associated with run-off. Evaporation is alsodecreased and up to 8 mm of soil water can besaved where more than 80 per cent of the soilsurface is covered with residues compared withbare soil. If this water was available to plants it couldreturn the equivalent of about 120 kg grain perhectare in wheat.CALCULATING HOW MUCH WATERCAN BE STOREDWhile soil organic matter can hold between 2-5times its weight in water, the impact of increasingsoil organic matter on water holding capacitydepends on the mineral composition of the soil, thedepth to which organic matter has increased andthe contribution of organic inputs to the various soilorganic fractions (see Chapter 1).As a general rule, each one percentage increasein soil organic matter increases water holdingcapacity in agricultural soils by an average of 2-4per cent (0.8–8.0 percentage range; Hudson 1994).This is the equivalent of less than two per cent onaverage for a one per cent increase in soil organiccarbon. For a soil that held 200 mm of soil waterthis would be equal to an additional 4 mm of water.However, as most of the soil organic matter will bein the top 10 cm of the soil, reports of increases inwater holding capacity beyond 10 cm are likely tobe an over-estimation.Similarly, changes in water holding capacity shouldbe considered in context to the likely changes in soil41MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDE

Figure 5.2 Adjustment of organic carbon content for an equivalent soil mass associated with changes inbulk density and sampling depth.42MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDECalculating the change in water-holding capacity due to organic matterScenario 1A one per cent increase in soil organic carbon (SOC) in the top 10 cm of a sandy soil, with a bulkdensity (BD) of 1.4 g/cm 3 and no gravel or stone content.Calculation (assuming one part soil organic carbon retains four parts water)[SOC (%) x BD (kg soil/ha to 10 cm depth*)] x retention factor= [0.01 x (1.4 x 100)] x 4= 1.4 kg/m 2 x 4= 5.6 litres/m 2 (or the equivalent of 5.6 mm)* While unlikely, if this change was observed to 30 cm then = [0.01 x (1.4 x 300)] x 4 = 16.8 mmorganic matter. For example, the likelihood of soilorganic carbon being increased by more than oneper cent is low because this level of change wouldrequire an additional 111 t/ha of organic matter to beadded to the soil. This is calculated assuming a soilbulk density of 1.5 g/cm 3 to 10cm, carbon contentof 45 per cent in plant material and a microbialefficiency of 70 per cent. Such an input level is highlyunlikely in Australian agricultural systems even ifconsidered over a 10-year time frame.Often reported ‘changes’ in soil organic carbonstocks are inaccurate having not adjusted forchanges in soil bulk density or stone content.Similarly, changes in the surface concentration(%) of organic carbon do not necessarily reflecta change in carbon stock over a deeper soilprofile where the distribution of carbon withinthe profile may have altered.

43Reported changes in soil water content due toincreases in organic matter often relate to changesin water holding capacity and are not necessarilyindicative of an increase in plant-available water. Thedifference between soil water holding capacity andplant-available water can vary widely depending onthe mineral composition of the soil, the form andlevel of organic matter and whether associatedchanges in bulk density have been considered. Ingeneral, increasing soil organic matter will have asmaller effect on plant available water in soils withincreasing clay content. It is therefore critical to takeall these factors into account when assessing thepotential impact of soil organic matter on soil watercontent.HOW IMPORTANT IS ‘EXTRA’ WATER TOAGRICULTURAL PRODUCTIONThe value of ‘extra’ soil water storage to crop andpasture yield depends on the amount and frequencyof rainfall or irrigation as well as the water demandof the crop or pasture. If the water is available at atime when crops are otherwise water stressed, evena small amount of water could lift crop or pastureproductivity. As a rule, each additional millimetreof rainfall (where water limited) can produce anadditional 20 kg grain per hectare of wheat (Frenchand Schulz 1984). However, where there is sufficientwater availability or when soil constraints preventplants accessing water then any extra soil moisturemay not be used.RELATIONSHIP BETWEEN ORGANICMATTER AND WATER REPELLENCEOrganic matter is not always associated withimproved water holding capacity because organiccompounds that exhibit water repellent properties(see Plate 5.3 a) such as plant waxes can beimplicated in the development of non-wetting soils(see Plate 5.3 b).Although not restricted to any one climate or soiltype, water repellence most often affects sands.An estimated 2-5 million hectares of agriculturalland in southern Australia exhibits signs of soilwater repellence (Roper 2004), with coastal sandplainsoils especially prone. Water repellence is asignificant issue in agricultural systems becauseit slows water infiltration and constrains soil waterstorage. This often results in poor or uneven cropestablishment and development, variable weedcontrol, increased risk of soil erosion and decreasedgrain yield. Nationally, estimated losses for crop andpasture production due to water repellent soils isabout $100-$250 million per year.An easy to use web-based tool for adjustingyour soil carbon and nutrient concentrationresults for either bulk density or gravel/stonecontent is available at www.soilquality.org.au/calculators/gravel_bulk_densityWHAT CAUSES WATER REPELLENCE?While all plants have waxes to prevent theirdesiccation, a range of more than 50 plant speciesincluding eucalyptus, acacia, clover and lupin andvarious organic compounds, including fungal derivedsources, have been more closely associated withMANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDE

44MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEwater repellence. These materials break down andeither mix with or coat soil particles making themhydrophobic, or water repellent. Most often waterrepellence is associated with the soil surface becausethis is where most of the organic matter is located.Despite being associated with organic matterthere is generally no direct relationship betweentotal soil organic matter and water repellence. Thisis because organic matter content is one of severalinteracting variables that impact on the severity ofsoil water repellence (Harper et al. 2000). Ratherit is a combination of the amount, type of organicmatter present and soil texture that influences thesusceptibility of soils to water repellence.Blue lupins (Lupinus consentinii) are renownedfor their capacity to cause severe water repellence.This is most likely not only associated with plantwaxes, but also the large biomass associated withthis plant type in many environments. For example,sandplain soils will develop moderate to severewater repellence following five years of continuousblue lupin production. Heavier soil types generallyrequire higher amounts of organic matter to inducewater repellence and as such it is less prevalent onthese soil types. Sheep camps also tend to be morewater repellent because they concentrate repellentorganic matter and waxy substances that have notbeen broken down during sheep digestion. Similarly,in native vegetation a strong link exists betweenparticular species such as Eucalyptus astringens(brown mallet), E. patens (blackbutt) and Banksiaspeciosa (showy banksia) and the occurrence ofwater repellence (Blackwell 1996).Only a relatively small amount of organic matter(between 1-4 per cent of the soil mass) is requiredto inhibit water absorption and slow infiltration insands. Water repellence has a similar impact onboth deep and shallow sands in terms of limitingwater entry (Moore 2001).In general, coarse textured sands require lessparticulate organic material and develop repellencemore rapidly than finer textured clay soils, but thesemay still exhibit water repellence (see Figure 5.3).It is also evident that increasing amounts of soilorganic matter is associated with the developmentof more severe repellence for any given clay content(see Figure 5.3).These results contrast to some extent with previousstudies that found water repellence only occurred insoils with less than 10 per cent clay and was mostsevere in coarse textured sandy soils, with less thanfive per cent clay (Harper and Gilkes 1994). Figure 5.3 A complex relationship exists betweensoil organic carbon, clay content and the severityof water repellence as measured at 400 sitesacross Western Australia (Hoyle and Murphyunpublished) for MED severity (nil – black, very low– red, low – green, moderate – dark blue, severe –light blue, very severe – pink; King 1981).

45MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEPlate 5.1 A sub-surface compaction layer shows a dense impenetrable soil layer.Source: Tim Overheu, DAFWAPlate 5.2 Bulk density core.Plate 5.3 a) Water droplet sitting on thesurface of a non-wetting soil (Van Gool etal. 1999) and b) typical sub-surface drynessobserved after rain in water repellent sand.

0646ORGANIC MATTER LOSSESFROM SOILMANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEAT A GLANCE• Organic carbon in soil is concentrated at the soil surface (0-10 cm). Protecting this soilfrom loss is therefore critical to maintaining soil organic carbon levels.• Much of the original loss of soil organic carbon was associated with the clearing andsubsequent tillage of land for agricultural pursuits.• Building soil organic carbon in coarse textured sandy soils is more challenging than infiner textured clay soils.• Warm, moist soils increase the decomposition rate of soil organic matter.

When soils under natural vegetation are convertedto agricultural land there is an important loss of soilorganic carbon mainly in the form of carbon dioxide.Organic matter levels in many Australian croppingsoils have declined by between 10-60 per centcompared to pre-clearing levels (Dalal and Chan2001). Based on a total arable soil area of 41 millionhectares and assuming the carbon component ofthis organic matter measured between 30-60 tonnescarbon per hectare (top 30 cm of soil), the totalhistorical loss in soil organic carbon is 646 milliontonnes carbon (Chan et al. 2009). This representsthe equivalent of nearly 2.4 billion tonnes of carbondioxide emissions.While soil forms and regenerates very slowly,it can degrade rapidly and could in essence beconsidered a non-renewable resource. Soil organicmatter is in a constant state of turnover whereby itis decomposed and then replaced with new organicmaterial. The balance between these additions andlosses determines the relative flux and amount ofsoil organic matter present at any point in time.Below-ground organic residues and root turnoverrepresent direct inputs of organic matter into thesoil system and have the potential to make majorcontributions to the soil organic matter stock (seePlate 6.1). The tight coupling between root distributionand the distribution of organic matter with depth isoften cited as evidence of the importance of rootinputs in maintaining stocks of soil organic carbon.In addition, roots generally decay more slowly thanabove-ground residue because of differences inlitter quality and environmental factors (Sandermanet al. 2010).DIRECT LOSSESSoil erosionIn Australia, annual soil losses from erosion arenegligible under a good pasture, but can be upto eight tonnes per hectare under planted crop.Erosion risk is strongly influenced by the amountof ground cover and the highest risk scenarios aremost often associated with bare fallow, under whichtypical soil losses in a single year can reach between60-80 tonnes per hectare. While less common, windand water erosion resulting from single, high-intensitystorms can erode up to 300 tonnes per hectare(see Plate 6.2). Since a 1 mm depth of soil weighsbetween 10-15 tonnes per hectare (assuming abulk density of 1.0 to 1.5g/cm 3 ), erosion events incropped soils represent a significant loss of topsoilalong with its associated carbon and nutrient-richfractions (Hoyle et al. 2011). Soil physical attributesassociated with high organic matter content suchas more stable soil aggregates, greater porosity,improved water infiltration and improved workabilityat high moisture content (plastic limit) all contributeto a lower risk of soil loss from erosion.INDIRECT LOSSESLosses of soil organic carbon occur primarily whenorganic matter is decomposed and mineralised tocarbon dioxide. The rate at which organic matteris decomposed is driven by factors that regulatemicrobial activity, including climate (soil moisture andtemperature), soil disturbance and the managementof organic inputs.ClimateIn moist soils, organic matter breaks down morerapidly as average temperatures increase. Asa general rule for every 10°C rise in averagetemperature between 5°C and 40°C the rate ofmineralisation will nearly double where carbonsubstrates are not limited (see Figure 6.1; Hoyleet al. 2006). Therefore, it is more difficult to storelarge amounts of organic carbon in soils subject tohigh temperatures and even more difficult in soilsexposed to high temperatures and extended periodsof adequate soil water. In cooler environments,decomposition does not occur year-round and isconstrained at low temperature. Figure 6.1 The effect of increasing temperatureon the amount of carbon lost from soil (kg carbonper tonne of soil per day) where stubble has beenretained (adapted from Hoyle et al. 2006).47MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDE

48MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEDrying soils increasingly inhibit microbial activityand therefore decomposition of organic matterbecause there are fewer substrates and nutrientsfor microbial growth and reproduction. Soil moisturebetween 20-60 per cent of water holding capacity isconsidered optimal for microbial activity, with wettersoils inhibiting biological activity due to low oxygenavailability.As soils warm up in spring, the microbial biomassincreases in size as well as activity. In general,population size and activity of soil microorganismsare highest during spring and lowest during winter.This means warm, moist environments can supporthigh levels of microbial activity and soil organicmatter can be lost quickly in these systems if organicinputs stop. Conversely, in soils with very low levelsof soil microbial activity organic carbon can slowlyaccumulate and build to relatively high levels, despitebeing in an environment of poor productivity. Forexample, in highly acidic, waterlogged or clay soils,organic matter can accumulate but does not breakdown. Highly alkaline and in particular sodic soils donot support high organic carbon stocks.Factors that control how sensitive organic matteris to decomposition include:(1) Physical protection. Organic matter can beprotected inside soil aggregates limiting accessto it by microorganisms and their enzymes(Tisdall and Oades 1982). Micro-aggregates(53–250 mm) slow the turnover of soil organicmatter, withstand physical disturbance andprotect carbon more effectively than largermacro-aggregates (Angers et al. 1997;Six et al. 2002).(2) Chemical protection. Organic matter canbecome adsorbed on to mineral surfacesprotecting it from decomposition.(3) Drought. Low soil moisture results in thinningor absent water films in soil, slowing the flowof extracellular enzymes and soluble carbonsubstrates. Organic compounds in dry orhydrophobic soils are isolated from degradationby water-soluble enzymes.(4) Flooding. Flooding slows the diffusion of oxygenand constrains aerobic decomposition oforganic matter.(5) Freezing. The diffusion of substrates andextracellular enzymes within the soil below 0°Cis extremely slow and this, in turn, slows thedecomposition of organic matter (Davidson andJanssens 2006).SOIL DISTURBANCESoil disturbance and cultivation can accelerate thedecomposition of organic matter, increasing its rateof mineralisation. Cultivation and soil disturbanceexposes previously protected organic matter to soilbiota increasing its decomposition. Minimum tillagehas the greatest potential to maintain, or perhapsincrease levels of organic matter in Australiancropping soils over the long-term, especially insurface soils.The increasing use of soil management practicessuch as mouldboard ploughing is likely to have aprofound effect on the amount and distribution ofsoil organic matter and needs further study (seePlate 6.3).MANAGEMENT OF ORGANIC RESIDUESSoil organic carbon declines rapidly under fallowbecause of increased microbial attack on storedsoil organic carbon supported by soil moistureconservation, a lack of plant production and, wherepracticed, due to cultivation for weed control whichexposes previously protected organic matter todecompositionCrop type, rotation and management influencesoil organic carbon content. In general, soils underpasture have a higher soil organic content than thoseunder cropping (Blair et al. 2006), while minimumtillage and stubble retention can either maintain orincrease soil organic carbon in cropped soils (Chanand Heenan 2005). Applying inorganic fertilisers tolow fertility soils can sometimes promote microbialactivity and soil organic matter decompositionwhere nutrients are limiting, but also support greaterplant productivity.Loss of topsoil from erosion results in a directloss of soil organic matter. Soil organic matter canalso be affected indirectly by erosion when exposedsub-surface soil layers are subject to highertemperatures leading to an increase in organicmatter mineralisation (Liddicoat et al. 2010).Grazing can remove a significant amount ofabove-ground biomass — a proportion of which isreturned to the soil as manure. Plant growth stageand grazing intensity can impact on the ability ofpastures to recover and therefore the amount ofabove-ground biomass that makes its way intosoil organic matter. Model estimates show a 10per cent loss of organic carbon stocks over 30 cmassociated with the net removal of 30 per cent of drymatter from an annual pasture paddock in WesternAustralia (Roth-C initialised at five per cent clay,

450 mm annual growing season rainfall, 75 tonnescarbon per hectare and no erosion loss).THE FATE OF CAPTURED CARBONIN SOILSThe contribution of recently fixed carbon to soilcarbon stocks depends on whether plant productsstay on the land and are incorporated into soil, orare exported as hay and grain (see Plate 6.4).In most farming systems a proportion of thecarbon fixed during photosynthesis will be removedas grain. For grain crops, 30-50 per cent of theabove-ground dry matter is typically removed fromthe farming system as grain or hay. Depending onhow the stubble is managed the balance of the drymatter remains as above and below-ground (root)residues. Some carbon is transferred into the soil asroot and mycorrhizal biomass and exudates.Incorporating organic matter into the soil can, insome cases, increase the amount and persistenceof organic carbon at depth. In farming systems,the majority of surface residues are mixed into soilduring tillage. In natural systems, soil fauna such asearthworms and litter arthropods (e.g. mites andants) fragment and mix surface residues into thesoil. Upwards of 30 per cent of the mass of surfaceresidues are leached into the soil. A proportionof this soluble organic carbon will be rapidly lost,while the remainder enters the soil to eventuallybecome humus.49MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEPlate 6.1 Canola roots contribute organicmatter to soil.Source: GRDCPlate 6.2 Soil erosion resulting from poorground cover and compaction.Source: Paul Blackwell, DAFWAPlate 6.3 Mouldboard plough in operation forthe treatment of non-wetting soil.Source: Evan CollisPlate 6.4 The removal of products such asgrain or hay can decrease organic matterinputs and contribute to soil acidification.Source: Kondinin Group

5007MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEGREENHOUSE GAS EMISSIONSAT A GLANCE•Methane, nitrous oxide and carbon dioxide are the main greenhouse gases associated withagricultural production.•Greenhouse gas emissions are reported in carbon dioxide equivalents (CO 2 -e).•One tonne of carbon is equal to 3.67 tonnes of carbon dioxide.•Soils act as both a source and sink for carbon.•Increases in soil organic carbon must be permanent and verifiable to be traded.

Australia’s greenhouse gas emissions are largelyassociated with methane (CH 4 ) from ruminantlivestock digestion, nitrous oxide (N 2O) from soilsand carbon dioxide (CO 2 ) from fossil fuel use andsoils (Australian National Greenhouse Accounts2011).The contribution of different greenhouse gasesto global warming is measured in carbon dioxideequivalents (CO 2 -e), which allows all greenhousegases to be compared with a common standard(that of carbon dioxide) and reflects how long thegases remain in the atmosphere and their ability totrap heat.Globally, fossil fuel combustion, land useconversion, soil cultivation and cementmanufacturing have largely been associated with a36 per cent increase in atmospheric carbon dioxideconcentrations from a pre-industrial level of 280ppm to 380 ppm in 2006 (Lal and Follet 2009).For example, over 100 years the globalwarming potential of:• 1 tonne of methane = 25 tonnes of carbondioxide• 1 tonne of nitrous oxide = 298 tonnes ofcarbon dioxideSource: IPCC, 4th Assessment 2007In Australia, agricultural industries are the dominantsource of methane (58 per cent) and nitrous oxide(77 per cent) emissions. In 2011, agriculturalemissions were estimated at 84.1 million tonnes ofcarbon dioxide equivalents (CO 2 -e) of which 65 percent was methane and 19 per cent nitrous oxide.However, while agriculture contributes significantlyto methane and nitrous oxide emissions it accountedfor just 15.2 per cent of Australia’s total greenhousegas emissions in 2011. By comparison, the energysector accounted for 76.4 per cent (417.4 Mt CO 2 -e)of Australia’s net emissions (Australian NationalGreenhouse Accounts 2011).SOIL ORGANIC MATTER ANDGREENHOUSE GAS EMISSIONSGlobally, soils are estimated to contain about threetimes as much carbon as that found in the world’svegetation. Soil organic matter and the carboncontained within it therefore play a critical role in theglobal carbon balance.Historical conversion of native land to agriculturalproduction has decreased the world’s soil organiccarbon stocks by between 40-60 per cent (Guo andGifford 2002), or the equivalent of 78 Petagrams (Pg)of carbon (Lal 2004). These large losses coupledwith concerns over rising atmospheric levels ofcarbon dioxide has driven interest in the possibilityof the world’s soils being used to store carbon andhelp mitigate climate change.Agricultural soils have a high potential for carbonsequestration (albeit slowly in many cases) atrelatively low cost. Increasing soil organic carboncould mitigate Australia’s greenhouse gas emissionsand the effects of climate change with co-benefitsfor productivity. Increasing soil organic carbon isalso associated with biological resilience, whichprovides agro-ecosystems with the capacity torecover critical soil functions following short-termclimate or environmental stresses.A one per cent change in the amount of stored soilorganic carbon equates to a change in atmosphericcarbon dioxide concentration of about eight partsper million (Baldock et al. 2012).Carbon dioxideFor every tonne of organic carbon that isdecomposed, 3.67 tonnes of carbon dioxide isreleased to the atmosphere. Conversely, for everytonne of soil organic carbon created, 3.67 tonnesof carbon dioxide is removed from the atmosphere.For example, a one per cent increase in organiccarbon in the top 30 cm of soil (with a bulk densityof 1.2 g/cm 3 ), is equivalent to 36 tonnes per hectareof organic carbon or 132 tonnes carbon dioxidesequestered per hectare.1 tonne of carbon is the equivalent of 3.67tonnes of carbon dioxide.One option available to lower atmosphericgreenhouse gases is to remove carbon from theatmosphere by sequestering carbon dioxide inorganic matter in a stable form (trees or soil carbon).Under current carbon accounting requirements thesequestered carbon must remain stored for at least100 years and be verifiable for accounting purposes(Noble and Scholls 2001; Smith 2004).Freshly deposited soil organic matter tendsto readily oxidise to carbon dioxide unless it isconverted to a more stable form. Stable forms ofcarbon take time to form and in many cases it cantake years to rebuild a bank of stable carbon toprevious levels.51MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDE

52MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEThe effect of nitrogen fertilisers on soil organiccarbon stocks should be assessed on thenet balance between increased carbon inputsfrom increased production versus increaseddecomposition if it occurs. In some cases, nitrogeninputs can actually slow down carbon loss becausemore carbon is stabilised. In other cases, nitrogenfertilisers can indirectly degrade soil organic carbonreserves because their addition stimulates a rangeof bacteria that feed on carbon for their growthand reproduction. For every tonne of fertilisernitrogen applied, bacteria consume about 30tonnes of carbon (based on a carbon to nitrogenratio of 30 to 1).Nitrous oxideNitrous oxide (N 2 O) emissions account for about10 per cent of global greenhouse gas emissions,with 90 per cent of these emissions derived fromagricultural practices (Smith et al. 2007). The mainsource of nitrous oxide emissions worldwide ismineral nitrogen fertilisation and its influence on thenitrogen cycle.As a consequence of its high global warmingpotential, nitrous oxide emissions from land can havea large bearing on the assessment of greenhousegases from cropping systems (Australian NationalGreenhouse Accounts 2011). Barton et al. (2010)report nitrous oxide emitted after the applicationof synthetic nitrogen fertilisers to land under graincropping systems to be 17 times lower than theIntergovernmental Panel on Climate Change (IPCC)default value of 1.0 per cent.Nitrous oxide (N 2 O) emissionsaccount for about 10 per cent ofglobal greenhouse gas emissions,with 90 per cent of theseemissions derived from agriculturalpractices (Smith et al. 2007).Nitrous oxide in soils is associated withmicrobial processes associated with nitrogentransformations. Denitrification occurs underanaerobic (waterlogged) conditions and involves thereduction of nitrate (NO 3 -) to nitrogen gas (N 2 ), withnitrous oxide as a by-product (de Klein and Eckard2008). Nitrification contributes to a lesser extent tonitrous oxide emissions by oxidising ammonium(NH 4 +) to nitrate (NO 3 -), with nitrous oxide as aby-product. Nitrification can be favoured at hightemperatures and inhibited at acid pH values(Mengel and Kirkby 1987).The influence of organic matter on nitrous oxideemissions is related to substrate availability. Currentevidence suggests this is most closely associatedwith the carbon to nitrogen ratio of the dissolved(soluble) organic matter fraction, which differsamong species and decomposition stages. A lowercarbon to nitrogen ratio provides more mineralisablenitrogen substrate for microbial nitrous oxideproduction and increases the bioavailability ofdissolved organic carbon.MethaneDespite a short residence time (about 10 years) inthe atmosphere, methane is the main hydrocarbonpresent in the atmosphere and due to its ability toabsorb infrared radiation has 20-30 times the globalwarming potential of carbon dioxide (Rodhe 1990).Nearly 16 per cent of Australia’s greenhouse gasemissions are associated with methane productionfrom agriculture. A large proportion (just over 67per cent) of this comes from methane producedby Australia’s cattle and sheep industries (NationalGreenhouse Gas Inventory 2010).Methane (CH 4 ) is a natural by-product of ruminantdigestion in animals, wetland rice paddy farmingand anaerobic decomposition of biological material.Its global warming potential is 21 times that ofcarbon dioxide. In soils, methane emissions are thenet result of two bacterial processes influenced byland use, management and soil type — methaneproduction and methane consumption. Methane isproduced by methanogens in anaerobic soils thatconstrain oxygen diffusion such as in water-loggedsoils and methane consumption in aerobic soilsby methane-oxidizing bacteria (Le Mer and Roger2012).CARBON OFFSETS FOR GREENHOUSEGAS EMISSIONS AND CARBONTRADINGGlobal markets for greenhouse gas emissionsCarbon trading markets have been introduced insome countries as a response to climate changeimperatives and typically greenhouse gas emissionsare limited by tradeable permits. Hence, a price fora set amount of emissions is determined by marketforces, which should promote changes in productiontowards a lower-emission producing industry. In a

number of European Union (EU) countries this hasoperated as a mandatory cap and trade scheme,which despite initially high permit prices and becauseof the recession, in May 2013 is currently tradingat around 3.5 Euros ($4.67 based on an exchangerate of 0.75 Euros per Australian dollar) per metricton on London’s ICE Futures Europe exchange.Each carbon credit represents one tonne of carbondioxide equivalents (CO 2 -e).The Australian situationThe Carbon Farming Initiative (CFI) aims to helpAustralia meet its international greenhouse gasobligations by undertaking land sector abatementprojects that generate saleable carbon credits(Australian carbon credit units, ACCUs) or offsets.These offsets are either Kyoto compliant and cancontribute towards Australia’s national inventor,or be exchanged for Kyoto consistent credits andexported overseas, or are non-Kyoto compliant andcommonly termed ‘voluntary’ carbon credits.The Australian government introduced a fixedprice of $23 per tonne of carbon dioxide for permitsin July 2012 and is set to increase by five per cent ayear through 2015 before shifting to a cap and tradesystem linked to the EU market. In Australia, around500 companies have a mandatory obligation to payfor, or offset their direct emissions using carboncredits.Currently, soil organic carbon can also be tradedvia voluntary carbon trading schemes both inAustralia and internationally to offset greenhousegas emissions. In an emissions trading framework,the term ‘offset’ describes the reduction or removalof greenhouse gas emissions in a ‘non-covered’sector (i.e. not mandatory). Until very recently,the agricultural sector remained uncovered andconstituted the primary opportunity for landholdersto engage in the creation of carbon offsets.Trading in voluntary carbon offsets may provideadditional benefits to companies, includingpromotion and strengthening of environmentalcredentials, and promoting a perception within thecommunity that they are taking responsibility fortheir emissions. Landholders wanting to participatein voluntary offset schemes are encouraged to seeklegal and financial advice.Afforestation and reforestation activities, which areincluded under Article 3.3 of the Kyoto Protocol, canbe traded internationally and attract a premium overvoluntary trading markets. These activities includeas an example the establishment of planted treesof at least 2m in height in an area greater than 0.2hectare that has been previously cleared of naturalvegetation post December 31, 1989.Should agriculture become a covered sector,landholders taking part would need to consider thewhole farm implications of participating in carbonmarkets as they may then be required to reportnot only sequestration and mitigation gains, butalso ‘leakages’ and losses from the system (i.e.increasing use of nitrogen fertilisers associatedwith nitrous oxide emissions and livestock methaneemissions), which may result in a negative carbonbalance overall.In May 2013, the Australian government electedto include reporting of activities which includecropland management, grazing land managementand revegetation towards their national greenhousegas reduction target during the second commitmentphase of the Kyoto protocol. This means that foran approved methodology developed under theCarbon Farming Initiative (CFI), these activities willbe able to generate and sell Kyoto-compliant CFIcredits. They also remain eligible for use in voluntarymarkets.Offsets are assessed againstinternationally recognised standards toensure real and verifiable abatement,including:• Additionality (projects only happenedbecause the offsets market was available)• Permanence (carbon store must bemaintained for 100 years)• Accounting for leakage (emissions fromelsewhere that nullify abatement must beaccounted for)• Measureable and auditable• Conservative• Internationally consistent• Supported by peer reviewed science(where estimation methods are differentto those used in Australia’s NationalGreenhouse Accounts, peer reviewedscience must support the estimationmethods)53MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDE