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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
Page 1 and 2:
MANAGING SOILORGANIC MATTERA PRACTI
Page 3 and 4: FOREWORD1The one constant in agricu
Page 6 and 7: TABLES, FIGURES AND PLATES4MANAGING
Page 8 and 9: 6INTRODUCTIONMANAGING SOIL ORGANIC
Page 10 and 11: 018SOIL ORGANIC MATTERMANAGING SOIL
Page 12 and 13: Table 1.1 Size, composition, turnov
Page 14 and 15: Table 1.2 Functional role of soil o
Page 16 and 17: Figure 1.7 The influence of soil ty
Page 18 and 19: 16HOW MUCH ORGANIC CARBONREMAINS IN
Page 20 and 21: 0218BIOLOGICAL TURNOVEROF ORGANIC M
Page 22 and 23: 20MANAGING SOIL ORGANIC MATTER: A P
Page 26 and 27: 2403MANAGING SOIL ORGANIC MATTER: A
Page 29: less than 15 per cent of carbon inp
Page 36 and 37: Figure 4.1 Nitrogen cycling in soil
Page 39: mycorrhizal associations have been
Page 42 and 43: Table 5.1 Influence of soil charact
Page 44 and 45: Figure 5.2 Adjustment of organic ca
Page 48 and 49: 0646ORGANIC MATTER LOSSESFROM SOILM
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