managing soil organic matter - Grains Research & Development ...

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