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

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,