48MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEDrying <strong>soil</strong>s increasingly inhibit microbial activityand therefore decomposition of <strong>organic</strong> <strong>matter</strong>because 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 wetter<strong>soil</strong>s inhibiting biological activity due to low oxygenavailability.As <strong>soil</strong>s warm up in spring, the microbial biomassincreases in size as well as activity. In general,population size and activity of <strong>soil</strong> microorganismsare highest during spring and lowest during winter.This means warm, moist environments can supporthigh levels of microbial activity and <strong>soil</strong> <strong>organic</strong><strong>matter</strong> can be lost quickly in these systems if <strong>organic</strong>inputs stop. Conversely, in <strong>soil</strong>s with very low levelsof <strong>soil</strong> microbial activity <strong>organic</strong> carbon can slowlyaccumulate and build to relatively high levels, despitebeing in an environment of poor productivity. Forexample, in highly acidic, waterlogged or clay <strong>soil</strong>s,<strong>organic</strong> <strong>matter</strong> can accumulate but does not breakdown. Highly alkaline and in particular sodic <strong>soil</strong>s donot support high <strong>organic</strong> carbon stocks.Factors that control how sensitive <strong>organic</strong> <strong>matter</strong>is to decomposition include:(1) Physical protection. Organic <strong>matter</strong> can beprotected inside <strong>soil</strong> aggregates limiting accessto it by microorganisms and their enzymes(Tisdall and Oades 1982). Micro-aggregates(53–250 mm) slow the turnover of <strong>soil</strong> <strong>organic</strong><strong>matter</strong>, withstand physical disturbance andprotect carbon more effectively than largermacro-aggregates (Angers et al. 1997;Six et al. 2002).(2) Chemical protection. Organic <strong>matter</strong> canbecome adsorbed on to mineral surfacesprotecting it from decomposition.(3) Drought. Low <strong>soil</strong> moisture results in thinningor absent water films in <strong>soil</strong>, slowing the flowof extracellular enzymes and soluble carbonsubstrates. Organic compounds in dry orhydrophobic <strong>soil</strong>s are isolated from degradationby water-soluble enzymes.(4) Flooding. Flooding slows the diffusion of oxygenand constrains aerobic decomposition of<strong>organic</strong> <strong>matter</strong>.(5) Freezing. The diffusion of substrates andextracellular enzymes within the <strong>soil</strong> below 0°Cis extremely slow and this, in turn, slows thedecomposition of <strong>organic</strong> <strong>matter</strong> (Davidson andJanssens 2006).SOIL DISTURBANCESoil disturbance and cultivation can accelerate thedecomposition of <strong>organic</strong> <strong>matter</strong>, increasing its rateof mineralisation. Cultivation and <strong>soil</strong> disturbanceexposes previously protected <strong>organic</strong> <strong>matter</strong> to <strong>soil</strong>biota increasing its decomposition. Minimum tillagehas the greatest potential to maintain, or perhapsincrease levels of <strong>organic</strong> <strong>matter</strong> in Australiancropping <strong>soil</strong>s over the long-term, especially insurface <strong>soil</strong>s.The increasing use of <strong>soil</strong> management practicessuch as mouldboard ploughing is likely to have aprofound effect on the amount and distribution of<strong>soil</strong> <strong>organic</strong> <strong>matter</strong> and needs further study (seePlate 6.3).MANAGEMENT OF ORGANIC RESIDUESSoil <strong>organic</strong> carbon declines rapidly under fallowbecause of increased microbial attack on stored<strong>soil</strong> <strong>organic</strong> carbon supported by <strong>soil</strong> moistureconservation, a lack of plant production and, wherepracticed, due to cultivation for weed control whichexposes previously protected <strong>organic</strong> <strong>matter</strong> todecompositionCrop type, rotation and management influence<strong>soil</strong> <strong>organic</strong> carbon content. In general, <strong>soil</strong>s underpasture have a higher <strong>soil</strong> <strong>organic</strong> content than thoseunder cropping (Blair et al. 2006), while minimumtillage and stubble retention can either maintain orincrease <strong>soil</strong> <strong>organic</strong> carbon in cropped <strong>soil</strong>s (Chanand Heenan 2005). Applying in<strong>organic</strong> fertilisers tolow fertility <strong>soil</strong>s can sometimes promote microbialactivity and <strong>soil</strong> <strong>organic</strong> <strong>matter</strong> decompositionwhere nutrients are limiting, but also support greaterplant productivity.Loss of top<strong>soil</strong> from erosion results in a directloss of <strong>soil</strong> <strong>organic</strong> <strong>matter</strong>. Soil <strong>organic</strong> <strong>matter</strong> canalso be affected indirectly by erosion when exposedsub-surface <strong>soil</strong> layers are subject to highertemperatures leading to an increase in <strong>organic</strong><strong>matter</strong> mineralisation (Liddicoat et al. 2010).Grazing can remove a significant amount ofabove-ground biomass — a proportion of which isreturned to the <strong>soil</strong> 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 into<strong>soil</strong> <strong>organic</strong> <strong>matter</strong>. Model estimates show a 10per cent loss of <strong>organic</strong> carbon stocks over 30 cmassociated with the net removal of 30 per cent of dry<strong>matter</strong> 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 <strong>soil</strong>carbon stocks depends on whether plant productsstay on the land and are incorporated into <strong>soil</strong>, 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 <strong>matter</strong> is typically removed fromthe farming system as grain or hay. Depending onhow the stubble is managed the balance of the dry<strong>matter</strong> remains as above and below-ground (root)residues. Some carbon is transferred into the <strong>soil</strong> asroot and mycorrhizal biomass and exudates.Incorporating <strong>organic</strong> <strong>matter</strong> into the <strong>soil</strong> can, insome cases, increase the amount and persistenceof <strong>organic</strong> carbon at depth. In farming systems,the majority of surface residues are mixed into <strong>soil</strong>during tillage. In natural systems, <strong>soil</strong> fauna such asearthworms and litter arthropods (e.g. mites andants) fragment and mix surface residues into the<strong>soil</strong>. Upwards of 30 per cent of the mass of surfaceresidues are leached into the <strong>soil</strong>. A proportionof this soluble <strong>organic</strong> carbon will be rapidly lost,while the remainder enters the <strong>soil</strong> to eventuallybecome humus.49MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEPlate 6.1 Canola roots contribute <strong>organic</strong><strong>matter</strong> to <strong>soil</strong>.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 <strong>soil</strong>.Source: Evan CollisPlate 6.4 The removal of products such asgrain or hay can decrease <strong>organic</strong> <strong>matter</strong>inputs and contribute to <strong>soil</strong> acidification.Source: Kondinin Group
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