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

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