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

When soils under natural vegetation are convertedto agricultural land there is an important loss of soilorganic carbon mainly in the form of carbon dioxide.Organic matter levels in many Australian croppingsoils have declined by between 10-60 per centcompared to pre-clearing levels (Dalal and Chan2001). Based on a total arable soil area of 41 millionhectares and assuming the carbon component ofthis organic matter measured between 30-60 tonnescarbon per hectare (top 30 cm of soil), the totalhistorical loss in soil organic carbon is 646 milliontonnes carbon (Chan et al. 2009). This representsthe equivalent of nearly 2.4 billion tonnes of carbondioxide emissions.While soil forms and regenerates very slowly,it can degrade rapidly and could in essence beconsidered a non-renewable resource. Soil organicmatter is in a constant state of turnover whereby itis decomposed and then replaced with new organicmaterial. The balance between these additions andlosses determines the relative flux and amount ofsoil organic matter present at any point in time.Below-ground organic residues and root turnoverrepresent direct inputs of organic matter into thesoil system and have the potential to make majorcontributions to the soil organic matter stock (seePlate 6.1). The tight coupling between root distributionand the distribution of organic matter with depth isoften cited as evidence of the importance of rootinputs in maintaining stocks of soil organic carbon.In addition, roots generally decay more slowly thanabove-ground residue because of differences inlitter quality and environmental factors (Sandermanet al. 2010).DIRECT LOSSESSoil erosionIn Australia, annual soil losses from erosion arenegligible under a good pasture, but can be upto eight tonnes per hectare under planted crop.Erosion risk is strongly influenced by the amountof ground cover and the highest risk scenarios aremost often associated with bare fallow, under whichtypical soil losses in a single year can reach between60-80 tonnes per hectare. While less common, windand water erosion resulting from single, high-intensitystorms can erode up to 300 tonnes per hectare(see Plate 6.2). Since a 1 mm depth of soil weighsbetween 10-15 tonnes per hectare (assuming abulk density of 1.0 to 1.5g/cm 3 ), erosion events incropped soils represent a significant loss of topsoilalong with its associated carbon and nutrient-richfractions (Hoyle et al. 2011). Soil physical attributesassociated with high organic matter content suchas more stable soil aggregates, greater porosity,improved water infiltration and improved workabilityat high moisture content (plastic limit) all contributeto a lower risk of soil loss from erosion.INDIRECT LOSSESLosses of soil organic carbon occur primarily whenorganic matter is decomposed and mineralised tocarbon dioxide. The rate at which organic matteris decomposed is driven by factors that regulatemicrobial activity, including climate (soil moisture andtemperature), soil disturbance and the managementof organic inputs.ClimateIn moist soils, organic matter breaks down morerapidly as average temperatures increase. Asa general rule for every 10°C rise in averagetemperature between 5°C and 40°C the rate ofmineralisation will nearly double where carbonsubstrates are not limited (see Figure 6.1; Hoyleet al. 2006). Therefore, it is more difficult to storelarge amounts of organic carbon in soils subject tohigh temperatures and even more difficult in soilsexposed to high temperatures and extended periodsof adequate soil water. In cooler environments,decomposition does not occur year-round and isconstrained at low temperature. Figure 6.1 The effect of increasing temperatureon the amount of carbon lost from soil (kg carbonper tonne of soil per day) where stubble has beenretained (adapted from Hoyle et al. 2006).47MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDE