Spatial and temporal variability in organic carbon observed in soil under lucerne pastures

Two field trials established in 2005 were sampled during 2008-09 to observe temporal and spatial variability in soil organic carbon levels under mature lucerne (Medicago sativa) stands. The study was conducted at Binalong and Gerogery in the high-rainfall cropping zone of southern New South Wales, Australia. Four replicate plots were selected at each location and sampled monthly between February 2008 – January 2009 by taking 10 soil cores at 0-0.1m depth and analysing the samples using the Walkley–Black and Heanes methodologies. Plot area was no more than 10 m 2 , and the 4 plots sampled at each location were no more than 20 m apart. Temporal variation in organic carbon levels during the 12 month sampling period was 24-76% (mean 37%) and spatial variation was observed to be 13-46% (mean 30%) using the standard Walkley–Black test with samples sieved to <2 mm. Analysing samples using the Heanes method reduced the mean temporal variability from 37% to 29% and mean spatial variability from 30% to 28%. A subset of samples were ground to a fine powder to increase the homogeneity of the sample before being reanalysed, but temporal and spatial variation in organic carbon values were still shown to be 29% and 24%, respectively. This study demonstrates the magnitude of sampling variability likely to be encountered when quantifying soil organic carbon for possible inclusion in future carbonemissions trading schemes. Aspects requiring further research are discussed. Introduction Farmers and scientists have long been aware of the importance of soil organic carbon (SOC) to soil health and sustainable agriculture. Recently there has also been an increasing interest in the possible role of SOC as a carbon sink for the mitigation of climate change and therefore the possibility of its inclusion in emission trading schemes (ETS). It has been estimated that SOC sequestration has the potential to mitigate 5-14% of global annual greenhouse gas emissions during the next century (Chan et al. 2009). However, for the credible inclusion of soil carbon in trading schemes it is essential to establish robust methodologies by which SOC can be measured and monitored over time. An accounting system for an ETS must ultimately be based on one or more laboratory methodologies for physically measuring carbon or carbon fractions in soil. The most common test for soil organic carbon in Australian laboratories is the Walkley–Black method (Walkley and Black 1934), but it only measures readily oxidisable/decomposable carbon rather than total SOC (Chan et al. 2010). The Heanes modification (Heanes 1984) of the original Walkley–Black method measures up to 100% of total SOC, including a proportion of the charcoal in the sample. Estimates of SOC using either of these tests will detect labile organic residues including fresh or partly decomposed organic matter (roots, leaves, manure etc), only a proportion of which is ultimately incorporated as SOC (Chan et al. 2010). The current study aimed to monitor the spatial and temporal variability of SOC using conventional sampling and laboratory methodologies to assess how SOC could be accurately assessed in the context of an ETS. Methods