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A METHOD FOR MEASURING THE EFFECT OF DAIRY AND DRYSTOCK GRAZED PASTURES ON SOIL CARBON STOCKS

Soil is the largest terrestrial store of carbon (C) with some 2000 Pg to a depth of 1 m compared to 500 Pg in the atmosphere. Quantifying soil C stocks is a key requirement of the Intergovernmental Panel on Climate Change (IPCC). The IPCC recommends that C stocks be measured using the depth based approach where the C stock (tC ha -1 ) is the product of sample depth (m), bulk density (t m -3 ) and percent C. However, soil properties such as bulk density may vary spatially and temporally in response to land use change and management practices. For greater accuracy in determining the effects of land use change on soil C, a mass based coring approach coupled to equivalent soil mass calculations (ESM) of soil C stocks may be more accurate as it accounts for differences in bulk density. Barnett et al. (2014) used a paired pit approach to sample 25 adjacent dairy and drystock pastures to a fixed depth of 0.6 m and showed that drystock grazed soils had about 8.6 t.ha -1 more C in the top soil than adjacent dairy sites (P<0.05). However, there was no significant difference between land uses when C was accumulated to 0.6 m. Our objective was to test a potentially more accurate method for estimating C stocks between dairy and drystock grazed pastures. We resampled the paired dairy and drystock sites to a depth of 0.6 m by taking 5 soil cores from each of two plots (5x5 m) within a paddock of each land use. Preliminary results have shown that dairy sites have on average 72 t ha -1 more soil (P<0.05) than drystock sites in the 0-0.25 m layer, outlining the importance of applying ESM calculations. Resampling of sites using the replicated coring approach was found to yield lower estimations of total C compared to the pit approach which was used for the initial sampling. Using a replicated coring approach takes into account the inherent spatial variability in C stocks, thus giving a more accurate estimation of the field mean and increasing the power to detect small differences in C stocks between land uses. Furthermore, C stocks can be estimated efficiently and cost effectively which is important for C monitoring and accounting purposes. Introduction The amount of carbon (C) stored in soil is equivalent to the amount stored in both the atmosphere and terrestrial vegetation (Conant et al., 2003). Soil plays a fundamental role in the global C cycle with the upper 1 m containing 1500–2000 Pg of organic C (Don et al., 2007; Schipper et al., 2007). Soil C is also an essential component of soil quality as it enhances soil structure, nutrient cycling and soil moisture holding capacity (Han et al., 2010). The United Nations Framework Convention on Climate Change (UNFCCC) made it mandatory to report greenhouse gas removals and emissions for soil C pools at the national scale (Hewitt et al., 2012). As a consequence, the Soil Carbon Monitoring System (CMS) was developed as a national inventory for soil C stocks in New Zealand (Scott et al., 2002; Tate et