Assessing carbon lability of particulate organic matter from d13C changes following land-use change from C3 native vegetation to C4 pasture

Land-use change from C3 vegetation (d13C values, –30‰ to –24‰) to C4 vegetation (d13C values, –14‰ to –11‰) provides a useful quantitative technique for estimating organic C turnover in soil, even when total organic C changes are negligible. We utilised this technique to estimate C turnover in physically fractionated soil organic matter, particulate organic matter C (POM C >250 µm fraction and POM C 250–53 µm fraction), and the <53 µm fraction. There were small changes in total soil organic C (SOC) after 23 years of land-use change from native vegetation (mixed vegetation of Acacia harpophylla and Casuarina cristata) to buffel grass (Cenchrus ciliaris L. cv. Biloela) pasture grown on Vertosol–Dermosol–Sodosol soil types. The SOC values (t/ha) under native vegetation were: 31 ± 3 for the 0–0.1 m depth, 21 ± 1 for the 0.1–0.2 m depth, 15 ± 3 for the 0.2–0.3 m depth, and 16 ± 2 for the 0.3–0.4 m depth; the corresponding SOC values under pasture were 25 ± 2, 19 ± 2, 14 ± 2, and 13 ± 1 t/ha. The respective d13C values in 0–0.1 m depths of the whole SOC and POM C >250 µm fraction changed from –25.5 ± 0.1‰ and –25.5 ± 0.3‰ under native vegetation to –20.1 ± 0.5‰ and –19.4 ± 0.2‰ under pasture. Similar, although smaller, differences were observed for other depths and SOC fractions. The SOC turnover periods (years) were 31 ± 6 for the 0–0.1 m depth, 60 ± 5 for the 0.1–0.2 m depth, 55 ± 15 for the 0.2–0.3 m depth, and 63 ± 20 for the 0.3–0.4 m depth; the corresponding turnover periods for the POM C >250 µm fraction were 13 ± 2, 19 ± 5, 14 ± 4, and 12 ± 5 years. The turnover periods of SOC in the POM C 250–53 µm and <53 µm fractions were similar to, or longer than, for the whole SOC at all depths studied. Thus, the lability of the SOC and SOC pools was in the order: POM C >250 µm fraction > POM C 53–250 µm fraction = POM C <53 µm fraction = whole SOC.