Parent material and climate affect soil organic carbon fractions under pastures in south-eastern Australia

In the present field survey, 72 sites were sampled to assess the effect of climate (Monaro, Boorowa and Coleambally regions) and parent material (Monaro region only; basalt and granite) on soil organic carbon (OC) under perennial pastures. In the higher-rainfall zone (Monaro and Boorowa; >500 mm mean annual rainfall), OC stocks under introduced and native perennial pastures were compared, whereas in the lower-rainfall zone (Coleambally; <500 mm mean annual rainfall) OC stocks under crops and pastures were compared. Carbon fractions included total OC (TOC), particulate OC (POC), resistant OC (ROC) and humic OC (HUM). Higher OC stocks were associated with higher spring and summer rainfall and lower annual temperatures. Within a climatic zone, parent material affected the stock of OC fractions in the 0–30 cm soil layer. Within a climatic zone, when grouped by parent material, there was no difference in OC stock with vegetation type. There were significant correlations between soil factors associated with parent material and OC concentration, including negative correlations between SiO2 and HUM (P < 0.05) and positive correlations between cation exchange capacity and TOC, POC and ROC (P < 0.01). TOC was also positively correlated with total nitrogen (N) and available sulfur (S; P < 0.05), indicating organic matter in soil is important for N and S supply for plant production in the studied regions, and vice versa. Although ensuring adequate available S may increase OC stocks in south-eastern Australia, the large stock of OC in the soil under perennial pastures, and the dominating effect of climate and parent material on this stock, may mean that modest increases in soil OC due to management factors go undetected.