Sensitivity of soil carbon to management and environmental factors within Australian perennial pasture systems

Environmental factors such as parent material and climate can have a large effect on total carbon concentration and soil carbon stocks, yet unlike vegetation type, fertiliser use and grazing pressure, these cannot be changed by management. The relative effects of these environmental and land management factors were compared in the Monaro and Boorowa regions of New South Wales (NSW), Australia. Parent material, geographic region, soil depth and soil fertility had a significant influence on soil carbon stocks to 0.70 m while pasture type (introduced vs native pastures) did not. Parent material and soil depth significantly (P < 0.05) influenced the mean soil carbon stock (Mg C/ha) in the Monaro region; 159 (11 se) in basalt-derived soils, 77 (11 se) in deep granite-derived soils and 43 (3 se) in shallow granite-derived soils. Climate also significantly (P < 0.05) influenced the mean carbon stock, with deep granite-derived soils in the Monaro region having 76.5 (11 se) compared with 51.8 (3 se) Mg C/ha in the Boorowa region. A considerable proportion of the total carbon stock to 0.70 m for all sites was measured in the subsoil (0.30 to 0.70 m). In the Monaro region, basalt-derived soil contained 43% of the total carbon stock in the subsoil, compared with 28% in deep granite and shallow granite-derived soil. In the Boorowa region, deep granite-derived soil contained 33% of the total carbon stock in the subsoil. Restricting soil carbon measurements to the surface 0.30 m of soil may result in erroneous conclusions with respect to the influence of land management on the accumulation of carbon in soil. Total carbon concentration was positively correlated with labile carbon, total nitrogen, cation exchange capacity and extractable sulfur, suggesting that for a given parent material and climate, maintaining adequate pasture nutrition may substantially increase soil carbon stocks.