Total soil organic matter and its labile pools following mulga (Acacia aneura) clearing for pasture development and cropping 1. Total and labile carbon

Mulga (Acacia aneura) dominated vegetation originally occupied 11.2 Mha in Queensland, of which 12% has been cleared, mostly for pasture production, but some areas are also used for cereal cropping. Since mulga communities generally occupy fragile soils, mostly Kandosols and Tenosols, in semi-arid environments, clearing of mulga, which continues at a rate of at least 35 000 ha/year in Queensland, has considerable impact on soil organic carbon (C), and may also have implications for the greenhouse gas emissions associated with land use change in Australia. We report here the changes in soil C and labile C pools following mulga clearing to buffel pasture (Cenchrus ciliaris) and cereal (mostly wheat) cropping for 20 years in a study using paired sites. Soil organic C in the top 0.05 m of soil declined by 31 % and 35% under buffel pasture and cropping, respectively. Land use change from mulga to buffel and cropping led to declines in soil organic C of 2.4 and 4.7 t/ha, respectively, from the top 0.3 m of soil. Using changes in the delta(13)C values of soil organic C as an approximate representation of C derived from C-3 and C-4 vegetation from mulga and buffel, respectively, up to 31% of soil C was C-4-derived after 20 years of buffel pasture. The turnover rates of mulga-derived soil C ranged from 0.035/year in the 0-0.05 m depth to 0.008/year in the 0.6-1 m depths, with respective turnover times of 29 and 133 years. Soil organic matter quality, as measured by the proportion/amount of labile fraction C (light fraction, <1.6 t/m(3)) declined by 55% throughout the soil profile (0-1 m depth) under both pasture and cropping. There is immediate concern for the long-term sustainable use of land where mulga has been cleared for pasture and/or cropping with a continuing decline in soil organic matter quality and, hence, soil fertility and biomass productivity. In addition, the removal of mulga forest over a 20-year period in Queensland for pasture and cropping may have contributed to the atmosphere at least 12 Mt CO2-equivalents.