Long-term land use change in Australia from native forest decreases all fractions of soil organic carbon, including resistant organic carbon, for cropping but not sown pasture

Abstract Soil organic matter (SOM) performs an essential function in soil fertility, biomass and crop productivity, environmental sustainability, and climate change mitigation. We examined how land use change from native forest to either pasture [sown buffel (Cenchrus ciliaris cv. Biloela)] or cropping [primarily wheat (Triticum aestivum L.) and sorghum (Sorghum bicolor L.)] affected total soil organic C (SOC) stocks as well as stocks of three SOC fractions, particulate organic C, humus organic C and resistant organic C. Furthermore, for the cropping system, we also examined whether the use of a ley pasture phase could reverse the loss of SOC. It was found that land use change from native forest to pasture decreased SOC stocks by 12.2 % and soil total N (STN) stocks by 24.6 % during the land development to pasture establishment (≤ 1.75 y), although there were no significant (P > 0.05) changes thereafter up to 33 y and final values were generally similar to initial values. Furthermore, stocks of the three SOC fractions did not change with time in this pasture system. In contrast to these modest changes following conversion to pasture, for land use change to cropping, SOC decreased by 48 % at 0−0.1 m and 38 % (from 54 to 33 Mg ha−1) at 0−0.3 m, due mainly to insufficient C inputs to maintain SOM at steady state. Moreover, stocks of all three SOC fractions decreased with time, including the resistant organic C fraction, indicating that this fraction was not recalcitrant under cropping. The biomass C inputs by crops, mainly as root biomass, were not sufficient to reverse or slow down the rate of decrease of SOC in this soil. However, the introduction of pasture during the last 4 y indicated that the decreases in the stocks of SOC could be arrested by a ley pasture phase.