Carbon inventory for a cereal cropping system under contrasting tillage, nitrogen fertilisation and stubble management practices

Conservation farming practices are often considered effective measures to increase soil organic C (SOC) sequestration and/or to reduce CO2 emissions resulting from farm machinery operation. The long-term CO2 mitigation potentials of no-till (NT) versus conventional till (CT), stubble retention (SR) versus stubble burning (SB) and N fertilisation (NF) versus no N application (NO) as well as their interactions were examined on a Vertosol (Vertisol) in semi-arid subtropical Queensland, Australia by taking into account their impacts on SOC content, crop residue C storage, on-farm fossil fuel consumption and CO, emissions associated with N fertiliser application. The experimental site had been cropped with wheat (Triticum aestivum L.) or barley (Hordeum vulgare L.) with a summer fallow for 33 years. Where NT, SR or NF was applied alone, no significant effect on SOC was found in the 0-10, 10-20 and 0-20 cm depths. Nonetheless, the treatment effects in the 0-10 cm depth were interactive and maximum SOC sequestration was achieved under the NT + SR + NF treatment. Carbon storage in crop residues decreased substantially during the fallow period, to a range between 0.4 Mg CO2-e ha(-1) under the CT + SB + NF treatment and 2.4 Mg CO2-e ha(-1) under the NT + SR + NO treatment (CO2-e stands for CO2 equivalent). The cumulative fossil fuel CO2 emission over 33 years was estimated to be 2.2 Mg CO2-e ha(-1) less under NT than under CT systems. Cumulative CO2 emissions from N fertiliser application amounted to 3.0 Mg CO2 ha(-1). The farm-level C accounting indicated that a net C sequestration of 4.5 Mg CO2-e was achieved under the NT + SR + NF treatment, whilst net CO2 emissions ranging from 0.5 to 6.0 Mg CO2-e ha(-1) over 33 years occurred under other treatments. (c) 2005 Elsevier B.V. All rights reserved.