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 (N0) 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 CO2 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. 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 + N0 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.