Effect of increasing soil carbon on N2O emissions and fertiliser requirements. Buntine, Western Australia, 2012-2013 [Theme: Soil Carbon in N2O Emissions]

Increasing soil organic carbon (SOC) is promoted as a strategy for sequestering carbon dioxide (CO2) and mitigating anthropogenic greenhouse gas (GHG) emissions. Agricultural management practices such as the conversion from conventional to no-till or reduced tillage, residue retention and addition, and crop rotations have the potential to increase SOC both internationally and in Australia. It is estimated widespread adoption of these recommended practices would increase SOC in croplands by 0.4 to 0.8 Pg C per year globally, with up to 0.99 Pg C in Australia cropping soils. However, while these agricultural practices may benefit soil C sequestration, their contribution to mitigating global climate warming may be offset by emissions of other GHGs.

Increasing SOC can increase nitrous oxide (N2O), a potent GHG, by increasing the availability of nitrogen (N) and carbon (C) to soil microoganisms. Crop residues are subject to N mineralisation, and in turn nitrification and denitrification; microbial process that lead to N2O production. For example, nitrifying microbes convert soil ammonium (NH4+) to nitrate (NO3-) under aerobic conditions, which may result in N2O-formation as a by-product of the N-transformation. Likewise, anaerobic denitrifiers sequentially reduce nitrogen oxides (e.g. NO3-) to nitric oxide, N2O and finally N2; with N2O emissions resulting from an incomplete conversion. Increasing soil N concentrations also inhibits methane (CH4) uptake by soil. While soil C sequestration would be expected to reach a maximum threshold, the effects on soil N2O and CH4 emissions could continue if SOC concentrations were maintained. Understanding the interaction between increasing SOC and GHG emissions is critical when assessing the effectiveness of land management practices to abate GHG emissions from the agricultural land sector.

The effect of increasing SOC on GHG emissions and N fertiliser requirements is poorly understood for Australia’s infertile dryland cropping soils. Our knowledge of the interaction between soil C and GHG emissions is mainly derived from agricultural systems in the Northern Hemisphere. The majority of studies have investigated these effects on medium fertility soils, with GHG emissions often measured for only one to two growing seasons. The overall objective of this project was to investigate if increasing SOC altered GHG emissions (N2O and CH4). Consequently, measured GHG emissions from a research site where growers have been increasing soil carbon via organic matter inputs since 2003. We hypothesised increasing SOC would increase soil N2O and CH4 emissions, but at the same time lower the amount of N fertiliser required for grain production.