Nitrous oxide fluxes from cropping soils in a semi-arid region in Australia: A 10 year perspective

Understanding nitrous oxide (N2O) fluxes from agricultural soils in semi-arid regions is required to better understand global terrestrial N2O losses. Nitrous oxide fluxes were measured from three rain-fed, cropped soils in a semi-arid region of south-western Australia on a sub-daily basis from 1995 to 2014 using automated chambers. Western Australia’s grain-belt includes 7 million hectares of arable land, with cropping confined to winter and soils fallow at other times. Nitrogen fertilizer (up to 100 kg N ha yr) was applied at planting and during the growing season depending on crop requirements. In situ N2O measurements were consistently small from all sites (0.04–0.27 kg N ha yr), representing 0.01 to 0.12% of applied N fertilizer. Increasing soil organic matter (OM) increased soil N2O fluxes, but losses represented <0.12% of the N fertilizer applied. While including grain legumes in cropping rotations also did not enhance soil N2O fluxes in the growing season or post-harvest. Developing strategies for mitigating N2O fluxes from cropping soils in our region is challenging as most losses occur post-harvest, when there is no active plant growth, and in response to summer rainfall. Increasing the efficiency of the nitrification process by increasing soil pH (via liming) decreased N2O fluxes from sandy, acidic soils following summer rainfall, and is a potential strategy for mitigating N2O fluxes from nitrification. Accurately accounting for N2O fluxes in our region has refined Australia’s national greenhouse gas inventory and demonstrated annual fluxes can be low from cropped soils in semi-arid regions.