Nitrous oxide emissions from a cropped soil in a semi‐arid climate

Abstract

Abstract Understanding nitrous oxide (N 2 O) emissions from agricultural soils in semi‐arid regions is required to better understand global terrestrial N 2 O losses. Nitrous oxide emissions were measured from a rain‐fed, cropped soil in a semi‐arid region of south‐western Australia for one year on a sub‐daily basis. The site included N‐fertilized (100 kg N ha −1 yr −1 ) and nonfertilized plots. Emissions were measured using soil chambers connected to a fully automated system that measured N 2 O using gas chromatography. Daily N 2 O emissions were low (−1.8 to 7.3 g N 2 O‐N ha −1 day −1 ) and culminated in an annual loss of 0.11 kg N 2 O‐N ha −1 from N‐fertilized soil and 0.09 kg N 2 O‐N ha −1 from nonfertilized soil. Over half (55%) the annual N 2 O emission occurred from both N treatments when the soil was fallow, following a series of summer rainfall events. At this time of the year, conditions were conducive for soil microbial N 2 O production: elevated soil water content, available N, soil temperatures generally >25 °C and no active plant growth. The proportion of N fertilizer emitted as N 2 O in 1 year, after correction for the ‘background’ emission (no N fertilizer applied), was 0.02%. The emission factor reported in this study was 60 times lower than the IPCC default value for the application of synthetic fertilizers to land (1.25%), suggesting that the default may not be suitable for cropped soils in semi‐arid regions. Applying N fertilizer did not significantly increase the annual N 2 O emission, demonstrating that a proportion of N 2 O emitted from agricultural soils may not be directly derived from the application of N fertilizer. ‘Background’ emissions, resulting from other agricultural practices, need to be accounted for if we are to fully assess the impact of agriculture in semi‐arid regions on global terrestrial N 2 O emissions.