Three Years of Continuous Nitrous Oxide Emissions From a Cropped Soil In a Semi-Arid Climate (Australia).

Nitrous oxide (N₂O) emissions from rain-fed agricultural soils in semi-arid regions have rarely been reported, but are required to understand global terrestrial N₂O losses.  Nitrous oxide emissions were measured from a rain-fed, cropped soil in a semi-arid region of south-western Australia for three years on a sub-daily basis.  The site included N fertilizer (75–100 kg N ha^-1 yr^-1) and no N fertilizer plots (control).  Emissions were measured using soil chambers connected to a fully automated system that measured N₂O using gas chromatography.  Daily N₂O emissions were low (-2.1–7.3 g N₂O-N ha^-1 day^-1) and culminated in 0.09–0.13 kg N₂O-N ha^-1 yr^-1 from the N fertilized soil and 0.07–0.09 kg N₂O-N ha^-1 yr^-1 from the control. The proportion of N fertilizer emitted as N₂O each year, after correction for the control fluxes, was 0.02–0.06%.  The emission factor  in the present study was up to 50 times lower than the IPCC default value for the application of synthetic fertilizers to land (1.0%), indicating the default value may not be suitable for cropped soils in semi-arid regions.  Daily N₂O emissions did not increase immediately in response N fertilizer applications.  Furthermore in the first year, over half (55%) the annual N₂O emission occurred from both N fertilizer treatments following a series of summer rainfall events while  the soil was fallow (i.e., no active plant growth). The timing of N₂O emissions suggests modifying N fertilizer practices are unlikely to decrease N₂O emissions from rain-fed, cropped soils in our region.