Response of soil nitrous oxide flux to nitrogen fertiliser application and legume rotation in a semi-arid climate, identified by smoothing spline models
Although large areas of semi-arid land are extensively cropped, few studies have investigated the effect of nitrogen (N) fertiliser on nitrous oxide (N2O) emissions in these regions (Galbally et al. 2010). These emissions need to be measured in order to estimate N losses and calculate national greenhouse gas inventories. We examined the effect of different agronomic management practices applied to wheat (Triticum aestivum) grown on an alkaline Vertosol in south-eastern Australia on N2O emissions. In 2007, N2O emissions were measured over 12 months, during which N fertiliser (urea) was applied at sowing or N fertiliser plus supplementary irrigation (50 mm) was applied during the vegetative stage and compared with a treatment of no N fertiliser or irrigation. In a second experiment (2008), the effect of source of N on N2O emissions was examined. Wheat was grown on plots where either a pulse (field peas, Pisum sativum) or pasture legume (barrel medic, Medicago truncatula) crop had been sown in the previous season compared with a non-legume crop (canola, Brassica napus). To account for the N supplied by the legume phase, N fertiliser (50 kg N ha-1 as urea) was applied only to the wheat in the plots previously sown to canola. Fluxes of N2O were measured on a sub-daily basis (up to 16 measurements per chamber) by using automated chamber enclosures and a tuneable diode laser, and treatment differences were evaluated by a linear mixed model including cubic smoothing splines. Fluxes were low and highly variable, ranging from -3 to 28 ng N2O-N m-2 s-1. The application of N fertiliser at sowing increased N2O emissions for ~2 months after the fertiliser was applied. Applying irrigation (50 mm) during the vegetative growth stage produced a temporary (~1-week) but non-significant increase in N2O emissions compared with plots that received N fertiliser at sowing but were not irrigated. Including a legume in the rotation significantly increased soil inorganic N at sowing of the following wheat crop by 38 kg N ha-1 (field peas) or 57 kg ha-1 (barrel medic) compared with a canola crop. However, N2O emissions were greater in wheat plots where N fertiliser was applied than where wheat was sown into legume plots where no N fertiliser was applied. Over the 2 years of the field study, N2O emissions attributed to fertiliser ranged from 41 to 111 g N2O-N ha-1, and averaged of 75 g N2O-N ha-1 or 0.15% of the applied N fertiliser. Our findings confirm that the proportion of N fertiliser emitted as N2O from rainfed grain crops grown in Australian semi-arid regions is less than the international average of 1.0%.