Soil nitrous oxide emissions under dryland N-fertilised canola and N₂-fixing chickpea in the northern grains region, Australia

Abstract

Nitrous oxide (N₂O) emissions from cropping soils contribute to increasing atmospheric N₂O. Planning to reduce emissions requires real-world measurements. Crop production systems that partially rely on nitrogen (N) fixed by legumes may emit less N₂O than systems that are totally dependent on fertiliser N inputs. We measured N₂O emissions from a dryland vertosol in northwest NSW, Australia during the growth of N-fertilised canola ('Brassica napus') and N₂-fixing chickpea ('Cicer arietinum'). At sowing, canola received 80 kg N/ha as urea, and chickpea was inoculated with effective rhizobia. Emissions of N₂O were monitored seven times per day using an automated system of chambers connected to a gas chromatograph. Daily N₂O emissions ranged from -1.7 to 39.6 g N₂O-N/ha/day in canola plots and -1.6 to 12.5 g N₂O-N/ha/day for chickpea. During crop growth, the N-fertilised canola plots emitted a total of 293 g N₂O-N/ha, equivalent to 0.37% of the urea N applied. Chickpea plots emitted 29 g N₂O-N/ha. The canola plots emitted a further 241 g N₂O-N/ha in the first months of the post-crop fallow, mostly during a short period of high rainfall, compared with 58 g N₂O-N/ha for chickpea. We hypothesise that the canola residue may have mineralised N earlier than chickpeas.