WHAT CAUSES NITROUS OXIDE EMISSIONS FROM SOME SUGARCANE CROPS TO BE SO HIGH?

NITROUS OXIDE IS a potent greenhouse gas, and emission from soils in sugarcane crops of NSW are some of the highest measured from cropping systems. Yet, not all emissions are this large and the reason for the range is unclear. The high emissions come from a site with acid sulphate soils in Australia, and chemo-denitrification in the acid subsoils have been suggested as important causes of the high emissions. However, emissions from acid sulphate soils are not always at the upper end of this range suggesting the explanation is not general. We used the APSIM model to investigate the degree to which the biological nitrous oxide-generating pathways represented in the model might (1) account for the high emissions measured in some sugarcane crops grown on acid sulphate soils, and hence (2) provide a broader understanding of the basis for the wide range of emissions associated with sugarcane. We found conditions at the site where the highest emissions were measured, particularly a combination of high soil carbon (~10%) and large applications of nitrogen fertiliser, gave simulated emissions similar to the high values measured. Sensitivity analyses showed that when soil carbon contents and/or nitrogen applications were lower, predicted emissions reduced to levels closer to those at other sites. Our results suggest that biological pathways are capable of producing the range of nitrous oxide emissions measured in sugarcane crops, and that the contribution of chemical pathways need not be great. The results have important implications for understanding both how nitrous oxide emissions from sugarcane may vary between different environments and how emissions can be mitigated; issues that are particularly important for the environmental sustainability of sugarcane production.