Dependence of riverine nitrous oxide emissions on dissolved oxygen levels

Abstract

Nitrous oxide is a potent greenhouse gas, and it destroys stratospheric ozone¹. Seventeen per cent of agricultural nitrous oxide emissions come from the production of nitrous oxide in streams, rivers and estuaries², in turn a result of inorganic nitrogen input through leaching, runoff and sewage. The Intergovernmental Panel on Climate Change and global nitrous oxide budgets assume that riverine nitrous oxide emissions increase linearly with dissolved inorganic nitrogen loads, but data are sparse and conflicting^2,3. Here we report measurements over two years of nitrous oxide emissions in the Grand River, Canada, a seventh-order temperate river that is affected by agricultural runoff and outflow from a waste-water treatment plant. Emissions were disproportionately high in urban areas and during nocturnal summer periods. Moreover, annual emission estimates that are based on dissolved inorganic nitrogen loads overestimated the measured emissions in a wet year and underestimated them in a dry year. We found no correlations of nitrous oxide emissions with nitrate or dissolved inorganic nitrogen, but detected negative correlations with dissolved oxygen, suggesting that nitrate concentrations did not limit emissions. We conclude that future increases in nitrate export to rivers will not necessarily lead to higher nitrous oxide emissions, but more widespread hypoxia most likely will.