Effect of dissolved oxygen and nitrogen on emission of N2O from rivers in China

Six rivers from three watersheds in China were chosen to study the temporal and spatial variations in nitrous oxide (N2O) concentrations and emissions in order to examine the link between N2O production and dissolved oxygen (DO) and nitrogen levels. These rivers can generally be divided into two types: runoff rivers with significant natural and agricultural runoff, and urban rivers with significant urban effluents. The results showed that N2O concentrations were 0.15-1.07 (mean 0.51) and 0.22-22.7 (mean 4.10) ug N L−1 in runoff rivers and an urban river, respectively. N2O was oversaturated in almost all the rivers, suggesting that the rivers were sources of atmospheric N2O. N2O emissions in the urban river (range from 1.53 to 2453, mean 529 ug N m−2 h−1) were significantly higher than those in runoff rivers (range from 0.51 to 80.9, mean 18.0 ug N m−2 h−1). We found a significant positive correlation of N2O production with NO3− (r2 = 0.30, p < 0.001) and a negative correlation of N2O production with DO concentrations (r2 = 0.22, p < 0.001) in runoff rivers. Particularly, there existed a significant positive relationship (r2 = 0.21, p < 0.001) between NH4+ and N2O production in an urban river. By using stepwise regression analysis, we found N2O production can be predicted by the dynamics of DO and N levels: for runoff rivers, NO3− and DO explained 47% variability in N2O production, while for the urban river, NH4+ and DO explained 64% variability in N2O production. We suggest that the IPCC method to calculate N2O emission factors should be revised in view of the importance of these multiple factors.