Is indirect N2O emission a significant contributor to the agricultural greenhouse gas budget? A case study of a rice paddy-dominated agricultural watershed in eastern China

The amount of indirect nitrous oxide (N2O) emission from agricultural watersheds with high intensive fertilizer nitrogen (N) application rate is supposed to be great. However, limited data restrict the understanding of N2O emission in these areas, especially in water-rich watershed dominated by rice paddies. Indirect N2O emission and its potential drivers were studied for two years in the surface water of a rice paddy-dominated agricultural watershed in eastern China. Results showed that nitrate concentration (mean 0.4 mg N L−1) and Eh (mean of −86.1 mV) in surface water were the most important drivers of indirect N2O emission. The N2O emission rates of rivers (mean = 12.9 ± SD 21.8 μg N2O–N m−2 h−1) were significantly higher than those of ponds (mean = 4.5 ± SD 16.3 μg N2O–N m−2 h−1) and the reservoir (mean = 7.9 ± SD 10.0 μg N2O–N m−2 h−1). The indirect N2O emission only accounted for 1.2% of the total indirect and direct N2O emissions and 0.53% of N removed via aquatic denitrification. The disproportionately low N2O emissions could have resulted from the limited inputs of N into waterways and low N2O/(N2O + N2) in removing N through denitrification under strong reductive conditions. We suggest that the N2O budget predictive modeling should consider water Eh because it may indirectly affect N2O emission rates by controlling the ratio of N2O to N2 via denitrification.