Is indirect N_2O emission a significant contributor to the agricultural greenhouse gas budget? A case study of a rice paddy-dominated agricultural watershed in eastern China

摘要

The amount of indirect nitrous oxide (N_2O) emission from agricultural watersheds with high intensive fertilizer nitrogen (N) application rate is supposed to be great. However, limited data restrict the understanding of N_2O emission in these areas, especially in water-rich watershed dominated by rice paddies. Indirect N_2O 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 N_2O emission. The N_2O emission rates of rivers (mean = 12.9 ± SD 21.8 μg N_2O-N m~(-2) h~(-1)) were significantly higher than those of ponds (mean = 4.5 ± SD 16.3 μg N_2O-N m~(-2) h~(-1)) and the reservoir (mean = 7.9 ± SD 10.0 μg N_2O-N m~(-2) h~(-1)). The indirect N_2O emission only accounted for 1.2% of the total indirect and direct N_2O emissions and 0.53% of N removed via aquatic denitrification. The disproportionately low N_2O emissions could have resulted from the limited inputs of N into waterways and low N_2O/(N_2O + N_2) in removing N through denitrification under strong reductive conditions. We suggest that the N_2O budget predictive modeling should consider water Eh because it may indirectly affect N_2O emission rates by controlling the ratio of N_2O to N_2 via denitrification.