[Indirect Nitrous Oxide Emissions from an Agricultural Headwater Stream During the Rainy Season in the Upper Reach of the Yangtze River].

Agricultural headwater streams have a close hydrologic connection with adjacent farmland ecosystems. Based on the aggravation of agricultural nonpoint source of nitrogen (N) pollution, these streams can become an important sink of N and source of indirect nitrous oxide (NO) emission. In this study, indirect NO emissions from an agricultural headwater stream in the hilly area of purple soil in the upper reach of the Yangtze River were measured using the closed static chamber-GC technique during the rainy season (June to September 2015). The results show that the headwater stream is a source of indirect NO emissions, with a mean emission rate of 12.8 μg·(m·h), which is close to the direct NO emission level from local farmland during the same season. The indirect NO emission factor (EF=0.01%) determined in this study is much lower than the default value proposed by the Intergovernmental Panel on Climate Change (0.25%; IPCC, 2006) for the estimation of indirect agricultural NO emissions and far lower than the recalculated mean value (0.20%) based on available global data. However, based on the limited number of studies on EF and the high spatial variations among them, more observations are needed and vital to generate more accurate EF values and reduce the uncertainty of indirect NO estimations calculated based on the EF. The indirect NO fluxes are positively correlated with the NO-N concentrations of the stream. Thus, denitrification is the main process of NO production. Moreover, the indirect NO fluxes could be notably promoted by the rapid increase of the NO-N concentrations that were driven by rainfall>9 mm during days with continuous rain.