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

Abstract

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 (N2O) emission. In this study, indirect N2O emissions from an agricultural headwater stream in the hilly area of purple soil in the upper reach of the Yangtze River were measured in situ 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 N2O emissions, with a mean emission rate of 12.8 μg·(m2·h)-1, which is close to the direct N2O emission level from local farmland during the same season. The indirect N2O emission factor (EF5r=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 N2O 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 EF5r and the high spatial variations among them, more in situ observations are needed and vital to generate more accurate EF5r values and reduce the uncertainty of indirect N2O estimations calculated based on the EF5r. The indirect N2O fluxes are positively correlated with the NO3--N concentrations of the stream. Thus, denitrification is the main process of N2O production. Moreover, the indirect N2O fluxes could be notably promoted by the rapid increase of the NO3--N concentrations that were driven by rainfall>9 mm during days with continuous rain.