Abstract
Headwater streams around agricultural farmlands can act as important sinks of active nitrogen (N) and potential sources of indirect nitrous oxide (N2O) emissions, as well as aggravating agricultural non-point source N pollution. In this study, the dynamic characteristics of the dissolved N2O concentration in an agricultural headwater stream in the hilly area of purple soil in the upper reach of the Yangtze River were observed during the period Dec. 2014-Oct. 2015 by measuring the headspace gaseous N2O concentration using headspace equilibration-gas chromatography, and the dissolved N2O concentration was calculated according to the related parameters. Simultaneously, the physical and chemical parameters of the stream water were also monitored to understand the factors that affect the dissolved N2O concentration. The results showed that the dissolved N2O concentration in the agricultural headwater stream ranged from 0.26 to 1.28 μg·L-1 with an annual mean value of 0.57 μg·L-1, with nitrate (NO3--N, with an annual mean concentration of 1.45 mg·L-1) as the predominant reactive N form. The seasonal mean concentrations of the dissolved N2O in winter, spring, summer, and autumn were 0.63, 0.45, 0.53, and 0.64 μg·L-1, respectively, without significant seasonal variations. The annual dynamics of the dissolved N2O concentration were primarily governed by the concentration of NO3--N in the stream water, with denitrification being the main process producing N2O. The saturation levels of the dissolved N2O in the stream water showed oversaturation, with an annual mean value of 203.9% (109.7%-546.5%), with a seasonal pattern in which the saturation levels in the summer and autumn were higher than those in the winter and spring, indicating that the agricultural headwater stream can release indirect N2O emissions throughout the year. The temporal variations in the saturation levels of the dissolved N2O were mainly controlled by the water temperature and the NO3--N concentration of the stream water. During April-October, the concentration of dissolved N2O in the stream fluctuated obviously as a result of heavy rainfall, which resulted in an increase of the concentration of NO3-N in the stream water in the short term after the rain, which promoted denitrification and then increased the dissolved N2O level correspondingly.
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