Tian Lin-Lin, Wang Zheng, Hu Lei, Ren Guang-Qian, Zhu Bo
Institute of Mountain Hazards and Environment, Chinese Academy of Sciences, Chengdu 610041, China.
Key Laboratory of Mountain Surface Processes and Ecological Regulation, Chinese Academy of Sciences, Chengdu 610041, China.
Huan Jing Ke Xue. 2019 Apr 8;40(4):1939-1949. doi: 10.13227/j.hjkx.201808251.
Headwater streams around agricultural farmlands can act as important sinks of active nitrogen (N) and potential sources of indirect nitrous oxide (NO) emissions, as well as aggravating agricultural non-point source N pollution. In this study, the dynamic characteristics of the dissolved NO 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 NO concentration using headspace equilibration-gas chromatography, and the dissolved NO 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 NO concentration. The results showed that the dissolved NO concentration in the agricultural headwater stream ranged from 0.26 to 1.28 μg·L with an annual mean value of 0.57 μg·L, with nitrate (NO-N, with an annual mean concentration of 1.45 mg·L) as the predominant reactive N form. The seasonal mean concentrations of the dissolved NO in winter, spring, summer, and autumn were 0.63, 0.45, 0.53, and 0.64 μg·L, respectively, without significant seasonal variations. The annual dynamics of the dissolved NO concentration were primarily governed by the concentration of NO-N in the stream water, with denitrification being the main process producing NO. The saturation levels of the dissolved NO 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 NO emissions throughout the year. The temporal variations in the saturation levels of the dissolved NO were mainly controlled by the water temperature and the NO-N concentration of the stream water. During April-October, the concentration of dissolved NO in the stream fluctuated obviously as a result of heavy rainfall, which resulted in an increase of the concentration of NO-N in the stream water in the short term after the rain, which promoted denitrification and then increased the dissolved NO level correspondingly.
农田周边的源头溪流可成为活性氮（N）的重要汇以及间接一氧化二氮（N₂O）排放的潜在源，同时还会加剧农业面源氮污染。本研究于2014年12月至2015年10月期间，通过顶空平衡-气相色谱法测量顶空气态N₂O浓度，并根据相关参数计算溶解态N₂O浓度，对长江上游紫色土丘陵区一条农业源头溪流中溶解态N₂O浓度的动态特征进行了观测。同时，还对溪流水体的理化参数进行了监测，以了解影响溶解态N₂O浓度的因素。结果表明，该农业源头溪流中溶解态N₂O浓度范围为0.26至1.28μg·L，年均值为0.57μg·L，其中硝酸盐（NO₃-N，年均浓度为1.45mg·L）为主要的活性氮形态。冬季、春季、夏季和秋季溶解态N₂O的季节平均浓度分别为0.63、0.45、0.53和0.64μg·L，无明显季节变化。溶解态N₂O浓度的年动态主要受溪流水体中NO₃-N浓度的控制，反硝化作用是产生N₂O的主要过程。溪流水体中溶解态N₂O的饱和度呈现过饱和状态，年均值为203.9%（109.7%-546.5%），具有季节变化规律，即夏季和秋季的饱和度高于冬季和春季，表明该农业源头溪流全年均可释放间接N₂O排放。溶解态N₂O饱和度的时间变化主要受水温及溪流水体中NO₃-N浓度的控制。4月至10月期间，由于暴雨，溪流中溶解态N₂O浓度波动明显，降雨后短期内溪流水体中NO₃-N浓度增加，促进了反硝化作用，进而相应提高了溶解态N₂O水平。
