长江上游农业源溪流雨季中N_2O间接排放特征

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

农业源溪流与农田生态系统有着紧密的水文连接,其会随着农业非点源氮(N)污染的加剧而成为重要的N汇和氧化亚氮(N_2O)间接排放源.本研究采用静态暗箱-气相色谱法于2015年6~9月(所研究区域的雨季)原位测定了长江上游紫色土丘陵区农业源溪流的N_2O间接排放通量.结果表明,农业源溪流雨季中N_2O平均排放通量为12. 8μg·(m^2·h)^(-1),接近其所在区域内同季节农田的N_2O直接排放水平,是重要的农业N_2O间接排放源.该农业源溪流中N_2O间接排放系数值(EF5r=0. 01%)远低于IPCC的建议值(0. 25%)和重新计算的全球平均值(0. 20%),然而,全球EF5r的现有数据量仍十分有限、且有较大的空间差异,应加强对此类N_2O间接排放的研究,从而进一步修正EF5r的精度、减少N_2O间接排放估算的误差.本研究的N_2O间接排放通量与水中NO-3-N浓度正相关,反硝化是N_2O的主要产生过程.雨季中较强的降雨(如连续降雨日内降雨> 9 mm)可促进溪流中NO-3-N浓度在雨后短期内急剧升高,进而激发水中N_2O间接排放通量的明显增加.

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 （N2 O） 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. （m^2· h）^ -1 , which is close to the direct N20 emission level from local farmland during the same season. The indirect N20 emission factor （EFsr = 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 EFsI 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 NO~ -N concentrations that were driven by rainfall 〉 9 mm during days with continuous rain.