太湖流域源头溪流氧化亚氮(N2O)释放特征

Characteristics of nitrous oxide(N_2O) emission from a headstream in the upper Taihu Lake Basin

采用密闭箱法研究太湖流域源头溪流N2O释放特征及其影响因素。结果显示:南苕溪N2O释放通量范围在-18.11—397.42μg.m-.2h-1,平均值为(30.37±10.87)μg.m-.2h-1。溪流N2O释放呈现明显的季节模式。冬季释放通量最低,仅为(9.69±7.10)μg.m-.2h-1,夏季释放通量较高,为(17.17±17.35)μg.m-.2h-1;而释放高峰发生于汛期,其N2O释放通量可达(125.97±90.77)μg.m-.2h-1。持续降雨带来的山洪爆发及大量径流输入是造成汛期N2O大量释放的主要原因。从上游源头区至下游农田与城区,N2O释放通量逐渐升高;流域污染梯度对N2O释放通量影响显著。统计分析表明:水体硝态氮负荷是控制流域N2O释放通量变化的主导因素,其他因素如磷含量、溶解氧、地势因素对通量也具有倾向性的显著影响。估算苕溪干流临安段N2O年释放通量可达到0.38 t/a。结果显示:河流人为污染负荷增加时可显著促进河流N2O的释放。

Anthropogenic sources of nitrous oxide（N2O） have received a great deal of attention in recent years.Agricultural fertilization has been shown to be one of the most important sources of N2O.However,the impact of urbanization in the agricultural watershed on regional N2O emission is currently not well understood.China has been undergoing rapid urbanization,which has had a stimulating effect on aquatic N2O emissions.Accordingly,there is an urgent need for assessment of the regional N2O budget.Therefore,this study was conducted to reveal the N2O emission characteristics and possible response mechanisms of a typical headstream under the human disturbed landscape gradients.The specific objectives were to investigate how nitrogen is transported and transformed with landscape gradient,as well as whether N2O emission fluxes are significant and responsive to the load gradient over spatial and temporal scales in the South Tiaoxi River（STR）,which is the most important headwater stream in the Taihu Lake Basin.N2O emission rates ranged from-18.11 to 397.42 μg · m-2 · h-1 and averaged（30.37 ± 10.87） μg ·m^-2·h^-1.Moreover,the emissions followed a seasonal pattern,with the lowest values（9.69 ± 7.10） μg · m^-2·h^-1 being observed in winter,moderate values（17.17 ± 17.35） μg · m^-2·h^-1 occurring in summer,and the highest values（125.97 ± 90.77） m^-2·h^-1 being recorded during the flood season.The significantly high N2O emission rates during the flood period were mainly ascribed to the large amount of overland runoff input caused by continuous rainfall.N2O emission rates responded to the landscape gradient in the watershed,gradually increasing along the stream from the upper headwaters region to the lower farmland and urban areas.Statistical analyses revealed that the nitrate（NO-3-N） load predominantly determined the variations in stream N2O emission rates,while other factors such as phosphorus content,dissolved oxygen,and topography played lesser roles.The South Tiaoxi River was found to be a net source of N2O to the atmosphere,with total annual N2O emissions from the main stream of the STR estimated to be as high as 0.38 t/a.This case study demonstrated that allochthonous pollution load inputs might dramatically stimulate aquatic N2O emissions from a montane headstream with relatively little sediment.Since the exchange of N2O across the gas-water interface is a complex process that involves river microbial activities,pollution loading,gas-water exchange dynamic factors as well as river hydrology and hydraulics,its mechanism requires deeper and more extensive study to enable more accurate estimation of N2O emission fluxes and relevant climate effects at the global scale.