通过氮氧同位素组成辨析太湖地区大气氮沉降来源

Nitrogen (N) deposition has been an important component in the global N cycle and it has sharply increased in recent decades. Excess N deposition has aroused concerns about its negative impacts on ecosystem health and services such as loss of biodiversity, eutrophication and N saturation, soil acidification, and increased susceptibility to secondary stresses. China is one of the global heavy N deposition areas, especially in the southeast. Identifying the origin of pollutants N deposition and analysis its spatial-temporal variations are important for N pollution control. However, the study of N dry deposition flux is limited, and the sources of N deposition are not well known. Measurements of the isotopic composition of the N in deposition have helped constrain the sources and processing of atmospheric N. The δ15N of some N compounds reflects their source, isotopic fractionation resulting from physical and chemical reactions, and biological reactions and functions. In addition, the stable isotopes of associated elements of N compounds, such as the δ18O of NO3-, can also be used as a powerful tracer. The Taihu Lake region is one of the most important industrially and agriculturally developed regions in China. The rapid development of the industry, excessive use of chemical N fertilizers, as well as the increasing amount of human and livestock excreta result in heavily N deposition in this region. To provide basic information on the management of atmospheric N pollution and the eutrophication of Taihu Lake, the objectives of this study are therefore to (1) use the dual isotopic composition of δ15N and δ18O as a tracer of the sources of N deposition in farming systems, rural and urban landscapes, (2) analyze the composition of δ15N and δ18O in N deposition and N emissions, (3) identify the main sources of N deposition and their spatial-temporal variations, and (4) discuss the relationship between N deposition and emissions.

大气氮沉降是全球氮循环的重要过程，通过化石燃料燃烧、氮肥施用、畜禽养殖等人为活动排放到大气中的氮又通过干湿沉降返回到陆地和水体。尽管氮沉降是重要的环境养分来源之一，但过量的氮沉降严重影响陆地及水生生态系统的生产力和稳定性。因此，从源头上控制大气氮沉降即辨别氮沉降的来源并对关键排放源进行调控，对于开展大气氮沉降及其减量措施的研究至关重要。针对目前氮沉降来源多采用推测和估算的方法及单一同位素组成无法准确辨析氮沉降来源等问题，本研究拟选取人为活动强烈的太湖地区作为研究区，通过分别对农田、城市及城乡结合处三种土地利用类型下大气氮沉降及主要氮排放源的长期野外观测和室内检测，分析大气氮干湿沉降及主要氮排放源的氮氧同位素组成、化学形态特征、氮沉降与排放通量和时空变化规律等，明确太湖地区大气氮沉降来源并评价氮沉降与氮排放的关系，最终为太湖地区大气氮污染和水体富营养化综合治理提供基础资料。

大气氮沉降是全球氮循环的重要过程，尽管氮沉降是重要的环境养分来源之一，但过量的氮沉降严重影响陆地及水生生态系统的生产力和稳定性。因此，从源头上控制大气氮沉降即辨别氮沉降的来源并对关键排放源进行调控，对于开展大气氮沉降及其减量措施的研究至关重要。本研究选取了太湖地区城市(Urban,SZ)、城郊(Suburban, CS)和农村(Rural, YX)三种土地利用方式，分析了太湖地区大气的时空变异特征，综合讨论各形态活性氮的影响因素并对主要来源和贡献进行了研究。研究结果表明：.观测期内3个监测点降水中年均TN浓度CS>SZ>YX，NH4+ -N浓度CS> SZ > YX，NO3--N浓度 SZ>YX>CS。CS、SZ、YX三个观测点大气湿沉降TN年均沉降通量为27.5kgN/ha、26.6 kgN/ha、和38.5 kgN/ha。.太湖地区大气气态活性氮浓度呈现较为明显的时空变异特征，城市、城郊、农村气态活性氮（NH3、HNO3、NO2）浓度分别为29.05、32.72、42.28 μgN∙m-3，平均值为34.68 μgN∙m-3。太湖地区大气NH3浓度受人为因素影响明显。太湖地区大气颗粒态NO3-月平均浓度为5.03 μgN∙m-3，CS、SZ、YX三个采样点分别为4.40、5.50、5.19 μgN∙m-3。大气颗粒态NH4+浓度月平均值为7.82 μgN∙m-3，其中SZ、CS、YX三个采样点月平均值分别为6.31、8.75、8.41 μgN∙m-3。.三个地点雨水中的δ15N-NH4+ 值分别为–4.86‰（SZ）、–3.99‰(CS) 和0.87‰（YX），采样期间颗粒态δ15N-NH4+的值均大于0‰, 显著高于气态与农田和养殖场挥发排放的氨同位素值。本研究中农田、养殖场排放的NH3 的δ15N-NH3值处于–35.09‰到–10.47‰之间。源解析模型模拟结果表明，农业源排放的氨是降雨和颗粒态NH4+ -N的主要贡献者，城市和城郊雨水中60%以上的NH4+ -N来自于农业排放。 另外，温度和降雨量也是影响大气中N-NHx同位素值的重要因素。本研究结果最终可为太湖地区大气氮污染和水体富营养化综合治理提供基础资料。