大气氮沉降对洞庭湖氮素入湖通量的贡献

China's atmospheric nitrogen (N) deposition has been increased by 50% in the last three decades, especially in the eastern and southern parts where it has reached 80-100 kg N per hectare per year. As important nutrient and acidic sources, more and more attention has been paid to the contribution of N deposition to the total N input, which results eutrophication of marine, coastal, river and lakes. Many studies have found that N deposition increased marine productivity, introduced costal and estuaries eutrophication, and changed lakes stoichiometry etc. Studies in China also found that N deposition input was much higher than the exceedence even without other N inputs taken into account. However, few research about this was concerned to full measurements of N deposition including both wet and dry depositions (dry deposition means both gases and aerosols), and most of researches were only concerned on inorganic N species without organic N components. It means that the real contribution of N deposition has been seriously underestimated even at current high levels. What’s more, no regional fine resolution model was conducted in China by now to estimate the contribution of N deposition to watershed, which results only average N deposition values or single site result cited for regional evaluation. Last but not the least, many studies only considered the input of atmospheric N deposition to the water surface instead of the watershed, which further underestimated the contribution of atmospheric N deposition to the water...In this study, Dongting lake watershed is selected as the research area. As the second largest fresh water lake, N concentration of lake water has been keeping increase in the last twenty years, and has become the main pollution species. N concentration of precipitation was also increased one third in the last thirty years, which is comparable to the lake water N concentration and about ten times that of the critical level of entrophication. The study aims to systematically estimate the flux of N deposition to the Dongting lake. Rain gauges, DELTA systems and OGAWA systems will be set up for collecting N deposition samples from precipitation, aerosols and gases. At the same time, the Fine Resolution Atmospheric Multi-pollutant Exchange (FRAME) model, will be applied to simulate spatial and temporal variations of N deposition in the region. With the calibration of monitoring results, simulated results will be used to estimate the contribution of N deposition not only directly deposit to lake water surface but also deposit to watershed and indirectly input to the lake from runoff. Finally, the Net Anthropogenic N Input (NANI) of the watershed will be calculated to estimate the fluxes of atmospheric N deposition in the sub-watersheds to the lake. Together with the N deposition directly deposit into the lake area, the total flux of N from atmospheric deposition to the Dongting Lake will be obtained.

针对我国东部和南部地区已成为高氮沉降区的特点，本研究选取洞庭湖为案例，以流域为源，湖泊为汇，通过点位监测和区域模型结合的方法，对该流域内大气氮沉降输入湖泊的氮素进行估算。采用系统、全面的大气氮沉降定量方法，对干、湿沉降的各种形式（降水、气溶胶、气体）进行采样，并对各种形态的氮素（无机和有机）进行完整的分析；通过大气氮沉降模型（FRAME）的应用，对流域内大气氮沉降的空间变异进行模拟，并通过监测结果进行校正，生成该流域大气氮沉降的空间分布图；根据大气氮沉降的生物地球化学循环特征，分别以流域和湖泊为研究范围，设定源和汇的边界，建立大气氮沉降输入湖泊的概念框架，将其定义为直接沉降到湖泊表面的氮素，与沉降到流域范围后间接输入到湖泊的氮素之和。并在此框架下，通过人为氮素净输入（NANI）估算洞庭湖流域大气氮沉降对湖泊氮素输入的贡献，为科学地评价大气氮沉降对湖泊（汇）水体富营养化的贡献提供理论基础。

在过去近二十年中，洞庭湖水体的总氮浓度从不足1.0 mg L-1 不断上升到2.0 mg L-1。整个湖区入水断面水质均低于Ⅲ类水标准，Ⅴ类和劣Ⅴ类水质占78.6%。根据从八十年代至今湖南省的降水化学分析发现，过去三十年降水中无机氮的浓度由1.5 mgN L-1 增长到了2.1 mg N L-1。针对我国东部和南部地区已成为高氮沉降区的特点，本研究选取洞庭湖为案例，以流域为源，湖泊为汇，通过点位监测和区域模型结合的方法，对该流域内大气氮沉降输入湖泊的氮素进行估算。采用系统、全面的大气氮沉降定量方法，对干、湿沉降的各种形式进行采样，并对各种形态的氮素（无机和有机）进行完整的分析；通过大气氮沉降模型（FRAME）的应用，对流域内大气氮沉降的空间变异进行模拟；根据大气氮沉降的生物地球化学循环特征，分别以流域和湖泊为研究范围，设定源和汇的边界，通过人为氮素净输入（NANI）估算洞庭湖流域内氮素通过地表径流输入湖泊的比例，由直接沉降到湖泊表面的氮素，与沉降到流域范围后间接输入到湖泊的氮素之和估算大气氮素沉降对洞庭湖富营养化的贡献。..研究结果表明，洞庭湖区大气氮素沉降的通量约为79.0 kg N hm-2 yr-1，其中氮素湿沉降通量51.5 kg N hm-2 yr-1，氮素干沉降通量27.5kg N hm-2 yr-1，远远超过以往研究工作中针对湿沉降中无机氮定量的结果，本研究湿沉降中有机氮占23%的比例，这部分在国内研究中往往被忽视的部分事实上占有相当的比例。根据NANI模型模拟的洞庭流域各子流域氮素的输入，结合主要入湖口监测的氮素入湖通量，该流域内通过地表径流由陆源输入湖泊的氮素通量比例为15%。根据该结果，流域内15%的大气氮素沉降间接输入湖泊，这部分氮素约为30.9万吨，结合直接沉降到湖泊表面的氮素通量2.1万吨，由大气氮素沉降输入的氮素总通量为33万吨。相当于提高了1.1 mg N L-1的湖水浓度，贡献了大约182万吨碳当量的初级生产力。 该研究结果不仅为科学地定量了大气氮素沉降的通量，而且为湖泊的氮素收支提供理论基础，也为湖泊富营养化的控制提供科学依据。