澜沧江流域大型梯级水库CH4产生与氧化的生物地球化学过程

Methane emission from reservoirs, which is disputed extensively and intensely as a hot environment issue, is a big problem that development of cascade powerstations in watersheds has to deal with. Methane production and emission is closely related to biogeochemical cycles of nutrients (C, N, and P) in systems of watershed-reservoir. On the other way round, CH4 is a key part in carbon cycle in reservoirs, especially in anaerobic environment in deep water. Therefore, to study the relationship between them is fundamentally important for us to understand the “conveyance and transformation of nutrients in rivers and relevant evolution of environmental quality”. Lancangjiang is a good place to study the relationship between accumulation of nutrients in rivers and CH4 in reservoirs because of big differences in the altitude and latitude of the cascade reservoirs in the catchment. In this proposal, we will conduct a study on concentration, emission flux, and carbon and hydrogen isotope composition of CH4 in 14 constructed reservoirs in the watershed, and with higher temporal and spatial resolution in 3 typical reservoirs among them. In the couple with the result from incubation, we will try to reveal the mechanism of methane production, methane oxidation rate and potential, and flux and pathways of methane emission. The relationship between reservoir methane and biogeochemical cycles of carbon, nitrogen and phosphorus will be studies, and biogeochemical mechanism to control the reservoir methane will be discovered. These are in well agreement with the core scientific problem of the Great Research Plan, “the accumulation effect of variation of environmental quality under the background of cascade powerstation construction”. Finally, research conclusion that will be taken is helpful to forecast the future change, and assess the overall environmental influence of methane emission from reservoirs.

水库CH4释放是流域梯级开发遇到的一个颇具争议的重大环境问题。西南地区大量的梯级水库在CH4释放方面的独特变化与流域水库系统C、N、P营养元素循环密切相关。为此，本项目选择具有极大落差和纬度差异的澜沧江流域作为研究对象，在对流域内梯级水库群调查的基础上，重点对上中下游三个代表性水库CH4浓度、通量及同位素变化开展系统研究工作，结合培养实验，认识大型梯级水库CH4的来源，形成机理，氧化速率及潜力，释放的主要途径和通量。通过与流域碳同位素、氮磷浓度的耦合关系分析，揭示营养盐的累积效应对水库CH4的地球化学调控机制。建立梯级水库CH4的质量平衡模型，预测未来的变化趋势，准确评估水库CH4释放的总体环境效应。项目申请符合指南的研究方向“大型水库生源物质的生物地球化学过程及其生态环境效应”，研究成果对认识重大研究计划科学目标“河流生源物质迁移转化及环境质量演变”具有重要的意义。

项目按计划执行，完成了各项研究内容，取得了预期研究成果，达到了预期目标。项目分别于2016年7－8月和2018年3－4月两次对澜沧江流域从上到下游干流、主要支流以及各梯级水库开展了系统的采样，供收集样品200多件，完成了现场观测、培养以及室内温室气体浓度、水化学、营养盐和同位素分析。依据表层浓度估算各水库CH4，CO2，N2O释放通量分别变化在25～800μmol/m2/d，-25～200mmol/m2/d，-0.15～15.4μmol/m2/d之间。项目同时也利用通量箱在野外现场观测了CO2和CH4的交换通量，分别为17.2±70.3 mmol/m2/d和1003±2099 umol/m2/d。CH4交换通量与CH4浓度变化完全一致，CO2交换通量主要受CO2分压以及pH控制。利用浓度估算的结果与利用通量箱直接观测的结果是可以对比的，表明数据是可信的。总体上，这些数值远较预期的要小，表明这些水库的温室气体释放不构成重要的环境问题。培养试验中，加入HgCl2，BES，CH3F以及CH2F2等不同抑制剂的样品在培养48小时后，CH4浓度与原始样品差异较小，表明可能水体中CH4的产率和氧化速率较低。.各水库温室气体浓度分布显示出一定的差异性，没有表现出明显的空间上的联系，温室气体主要受控于各水库内部的生物地球化学过程。同时，温室气体浓度变化与TN、TP之间存在较明显的联系，很多因素包括人为活动，水库内部的生物地球化学循环，水库的水动力条件等可能都会显著影响温室气体产生。CH4与DOC浓度表现出一致性变化，暗示出二者可能存在成因上的联系。水体CO2浓度与碳同位素表现出负相关，CO2产出的主要机理是有机质降解有关。从上游到下游，CO2浓度有增加的趋势，碳同位素降低的趋势更加明显。N2O的DO呈正相关，主要的产出机理为硝化作用，这与水体较高的氧浓度是一致的。