梯级水库关键界面氮动态对N2O排放的影响及因素识别

Environment effects of greenhouse gas emission due to river impouned by dams attracts global attention recently. Unprecedented numbers of hydroelectric development were to conduct in SW China, where the rivers commonly are high-nitrogen loading because of anthropogenic effects, while little is known on the effects of nitrogen dynamics on N2O emission in cascade developed river-reservoir systems, due to that the studies on these issues were scientifically and pratically important. Based on this, research on nitrogen cylces and key process in multi-interface in the cascade reservoirs will be carried out on presentive reservoirs in Wujiang river for evluating nitrogen release. By monitoring the flux, content and isotope composition of DIN and N2O especially in the water-sediment interface and water column of hypolimnion and epiliminon daily and seasonally, we compared these to water chemistry and the content and isotope composition of nitrogen, trying to indicate the nitrogen transfermation and transport in thermal stratification period and what make it happen. Also through this, we will reveal the key process in the water-sediment interface, gas-water interface and hypolimnion-eplimnion interface, and evaluate the releasing flux of N in canyon-reservoir in SW China. By the foundation, we want to do help in studying the regional nitrogen cycle and making a rules to eliminate the nitrogen balance.

河流筑坝导致水体温室气体排放的环境效应近年来在全球范围内引起持续广泛的关注，我国是水电开发大国且多数河流均具有高氮背景，然而目前对蓄水发电水库尤其是梯级开发水库中氮转化运移的动态过程及其对温室气体N2O释放影响的基础研究还十分薄弱，针对该问题的研究具有重要的科学意义和现实需求。本申请拟选取我国乌江干流梯级开发水库为研究对象，通过对不同水库沉积物-水、水-气和深水层-温跃层等三个关键界面沉积物、水体中氮形态、N2O/N2含量及其氮氧同位素的分析，结合水化学参数和水文资料，系统开展流域水库氮排放观测，明确水库中氮的界面过程及其影响因素，掌握水库多界面上氮运移和N2O源汇作用的规律及成因，探讨多界面氮的生物地球化学过程对N2O产生的影响及因素；并从水库氮收支角度评估N2O释放的总通量；研究成果将为评价河流梯级开发N2O排放的环境效应提供数据基础、研究范例和评估方法。

本课题开展了梯级开发河流-水库体系中关键界面的氮转化过程及其对N2O影响的针对性研究。选取了我国西南典型梯级开发河流及其水库群为研究对象，借助水化学、稳定同位素技术结合水文资料对水-气界面、沉积物-水界面、真光层、温跃层、深水层以及下泄水等水体N2O的产生机理、释放通量及影响因素进行系统研究。结果表明乌江中上游河流水库均是大气N2O的释放源，季节性温度分层引起的水环境参数包括温度、溶解氧、叶绿素等的分层导致了水库内部和河流-水库不同河段N2O显著的时空差异。N2O在从河流-河库过渡带-库区方向上逐渐升高，表明了水文条件，尤其是流速降低为水体中N2O的产生提供了适宜的环境，进而影响了河流-水库N2O的时空格局。水化学和同位素结果表明真光层和温跃层N2O的产生主要受控于硝化作用，而深水层至沉积物-水界面则受控于反硝化作用。分层期间乌江渡和洪家渡水库深水厌氧层的N2O含量水平有很大差异，同位素组成证明二者底部N2O主要受控于是否发生完全的反硝化作用，而深层水中的完全反硝化作用通过将N2O进一步还原为N2，可以成为水库去除N2O的重要的“汇”；同位素结果证明了非分层期间东风水库和洪家渡底部高含量的N2O是由界面上反硝化作用引起的。δ15N-N2O和δ18O-N2O及其择优占位同位素特征值SP在百花湖厌氧层的结果表明，底部剧烈的反硝化作用可以由SP值直接证明，是表征N2O产生机理的可靠工具。借助ΔN2O 和ΔN2发现非分层期间水库剖面具有显著偏高的N2O产率，同时温跃层和真光层产率高于深水层。各水库水-气界面N2O扩散通量与世界其他湖泊水库相比属于中等水平，但下泄水中释放的N2O不容忽视，可以达到水-气界面扩散通量的0.72到16.61倍。本课题的研究成果可以为评价我国梯级开发河流-水库体系N2O释放能力提供重要参考，而下泄水体较高的N2O释放通量与水库蓄水调节方式有着紧密关系，对于类似梯级开发河流来说，需要制定针对性措施以完善未来流域水环境管理和政策制定。