水塘系统是我国传统的农田水利设施，在南方山区分布广泛。水塘的人工修建使得山区小流域径流中携带的氮素被截留于塘内，经微生物转化以气体排出。特别是在降雨径流进入水塘的初段，塘内发生短时而快速的氮转化过程，形成水塘中氮素去除的“热区”现象。然而由于水塘个体面积小，其生态功能与反应机理常被忽略。但是水塘数量巨大，其总面积超出大型湖泊百倍。水塘氮循环研究的不足，导致其对流域污染消减乃至全球氮交换的整体贡献可能被严重低估，成为全球氮通量核算中缺失的一环。本研究通过对山区水塘系统进行原位观测，对关键氮循环过程开展实验模拟，分析水塘系统氮素转化的时空变化规律与微生物机制；针对径流汇入水塘初期氮素微生物转化的“热区”现象，揭示这一非常态但有重要影响的生态过程的触发机制与反应路径。本项目对于阐明山区水塘系统对流域氮污染的消减作用与内在机理、水塘景观在小流域中的生态功能，乃至淡水生态系统氮通量核算具有重要意义。

The ponds system is traditional farmland water conservancy facility, which is widely distributed in southern China. The man-made ponds retain the runoff in the small mountain watershed. The nitrogen carried in runoff is thus transferred into nitrogenous gas by microorganisms and eliminated from the ecosystem. It is noted a short time and rapid transforming of nitrogen occurs in pond at the initial time when the runoff get in, which is called a “hot moment”. Due to the relatively small individual area, the ecological function and reaction mechanism are apt to be neglected. But the total area of ponds is 100 time more than lake and river globally. Therefore, the lack of research on the nitrogen cycling in ponds may lead to a serious underestimation of the overall contribution in watershed pollution control and global nitrogen exchange. In this study, in situ observations and experimental simulation are set up targeting the ponds in small mountain watershed. Temporal and spatial variation of nitrogen transformation and microbial mechanism in pond system are investigated. Moreover, we aim to reveals the trigger mechanism and reaction pathway of the “hot moments” in nitrogen cycling which is unusual but potentially important ecological process. This project is of great significance for clarifying the pollution depletion and internal mechanism of the ponds system, the ecological function of the pond landscape in the small watershed, and even the estimation of nitrogen flux of the freshwater ecosystem.