基于氮氧双同位素解析大气沉降氮在典型水源林的迁移转化过程

Atmospheric nitrogen deposition has become an important part of water pollution. Studying the migration and transformation process of nitrogen deposition and its contribution to the surface non-point source pollution can help clarity nitrogen migration transformation mechanism, discriminate the pollutant sources and prevent the non-point source pollution. A small watershed, located in the upstream of Fushi reservoir which is the source of Taihu Lake, is chosen to be the research area for this project. Within the study area, three typical types of water conservation forests (bamboo, fir, evergreen broad-leaved forest) are chosen as the research objects. Meteorological factors, runoff and sediment，soil nutrient variation and vegetation growth status are monitored to analyze the interception effects of different forests on the nitrogen deposition and confirm the content and morphological characteristics of nitrogen on the surface runoff during the rainfall-runoff processes. Considering the advantages of nitrogen and oxygen isotope tracer technique on the composition and source identification, it will be applied to clarify the influences of nitrogen deposition on the surface water eutrophication, thus to reveal the different responses of woodland output nitrogen, including source, content and form (Ammonia nitrogen, Nitrate nitrogen, Dissolved organic nitrogen, etc.), to the meteorological factors, soil type, vegetation type and so on. This study can help provide scientific references for the regional ecological safety risk assessment and regional non-point source pollution prevention.

大气氮沉降已成为水体外源污染的重要组成部分。研究降雨-径流过程中沉降氮素在不同水源涵养林的迁移转化过程及其对地表面源污染的贡献，对于明晰氮素迁移转化规律，辨析地表氮素来源、防控水源区面源污染等具有重要意义。本项目以太湖源头赋石水库上游小流域为研究区，选取区域典型水源涵养林（毛竹林、杉木林、常绿阔叶林）作为研究对象，通过对气象因子、径流泥沙、土壤养分变化以及植被生长状况的定位监测，分析降雨-径流过程中不同水源涵养林对沉降氮素的截留分配作用，阐明各林地地表径流中输出态氮素的含量与形态特征，探讨林地类型对沉降氮素迁移的影响；并结合氮氧同位素示踪技术在成分和来源识别中的优势，分析氮沉降对地表水体富营养化的影响，揭示各林地输出态氮的来源、含量及形态（氨氮、硝氮、溶解性有机氮等）对气象因子、土壤特性、植被类型等的响应，从而为区域生态安全风险评估及区域面源污染防控等提供依据。

本项目聚焦沉降污染物在不同水源林的迁移转化过程及其对地表面源污染的影响，以区域典型水源林（毛竹林、杉木林、青冈阔叶林）为主要研究对象，基于野外定位观测，分析沉降污染物的时空动态及其在各林地的迁移过程，阐明水源林在截留净化沉降污染中的关键作用；在此基础上，结合稳定同位素技术在成分和来源识别中的优势，对地表硝酸盐来源进行分析，并采用贝叶斯混合模型定量分析各污染源的贡献比例，阐明不同水源林地表氮负荷来源及其影响，为区域生态安全风险评估及区域面源污染防控提供依据。本研究发现：（1）研究区大气降水中氮磷含量较高，且氮沉降主要以铵态氮形式存在，NH4+-N和NO3--N浓度之比为1.74:1，区域氮沉降受农业活动的影响较大；（2）森林群落对沉降污染物有明显的截留分配作用，其中，林冠层对TP和NH4+-N均表现出明显的截留作用，截留率高达84.68%；枯落物层则是林地氮、磷的主要释放源，对NH4+-N、NO3--N、TP和SO42-具有明显的淋溶作用；（3）基于输入（降雨）-输出（径流）过程的分析发现，林地输出态硝酸盐浓度变化趋势与大气氮沉降变化趋势相同，大气沉降对地表水质具有一定影响。此外，基于氮氧稳定同位素分析可知，大气氮沉降对地表硝酸盐的贡献度可高达35.08%，在农业源区，大气沉降氮对水体面源污染的影响不容忽视。