滇池流域退耕还林对侵蚀条件下土壤氮素流失响应的示踪研究

China's grain-for-green policy of converting steep cultivated land to forest is one of the most important initiatives for ecological remediation. It is important and necessary to evaluate the benefits of reducing nitrogen losses associated with soil erosion after returning farmland to forest. Elucidating its mechanism will be of benefit to our understanding and provide scientific evidences for policy-making of regional nitrogen non-point pollution control and grain-for-green planning. In this study, Dianchi watershed is selected as the study area and isotopic tracers techniques (such as 137Cs, 210Pbex and 15N) are used. Based on vertical distributions of soil nitrogen, nitrogen losses during soil erosion and its relationship to grain-for-green programme are analyzed. Temporal and spatial variability of isotopic composition of sedimentary organic nitrogen is used to record changes of nitrogen source. The contribution of eroded soil to sediment nitrogen is explored. Impact mechanism of grain-for-green programme on nitrogen source change in sediments is studied. Benefits of reducing nitrogen losses caused by soil erosion after grain-for-green programme are quantitatively evaluated. A theory and method system for understanding responses to nitrogen losses associated with soil erosion of grain-for-green programme in Dianchi Watershed using isotopic tracers is constructed.

退耕还林工程是我国生态修复的重要举措，客观评估退耕还林对侵蚀条件下的土壤氮素流失控制效益，阐明其作用过程，不仅具有重要的理论意义，也为流域氮素面源污染控制和退耕还林工程规划提供科学决策依据。本研究选择滇池流域为研究靶区，利用同位素（137Cs、210Pbex）示踪和氮同位素技术手段，在获得土壤氮素垂直分布的基础上，定量估算土壤侵蚀过程中氮素流失量，分析侵蚀条件下土壤氮素流失量与退耕还林的对应关系；根据沉积物有机质氮同位素组成时空特征，示踪沉积物氮素来源变化，获得侵蚀条件下的源地土壤氮素流失相对贡献，揭示退耕还林对沉积物氮源的影响及过程机制；定量评估退耕还林侵蚀条件下土壤氮素流失控制效益，构建一套利用多同位素示踪研究退耕还林对侵蚀条件下土壤氮素流失响应的理论与方法体系。

滇池流域水土流失严重和水体富营养化。本研究选择位于滇池流域的宝象河小流域和斗南实施种植基地为研究靶区，利用同位素（137Cs、210Pbex）示踪和碳氮同位素技术手段，着重研究了土壤氮素垂直分布，土壤侵蚀过程中氮素流失量，土壤氮素流失量与退耕还林的对应关系；根据沉积物有机质碳氮同位素组成时空特征，示踪沉积物碳氮素来源，揭示了退耕还林对沉积物碳氮源的影响及过程机制；分析了宝象河小流域水库沉积物及斗南实施耕地多环芳烃的垂直分布特征及来源；从土壤侵蚀和碳氮磷角度评估了滇池流域退耕还林的生态环境效益。结果表明，耕地的侵蚀模数远远大于非耕地，植被在减缓土壤侵蚀方面具有显著作用。宝象河小流域土壤TOC含量变化表现为：林地＞耕地＞草地，而TN则表现为：耕地＞林地＞草地；斗南地区设施农地的土壤TOC、TN平均含量均显著高于林地和草地。在0-40cm土壤深度内，宝象河小流域土壤C、N每公顷储存量分别为114.07×10７kg和131.53×10６kg。以林地为比较基准，8年时间斗南设施农地土壤平均C、N的累积率平均分别为4.638Mg ha–1y–1 和1.113Mg ha–1y–1。设施农业是较好的碳氮储存方式，但土壤中氮的积累可能会对滇池带来潜在的环境威胁。土壤137Cs和210Pbex比活度与SOC、TN 均显著相关。宝象河小流域耕地土壤SOC、TN 流失量分别为6.21 和1.07 t km-2 year-1，草地分别为4.08和0.40 t km-2 y-1，林地分别为2.51和0.21 t km-2 y-1。耕地由于人为干扰强度大，土壤侵蚀程度最高，土壤中面源污染组分输出潜力较大。距离入库河口远近的不同，受人类活动影响的强度不同，导致沉积柱芯有机碳氮的来源不同。燃烧过程是土壤和沉积物中PAHs的主要来源，PAHs含量与TOC之间有显著正相关关系，TOC影响宝象河水库沉积物中PAHs的分布。滇池流域退耕还林工程能大幅度降低土壤侵蚀速率，退耕还林后土壤总氮和总磷含量明显低于耕地，进而降低水土流失带来的土壤营养元素流失量，将为滇池流域农业面源污染控制发挥重要作用，具有明显的生态环境效益。