太湖上游茶园坡地氮磷流失过程及其对植被盖度变化的响应机制

The pruning of tea plants significantly reduces vegetation cover, exposing large areas of soil surface to rainfall and thus significantly changing the surface evapotranspiration and canopy interception. This can potentially trigger a series of changes in the pathways and processes of nitrogen (N) and phosphorus (P) losses. So far, however, little attention has been given to the mechanisms behind soil nitrogen & phosphorus losses in response to vegetation cover changes. In this study, a tea plantation, located in the upstream region of the Taihu Lake, is chosen as our research area. The gradient of vegetation cover of the tea hillslope will be obtained by using different pruning techniques. Long-term dynamics of N & P losses, as well as factors associated with crop growth, soil nutrient cycling and soil water in the plots with different vegetation cover will be monitored to analyze the characteristics of the temporal and spatial distribution of soil N & P contents, surface and subsurface runoff, and N & P loss processes. Furthermore, both the hot-spot areas of soil N & P content and soil water movement will be identified, and the pathway and the flux of N & P losses will be investigated. A comprehensive analysis of the sources, pathway, and transport processes of N & P losses in response to vegetation cover changes will be proposed; and in order to better understand the roles of vegetation cover, a transport model will be constructed to simulate the processes of N & P losses after the reduction of vegetation cover. Overall, the implementation of this proposal is expected to provide a theoretical basis for pollution simulation. It can also offer some important suggestions for improving the ability of water conservation, reducing nutrient losses, and formulating economic development policies that explicitly consider the benefits of environmental protection for the hilly area.

坡地茶园雨季修剪造成连片的地表裸露，导致地表蒸散、冠层截留等一系列变化，引发氮磷流失途径和流失过程的巨大改变。然而目前，人工干扰下植被盖度变化对茶园坡地氮磷流失的影响机制还有待进一步揭示。本项目拟选取太湖上游分布广泛的茶园坡地为研究对象，开展降雨-径流小区定位试验。通过茶园修剪方式的不同来设置植被盖度梯度，连续观测坡地作物生长、氮磷循环和土壤水分动态，分析土壤氮磷时空格局、坡面产流及氮磷流失特征，探明坡地氮磷富集热点区，辨识土壤水分运移活跃区和时段，明确氮磷流失通量及主要途径，综合揭示植被盖度对源强、产流和流失过程的影响机制；而后以此为基础，通过坡面尺度氮磷运移的过程模拟，探究植被盖度骤变下氮磷流失的过程效应，进一步揭示茶园氮磷流失的过程机理。本研究的开展不仅可为以茶园为代表植被盖度多变的坡地养分流失模拟提供基础信息，还可为丘陵山区非点源污染控制提供科学依据。

坡地茶园快速扩张驱动大量的化肥农资输入，并造成连片的地表裸露，引发丘陵水源涵养区面源氮磷的大量流失，并显著威胁下游河流水质。然而目前，茶园坡地水分输送过程和氮磷流失机制还有待进一步揭示。本项目拟选取太湖上游丘陵区分布广泛的茶园坡地为研究对象，依托天目湖流域生态观测研究试验站，开展降雨-径流小区定位试验。研究结果发现1）太湖上游丘陵区茶园种植密度一般可达为4万株/公顷，行间距约为1.5m。成年茶园植被盖度整体良好，但雨季来临前，茶园修剪和陡坡种植导致的地表裸露是水土流失和面源污染发生的重要风险窗口期；2）受高强度降雨和茶园土壤易渗漏的影响，茶园坡地土壤含水量波动大，水分侧向输送明显，地表径流是磷流失，壤中流是氮流失的重要途径；3）茶园坡地氮磷流失表现出了明显的初始冲刷效应，大量的面源污染物的流失主要发生在降雨初期，氮磷分别主要以硝态氮和颗粒态的形式流失；4）构建的模拟模型结果表明，当太湖上游天目湖小流域以茶园为代表的农业用地扩张面积达到流域面积的15％时，氮的流失通量增加44％，而磷流失通量将增加50％。在未来的管理中应加强土地利用管控并加强茶园耕作的管理以减少面源氮磷流失。