桉树种植对流域水循环及氮循环的作用机理及其长期影响研究

As one of the most important economic trees in the subtropical and tropical zone, the Eucalyptus has a large planting area in the Southeast, China. The effect of Eucalyptus plantation on hydrological and nitrogen cycle change in the hydrosphere and biosphere have become a research hotspot around the world. In this study, a typical watershed, the Zhuhai experimental station located in the hilly and mountainous area in the Southeast, China have been selected as the major study area to evaluate mechanism and long-term effect of Eucalyptus plantation on hydrological and nitrogen cycle change. Three major problems will be settled in this study: the mechanism of Eucalyptus plantation on hydrological cycle change, the mechanism of Eucalyptus plantation on nitrogen cycle change and the evaluation of its long term effect. To achieve our objects, the precipitation, soil water, groundwater and river water is treated as a continuum for both hydrological and hydrochemical analysis. Major tasks include: infiltration test for soils with different land covers to evaluate infiltration capability change; hydrological processes (precipitation amount, soil water potential, groundwater level and runoff) monitoring to make clear the hydrological cycle change; hydrochemical analysis for precipitation, soil water, groundwater and river water to study the spatial and temporal variation of nitrogen in different media and the nitrogen cycle change; analysis of δD and δ18O for water to discuss the change of groundwater recharge regime; measurement of nitrogen isotope to identify nitrogen sources, behavior and fate in both soil water and groundwater; CFCs analysis to trace groundwater flow pathways and evaluate groundwater ages. The main innovation point for this study is addressing the importance of groundwater flow system when consider the effect of Eucalyptus plantation on water and nitrogen cycle. By using the CFCs and other environmental traces, the historical influence of Eucalyptus plantation to hydrologcial and nitrogen cycle can be reconstructed. And the prediction of future change can also be evaluated.

桉树是我国华南地区种植面积最广的速丰经济树种，其对水循环及氮循环的影响是近年来的研究热点之一。本项目以华南地区桉树种植的山区丘陵为研究区，以大气降水-枯落物层水-土壤水-地下水-河水连续统一体为研究对象，利用水文观测、水化学及同位素解析等手段，阐述流域植被变化对水循环过程的影响机制，重点揭示土壤水运动过程、地下水补给时空变化规律以及地下水流动路径；以此为基础，研究按树种植对氮循环的作用机理，着重探讨氮在土壤水-地下水-地表水连续体中的迁移转化过程；结合地下水滞留时间评价，综合考察植被覆盖变化对水循环及氮循环的长期影响；最终建立以水循环为载体多示踪剂为手段从流域整体角度研究桉树种植对水氮循环作用机理及长期影响的科学体系，为流域水循环及水资源管理提供科学依据。

随着桉树人工林的逐渐增多，近年来桉树种植对流域水循环和物质循环逐渐引发关注。尤其是对区域水质和水化学特征的影响尤为突出。本项目通过流域对比法，对不同时空尺度水体、土壤、植被等进行了综合考察。结果表明，桉树种植后土壤透水系数在深度方向出现了很大的变化。这个变化不仅改变了流域产汇流特征，也改变了元素迁移速度和路径。土壤氮是桉树种植前原本储存的土壤中的氮可能性很大，且随着桉树生长逐渐生长成熟，土壤δ15N发生富集。碳氮同位素的差别表明，桉树人工林的种植在短期内不会对土壤有机碳造成明显影响，而氮在土壤中的转化与迁移比较活跃，土壤中无机氮大量向桉树体转移及在土壤环境中迁移。利用河流沉积柱样品和210Pb同位素定年技术重建了北江沉积物的沉积年代序列和沉积速率。结合δ15N变化，了解河流生态系统中不同时间尺度的养分来源和转化，解释河流中的氮循环特征，增进了对河流生态系统中不同时间尺度上的氮循环过程的理解。

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