种间配置对胶农（林）复合系统水土保持功能的影响

An important driver of land-use/land-cover change in Xishuangbanna has been the transformation of large-scale tropical seasonal rain forests to monocultural rubber plantations, which induced the earth suffered excessive cultivation and management, significant water loss and soil erosion, soil hardening and even soil water gradual depletion in watershed scale. In this study, based on some selected small closed watersheds, we attempt to comparatively evaluate the effect of different intercropping patterns on soil and water conservation functions in rubber agroforestry systems in the Xishuangbanna region, by using the stable isotope technique (stable hydrogen and oxygen isotope composition), and the method of comparative measurements in the paired watersheds and surface runoff plots. The experiment consisted of different intercropping systems (polyculture systems) that were thought to be economically viable in this region and, at the same time, ecologically sustainable. These selected experimental rubber plantations have been intercropped with cacao, coffee, pineapple, grapefruit, rauwolfia and cinchona, macrophylla and cambodiana, thyrsoideum, oxyphylla, etc., respectively. Monocultural rubber plantations serve as control plots in this study. Through measuring and observing the partitioning of gross rainfall on the forest canopy, water-holding capacity of the forest floor litter layer, soil water infiltration, water repellency on the surface soil, soil erosion and splash, soil water retention, rainfall-runoff generation and response (including the temporal variations of event water, pre-event water, surface runoff, groundwater runoff, soil water interflow and preferential flow), mean residence times of stream water at base flow and mean soil water residence times at different depth layers in the experimental watersheds, we will explore the regulating and controlling mechanisms of rubber plantations on water loss and soil erosion under different intercropping patterns. Meanwhile, we will analyse the differences in soil and water conservation functions between the rubber agroforestry systems and the monocultural rubber plantations. The objective of this study is to select some suitable intercropping models among the established rubber agroforestry systems in this region, which would provide greater capability in soil and water conservation and runoff regulation. Our results should be offering some important references for the local rubber planters, smallholders and state rubber farms, not only on the optimal allocation of cropping models, but also on the reduction of water loss and soil erosion in the watershed scale in the xishuangbanna region.

针对西双版纳地区大面积单层橡胶林所导致的显著水土流失、土壤板结缺水、水源逐渐枯竭的现状，以该地区近年来构建的橡胶-大叶千斤拔-龙血树、橡胶-萝芙木-金鸡纳、橡胶-可可、橡胶-益智、橡胶-咖啡、橡胶-菠萝、橡胶-柚子、橡胶-砂仁等典型胶农复合系统为研究对象，基于选取的多个闭合小流域，采用稳定同位素技术、集水区和地表径流场对照观测等方法，研究不同胶农复合系统的水土保持功能。通过对其林冠分配降水、枯落物截留、土壤水分入渗、土壤侵蚀和斥水性、土壤水分动态和蓄水能力、降雨-径流的形成机制、小流域内土壤水和溪水的存留时间和转换规律等方面的对比实验，揭示复合系统的种间配置对水土流失的调控机制及其与单层橡胶林的差别，明确不同胶农复合系统的涵养水源、减少水土流失的能力，并遴选一些固土保水性良好的复合胶林模式。研究结果为改造现存的大面积单层橡胶林、改善胶园环境、构建可持续发展的生态胶园模式，提供科学参考。

本项目以该地区近年来构建的橡胶-大叶千斤拔、橡胶-茶叶、橡胶-可可、橡胶-咖啡、橡胶-菠萝等典型胶农复合系统为研究对象，采用稳定同位素（δD、δ18O）技术、集水区和地表径流场对照观测等方法，研究不同胶农复合系统的水土保持功能。通过对其林冠分配降水、枯落物截留、土壤水分入渗、土壤侵蚀和斥水性、土壤水分动态和蓄水能力、降雨-径流的形成机制、小流域溪水的存留时间和转换规律等方面的对比实验，揭示复合系统的种间配置对水土流失的调控机制及其与单层橡胶林的差别。初步研究结果表明：1）与纯橡胶林相比，橡胶复合系统的林冠截留降水能力都得到了提高，其中以橡胶-茶叶复合模式的截留能力最大；2）纯橡胶林改造为复合橡胶林之后，土壤有机质、全氮含量、孔隙度、含水量、导水率显著增加，土壤容重降低；纯橡胶林增加了土壤斥水性，斥水指数比其他胶农复合系统显著增大；3）相比纯橡胶林，橡胶复合系统土壤团聚体稳定性指标重量直径（WMD）、几何直径（GMD）、分形维数D值则减小，土壤团聚体更加稳定、土壤质量得到了明显的改良；4）与纯橡胶林相比，各橡胶复合模式显著地削弱了土壤溅蚀力（橡胶-橘子除外），其中以橡胶-茶叶、橡胶-大叶千斤拔效果较好；复合橡胶林下的土壤溅蚀力显著地受控于间作植物的冠层结构；5）橡胶林地表枯落物在化解雨滴溅蚀力、消减水土流失的过程中发挥着重要作用，橡胶复合系统在水土保持方面远优于纯橡胶林，而林下枯落物在这个过程中扮演着极其重要的角色，间作植物的作用则相对较小；枯落物对土壤流失的调控能力明显强于对地表径流的消减能力；分析发现，随着地表覆被层次的增多，雨水的直接侵蚀作用变得愈趋不明显；因此在橡胶林经营管理、友好型胶园营造过程中，需注重对枯落物的有效保护和地表覆被层次的适时添补，尤其是台地内侧的排水沟部位；6）干季末期和雨季中期，热带雨林集水区溪流水中当场雨占的比率比较低（29%~31%），而纯橡胶林集水区内溪流水中当场雨占了较高的比率（62%~69%），热带雨林和纯橡胶林集水区内雨水的平均存留时间分别为0.9年和0.5年，表明热带雨林延缓雨水流失的效应远大于橡胶林；7）虽然橡胶-大叶千斤拔、橡胶-茶叶复合系统内的植物存在一定的种间水分竞争，但并没有对植物间的水分有效获取产生不利影响，相反加强了橡胶树的水分利用效率（WUE）、养分吸收，同时也显著地增强了土壤的保水能力。