复杂胶农复合系统内植物水分和养分的竞争与共存机制

Despite the rubber agroforestry system seems to provide a sustainable development way to ensure regional economic growth and simultaneously resolve the negative effects caused by rubber monoculture, the selection of intercropping plants and the design of intercropping patterns in the actual reconstruction and construction of rubber agroforestry system still exhibited some blindness, especially exhibited the limited understanding about the interspecific competition for belowground resources which negatively impacts the ecological and economic benefits of such intercropping system. However, our current understanding of the mechanisms that underlie plant interspecific competition for belowground resources is still limited because of the complexity of resource competition, and most of previous studies ignored the mechanisms of plant coexistence that emerge only in diverse competitive networks. Despite the potential for these mechanisms to create conditions under which the loss of one competitor triggers the loss of others, we still lack the knowledge needed to judge their importance for coexistence in competitive networks. For above reasons, this project would use stable isotope methods (D、18O、13C、15N) to investigate plant water and nutrient sources, use efficiency and their spatio-temporal variations in two-species community (rubber & tea, rubber & orange, rubber & devilpepper and rubber & fructus amomi agroforestry systems), in three-species community (rubber & tea & orange, rubber & devilpepper & fructus amomi agroforestry systems), in multi-species community (Jungle rubber), and in rubber monoculture and tropical rainforest which are designed as controls. We would like to use bayesian linear mixed models (MixSIAR) to analyse plant water (through δD and δ18O) and nutrient (through δ15N) use patterns; use exponentional piston model and dispersion model to analyse soil water residence time, transport and transformation mechanisms; use general linear model to analyse plant water use efficiency (through δ13C), drought and competition adaptation strategies; use correlation network (through carbon, nitrogen and phosphorus concentration) to analyse plant nutrient allocation and translocation, and nutrient cycling in each community. Therefore, to explore the relationship among community structure, plant water and nutrient use strategies, and the ecological hydrological environment. The ultimate purpose of this research is to explain how higher-order interactions maintain plant coexistence in competitive networks. Meanwhile, we will explore the ecological intercropping patterns which also show high production, and thus to provide scientific theoretical basis for the designment of sustainable rubber agroforestry system.

由于对植物地下界面种间关系的认识不足，人们对胶农复合系统内间作植物的选择与配置仍存在较大的盲目性，还需借鉴多物种持久共存的植物群落组成结构，并通过研究群落内植物的资源利用策略来了解竞争网络中物种共存的机制。因此，本项目采用稳定同位素（D、18O、13C、15N）等技术，以两层两种（橡胶-茶叶、橡胶-橘子、橡胶-萝芙木、橡胶-砂仁等）、三层三种（橡胶-茶叶-橘子、橡胶-萝芙木-砂仁等）、多层多种的胶农复合系统为研究对象，以纯橡胶林和热带雨林为对照，对各群落系统内植物的水分和养分利用模式、利用效率及其时空变化规律进行研究，同时分析土壤水分的驻留时间、运移和转换规律，阐明群落结构从简单到复杂变化过程中植物对水分和养分利用策略的调整过程，及其与群落生态水文环境之间的关系，揭示高阶交互作用下植物在水分和养分竞争中的共存机制，为改造大面积的纯橡胶林、构建可持续利用的生态胶园种植模式提供科学依据。

在橡胶种植区域推广橡胶农林复合系统有望协调区域的经济发展和生态环境保护之间的矛盾。然而，由于人们对橡胶农林复合系统内植物种间竞争机制的认识仍然有限，间作植物的选择以及植物资源竞争的管理成为了当前农林复合系统建设与推广的挑战。为了解决这些问题，本项目主要采用了稳定同位素的手段结合养分诊断的方法在西双版纳地区的多种橡胶农林复合系统内进行了植物地下资源竞争的研究。结果表明:（1）在植物群落由简单到复杂的变化过程中，土壤碳氮含量增加、矿物养分含量降低、水分驻留时间缩短、土壤含水量先增后减。其中，乔木、灌木、草本植物对土壤水分和养分的影响各不相同，有时甚至出现相反的结果。（2）植物竞争的优势和劣势总是随着环境的时空异质性而发生变化，竞争优势的逆转因此在群落的物种间形成了一个非传递性的竞争循环，从而限制了植物对地下资源的利用深度，避免了竞争排斥。且物种组成越多，植物资源利用范围越精细，种间竞争优势越小，物种共存越稳定。因此，多物种共存的橡胶复合系统是可以实现的。（3）橡胶树与草本植物的水分利用模式存在着互补的现象，可极大地减弱植物种间的水分竞争。然而，复合种植模式下橡胶树的水分利用效率并未得到显著的增强，且间作草本植物对橡胶树的氮吸收有较大的影响。因此，复合系统中应尽量避免选择需大片密集种植的草本植物。（4）种间竞争会促进植物的氮磷养分从枝条向根系转移，导致植物叶片氮磷养分供应的不足，并由此使得凋落物的初始质量随着竞争强度的增加而下降，进而减弱凋落物的养分释放并加剧土壤养分的损耗，从而深刻地影响复合系统内的养分循环过程。从植物组织的养分状态对竞争强度变化的反应来看，橡胶树根系的养分反应最为明显，可用于竞争强度的诊断。（5）将单层橡胶林转化为丛林式橡胶林后，土壤理化性质得到了较大的改善，尤其是土壤磷的周转效率。因此，构建丛林式橡胶林将是恢复由橡胶单一种植所造成的土壤退化的有效途径。