滇西北高寒草甸植物—土壤系统退化阈值量化及退化等级辨识

To diagnose the degradation level is the precondition for the restoration of grassland ecosystems. Quantifying degradation thresholds of the coupled plant and soil system is the key to determining the degradation level of grassland ecosystems. Currently, identification of grassland degradation bases on the characteristics of the plant community. It is a choke point to divide degradation levels of grassland ecosystems according to a coupled degradation thresholds of plant community and soil systems. A field survey, in this project, that using the standard methodology of grassland science will be conducted to investigate the structure and productivity of plant communities of alpine meadows in northwestern Yunnan province. Meanwhile, soil microbial activity and soil physicochemical properties of the alpine meadow will be analyzed as well. Basing on these data, a synthetic index of plant community, soil microbial activity and soil physicochemical properties will be constructed, respectively. The Mann-Kendall test will be used to identify abrupt thresholds of these synthetic indexes. And these thresholds will be coupled to diagnose degradation levels of meadow ecosystems. Based on the identification of degradation levels, the structural equation modeling (SEM) will be applied to analyze the shift of the relationships between plant communities and soil systems along the degradation gradient to reveal the mechanism of meadow degradation. This project will build a ‘plant community-soil microbe-soil physicochemical property’ framework to diagnose the degree of meadow degradation, shifting the identification of meadow degradation from a plant community dimension to a plant community and soil system coupled dimension. At the same time, it will provide a scientific basis for the restoration of alpine meadow ecosystems in northwestern Yunnan province.

明确草地退化等级是开展草地退化治理的前提，而划分草地退化等级的关键在于确定植物—土壤系统的退化阈值。目前主要依据植物群落特征判断草地退化程度，如何按植物—土壤系统退化阈值划分退化等级是草地退化评价研究中的瓶颈问题。本研究选取滇西北高寒草甸为研究对象，通过野外调查和实验室分析获取草甸植物群落、土壤微生物活性和土壤理化性状数据，构建植物、土壤微生物、土壤三个子系统的状态指标，用Mann-Kendall突变分析确定三者的退化阈值，通过植物—土壤系统复合的退化阈值判定草甸退化等级，并用结构方程模型（SEM）分析退化梯度上草甸植物—土壤系统各要素耦合关系的变化过程以揭示草甸退化机制。本研究尝试构建以“植物—土壤微生物—土壤”为框架、按系统退化阈值划分退化等级的方法，实现草甸退化阈值识别和退化等级划分从植物群落单一维度到植物—土壤系统整体维度的转变，为滇西北高寒草甸的退化治理提供理论参考。

近年来，气候暖干化、不合理的旅游开发、超载放牧等因素导致滇西北的高寒草甸呈现明显的退化趋势。为识别滇西北高寒草甸生态系统的退化阈值从而明确系统所处的退化阶段，本项目以香格里拉市受游客践踏和车辆碾压严重的典型高寒草甸为研究对象，以网格法确定调查样地、开展植物群落调查和土壤样品采集。基于野外调查和室内实验获取的数据，分析植物群落结构指标、植物群落功能指标、土壤理化性质指标以及土壤微生物活性指标的变化特征。采用熵权法构建植物群落综合状态指数（Vegetation status index，VSI）、土壤理化性质综合指数（Soil Synthesis Index，SSI）和土壤微生物活性指数（Soil microbial activity index，SMAI），然后基于地理编码和Mann-Kendall突变检验识别VSI、SSI以及SMAI的变化阈值，并根据这些退化阈值划分研究区的草甸退化阶段。研究结果表明：1）随着干扰强度增加，草甸植物群落的盖度、平均高度、物种丰富度均逐渐下降，优良牧草综合优势度、高度和盖度变异系数呈小幅上升趋势，均匀度指数呈小幅下降趋势，植物群落的地上生物量呈现显著下降趋势，VSI的变化阈值分别为0.54、0.59、0.61和0.67；2）除土壤硝态氮含量随干扰强度增加外，其他大部分土壤理化性质对外界干扰有较好的抵抗力，SSI的变化阈值分别为0.31、0.38、0.41和0.48；3）随着干扰强度增加，草甸土壤微生物量碳、微生物量氮、微生物量磷、脲酶活性、蛋白酶活性、蔗糖酶活性均呈上升趋势；酸性磷酸酶活性呈下降趋势；SMAI呈增加趋势，其变化阈值为0.53、0.61和0.67；4）整合VSI、SSI和SMAI的变化阈值，可将研究区内高寒草甸退化类型划分为“基本退化类型”和“过渡退化类型”两大类；5）研究区高寒草甸植物—土壤耦合关系在恢复序列上的变化过程比在退化序列上的复杂。本研究建立的高寒草甸退化阶段划分方法完善了当前草地退化阶段划分的理论体系，对退化草地的精准治理具有参考价值。