高寒草甸不同功能群土壤过程的差异性及其对禁牧与养分添加的响应

Grazing exclusion and nutrient addition have been recommended as two of major measures for restoring degraded grasslands in the Tibetan Plateau. However, current results on the effects of grazing exclusion and fertilizations on soil quality are unclear: they may increase, decrease, or not affect soil organic matter pool. We propose that for clarifying the effects of grazing exclusion and fertilization on soil quality, it is a key solution to distinguish differential influences on soil process between different plant species and their responses to grazing exclusion and fertilization. This is because grazing exclusion and fertilization usually induce changes in plant community composition. By reviewing the literature, it is clear that current studies on plant-soil processes mainly focus in the community level, and not target the differential effects of plant species on soil process. Therefore, in the present proposal, by using single-population quadrates and pot experiments, we intend to investigate: (i) differences in above- and below-ground biomass allocation, root biomass and production, and rhizodeposition and their responses to grazing exclusion and fertilization; (ii) differences in the size and composition of soil organic matter pool and their responses to grazing exclusion and fertilization; and (iii) soil organic carbon mineralization and nutrient transformations and their responses to grazing exclusion and fertilization, under different functional group plants. Our ultimate objective is to clarify mechanisms of the effects of grazing exclusion and fertilization on soil quality on the Qinghai-Tibetan Plateau.

禁牧和施肥一般被推荐为青藏高原防治草地退化和提高草地生产力的主要措施, 但是关于禁牧和施肥对对土壤质量影响的研究结果是模糊的。我们认为要厘清禁牧和施肥对土壤质量影响的关键是需要明确不同植物种类对土壤过程影响的差异性及其对禁牧和养分添加的响应。这是因为禁牧和施肥常常导致植物群落组成发生变化。而目前在青藏高原关于禁牧和施肥对土壤过程影响的研究都是在群落层次，关于土壤过程的种间差异性及这些差异性对禁牧和养分添加的响应还未见有研究。本项目拟建立单一种群的田间样方和盆栽实验，研究(i)不同植物生物量分配、根生物量及产量、根际沉积的差异性及其对禁牧和施肥的响应；(ii)不同植物对土壤有机质含量及组成影响的差异性及其对禁牧和施肥的响应；(iii) 不同植物对土壤有机碳和养分矿化过程及土壤微生物活性影响的差异性及其对禁牧和施肥的响应。最终目的是帮助从种群层次说明禁牧和养分添加对高寒草甸土壤质量影响的机理。

以往研究中，草地利用（禁牧、施肥、开垦等）以及全球变化对土壤质量影响的研究结果模糊不清。主要原因是没有考虑这些人为及自然因素所导致的植物群落组成的变化对土壤性质的影响。放牧、施肥及气候变化常常导致草地群落组成发生变化。本项目主要研究群落和种群对土壤过程影响的差异性，取得的主要进展为：（1）明确了青藏高原植物种对土壤性质的影响大于草地的施肥和放牧(刈割)的效应。（2）在群落水平，莎草科植物比禾本科植物向根系和土壤（通过根际沉积）分配更多的光合产物，说明高寒草地莎草科群落被禾本科群落替代会降低植物向土壤中碳的输入及固定。（3）在种群水平，作为青藏高原分布最广的两个植物种，矮嵩草比垂穗披碱草将更多的光合产物分配到地下（根系和土壤）；施肥显著提高了矮嵩草和垂穗披碱草的光合同化效率和根际沉积量。（4）莎草科植物残体在土壤中的分解速率慢于禾本科植物残体，莎草科植物的光合产物周转速率小于禾本科植物。（5）坡向—群落组合极其显著改变土壤生态过程（碳氮磷的输入输出、转化及其储量）。（6）群落组成改变及土壤扰动显著降低土壤中微生物合成物质的积累，影响稳定土壤有机质的形成。项目首次明确了青藏高原草地群落组成对土壤过程影响的重要性，群落组成对土壤过程的影响大于管理措施和环境因子。在评估放牧管理、施肥、全球变化对土壤过程的影响时必须考虑这些驱动因子对植被组成的影响。气候变化、氮沉降及施肥所导致的莎草科植物的优势度被禾本科植物替代增加土壤碳周转速率。项目培养博士研究生3名、硕士研究生1名，在SCI期刊发表研究论文5篇，部分结果正在整理待发表。