Response of soil fungal communities and their co-occurrence patterns to grazing exclusion in different grassland types

Overgrazing and climate change are the main causes of grassland degradation, and grazing exclusion is one of the most common measures for restoring degraded grasslands worldwide. Soil fungi can respond rapidly to environmental stresses, but the response of different grassland types to grazing control has not been uniformly determined. Three grassland types (temperate desert, temperate steppe grassland, and mountain meadow) that were closed for grazing exclusion for nine years were used to study the effects of grazing exclusion on soil nutrients as well as fungal community structure in the three grassland types. The results showed that (1) in the 0–5 cm soil layer, grazing exclusion significantly affected the soil water content of the three grassland types (P<0.05), and the pH, total phosphorous (TP) and nitrogen-to-phosphorous ratio (N/P) changed significantly in all three grassland types (P<0.05). Significant changes in soil nutrients in the 5–10 cm soil layer after grazing exclusion occurred in the mountain meadow grasslands (P<0.05), but not in the temperate desert and temperate steppe grasslands. (2) For the different grassland types, Archaeorhizomycetes was most abundant in the montane meadows, and Dothideomycetes was most abundant in the temperate desert grasslands and was significantly more abundant than in the remaining two grassland types (P<0.05). Grazing exclusion let to insignificant changes in the dominant soil fungal phyla and in α diversity but significant changes in the β diversity of soil fungi (P<0.05). (3) Grazing exclusion areas have higher mean clustering coefficients and modularity classes than grazing areas. In particular, the highest modularity class is found in temperate steppe grassland grazing exclusion areas. (4) We also found that pH is the main driving factor affecting soil fungal community structure, that plant coverage is a key environmental factor affecting soil community composition, and that grazing exclusion indirectly affects soil fungal communities by affecting soil nutrients. The above results suggest that grazing exclusion may regulate microbial ecological processes by changing the soil fungal β diversity in the three grassland types. Grazing exclusion is not conducive to the recovery of soil nutrients in areas with mountain meadow but improves the stability of soil fungi in temperate steppe grassland. Therefore, the type of degraded grassland should be considered when formulating suitable restoration programmes when grazing exclusion measures are implemented. The results of this study provide new insights into the response of soil fungal communities to grazing exclusion, providing a theoretical basis for the management of degraded grassland restoration.

过度放牧与气候变化是草地退化的两大主要诱因，而禁牧是全球范围内修复退化草地最常用的措施之一。土壤真菌可对环境胁迫做出快速响应，但不同草地类型对禁牧的响应尚未形成统一定论。本研究选取了实施9年禁牧的三类草地（温带荒漠、温带草原草地与山地草甸），探究禁牧对这三类草地土壤养分及土壤真菌群落结构的影响。研究结果显示：（1）在0~5 cm土层中，禁牧显著影响三类草地的土壤含水量（P<0.05）；三类草地的pH值、总磷（TP）以及氮磷比（N/P）均发生显著变化（P<0.05）。在5~10 cm土层中，禁牧后仅山地草甸草地的土壤养分出现显著变化（P<0.05），温带荒漠与温带草原草地的土壤养分无显著改变。（2）针对不同草地类型，古根霉菌纲（Archaeorhizomycetes）在山地草甸中丰度最高；座囊菌纲（Dothideomycetes）在温带荒漠草地中丰度最高，且显著高于另外两类草地（P<0.05）。禁牧并未显著改变土壤真菌优势门的组成及α多样性，但对土壤真菌的β多样性产生了显著影响（P<0.05）。（3）禁牧区域的平均聚类系数与模块化等级均高于放牧区域，其中温带草原草地禁牧区域的模块化等级最高。（4）本研究还发现，pH值是影响土壤真菌群落结构的主要驱动因子，植被覆盖度是影响土壤群落组成的关键环境因子；禁牧可通过改变土壤养分间接作用于土壤真菌群落。上述结果表明，禁牧或可通过改变三类草地的土壤真菌β多样性调控微生物生态过程。禁牧不利于山地草甸区域的土壤养分恢复，但可提升温带草原草地的土壤真菌群落稳定性。因此，在实施禁牧措施、制定适配的修复方案时，需考虑退化草地的类型差异。本研究结果为土壤真菌群落对禁牧的响应机制提供了新的认知，也为退化草地修复管理提供了理论依据。