4.1 | Changes in soil microbial biomass in response to grazing
In addition to topographic limitations, nutrient consumption caused by the mass proliferation of soil microorganisms under the influence of grazing stimulation is also an important reason for the decrease in soil nutrient content. Our results showed that the decrease in soil nutrient content was accompanied by an increase in soil microorganisms MBC and MBN at the same grazing intensity (FIG. 2), which also proved our second hypothesis that soil microorganisms may be affected by the stimulation effect, making MBC and MBN a grazing response opposite to that of soil factors. Soil microorganisms are extremely sensitive and important in the soil, and grazing disturbs the original stable environment of soil microorganisms. Studies have shown that grazing will promote the formation of microbial communities dominated by bacteria with rapid growth and high nutrient consumption, thus accelerating the conversion of soil carbon and the release of CO2. In other words, grazing will accelerate the decomposition and utilisation of soil organic matter by micro-organisms, which is called the grazing pair The stimulating effect of soil micro-organisms, in which soil water content, C/N ratio and soil organic matter content are the most important regulating factors(Li et al., 2023; Xun et al., 2018). In this study, it was observed that the surface soil water content increased significantly with the increase in grazing intensity, and the soil water content was the highest in the HG treatment. A similar phenomenon was also observed in desert steppe grazing experiment(Wang et al., 2012) .This increase in surface soil moisture content was mainly regulated by growing season precipitation rather than grazing(Gan Lei et al., 2015). Due to the limited environmental conditions of sloping land, the soil moisture content is already low, coupled with the strong solar radiation received, resulting in high surface soil evaporation. After the start of grazing, the trampling and excretion of the animals increases the compactness of the soil, making it easier to retain rainfall. In addition, the vegetation on the surface forms a protective cushion through trampling, which blocks some of the radiation evaporation. At the same time, the water content of the surface soil increases with increasing grazing intensity because the surface layer is less affected by root water uptake(Gan et al., 2012; Gan Lei et al., 2015). The surface soil is the most active part of microorganisms, and the improvement of water conditions is more conducive to the mass reproduction of microorganisms. Meanwhile, soil organic matter as a carbon source for microbial growth is the most important regulatory factor, and nitrogen is an essential element for microbial protein synthesis, which directly participates in microbial growth. Therefore, soil microorganisms are regulated by the C/N ratio(Wang Zexi et al., 2019). This also supports our observation results: TN, SOM, pH and SOM jointly affect the MBC and MBN of soil microorganisms, SOM is the most important regulatory factor, and TN also plays a regulatory role in soil microorganisms, which is also confirmed by the structural equation model analysis results (Figure 3). TN and SOM provide the necessary elements and energy for microbial growth, and the decrease in pH caused by grazing (Figure 1(b)) is more conducive to the metabolic accumulation of microorganisms, resulting in an increase in soil microbial biomass and a decrease in soil nutrient content. At the same time, the observed transient increase in TN and SOM under HG treatment (Figure 1(a-e)) and the transient decrease in CN (Figure 1(f)) also support this view. The stimulating effect of the micro-organisms leads to the accumulation of nutrients, while the growth potential of the vegetation is weak and the absorbed nutrients are less than the accumulated nutrients, resulting in a transient increase in TN and SOM levels, mainly due to the decomposition of organic carbon and nitrogen and the accumulation of dead microbial residues.