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.