4.1 | Changes in soil nutrient content in response to grazing
Our results showed that in the alpine meadow ecosystem with poor growth environmental conditions, grazing decreased the surface soil nutrient content and soil water content (FIG. 2(a)(c)(d)(e)) and showed a trend of decreasing with increasing grazing intensity, with the lowest value appearing under the MG treatment. This supports our initial hypothesis that the response of soil nutrient content to grazing shows a linear trend of decreasing soil function with increasing grazing intensity due to environmental constraints. The main reasons for the decrease in soil nutrient content are as follows. First of all, the geographical environment of QTP is complex, and the abiotic environmental conditions of the ecosystem are the decisive factors for the function of the grassland ecosystem. The alpine meadow limited by the geographical environment is mainly characterised by a thin soil layer, low soil nutrient and soil moisture content, large diurnal temperature difference and large soil evaporation, and other environmental conditions that are unfavourable to growth. When the grassland is disturbed by grazing, plants absorb a large amount of nutrients for their own recovery, growth and reproduction, resulting in a decrease in soil nutrient content. Our findings are also supported by previous research showing that topography is an important limiting factor for soil nutrients in geographically disadvantaged locations, making them more vulnerable to grazing and gradually decreasing (Li Ying et al., 2022). In addition, the decline in soil nutrient levels may also be related to the growth of soil microorganisms. As a nutrient input, the excrement of grazing livestock plays an active role in the activation of soil microorganisms and is the energy source for accelerating the reproduction process of microorganisms. Some studies have shown that there is nutrient competition between vegetation and soil microorganisms(Jiang Jing & Song Minghua, 2010) , the main source of energy for the growth of plants and microorganisms is soil nutrients. If plants are damaged after grazing and are in the growing season at the same time, stress responses may occur. Plants adopt accelerated growth to compensate for the damage and to ensure survival and reproduction, which consumes a lot of soil nutrients. Rapid reproduction was the main reason for the decrease in soil nutrient content. At the same time, the decrease in soil nutrient content may also be related to the effect of moderate disturbance. Under moderate disturbance, plant and soil micro-organisms will rearrange their species, and the dominance of the original dominant species may be reduced by disturbance, allowing non-dominant species to increase their reproduction. More species require more nutrients to sustain them, and the lack of environmental nutrients cannot support the increased fraction of ecosystem demand, which may be the reason for the lowest soil nutrient levels under moderate grazing. Similarly, under severe disturbance, only a few species can adapt to high disturbance, and plants may adopt survival strategies to survive first, and then expand their range once the damage has exceeded their carrying capacity. Plant activity is weakened, soil microbial species are single and soil nutrient supply species are reduced, which may be the reason why soil nutrient content is higher under heavy grazing than under moderate grazing.