1 | INTRODUCTION
On the Qinghai-Tibet Plateau (QTP), the area of alpine grassland is about 54-70% of the total area, and the soil carbon content of grassland is 94% of the total soil carbon. The soil carbon sequestration of the alpine meadow is about 63.99 ± 4.41 g kg-1 SOC and 4.11 ± 0.63 g kg-1 SIC, which is two to three times higher than the fixed stock of the alpine meadow ecosystem (19.78 1.98 g kg-1 SOC and 9.21 0.66 g kg-1 SIC)(Huang et al., 2022). As one of the four largest grazing areas in China, QTP supports nearly 5.3 million people, and livestock production is as important as environmental protection. In recent years, the impact of human activities on QTP ecosystems, especially on alpine meadows, has increased and overgrazing threatens the sustainable development of QTP alpine grassland systems(Liu Ronggao et al., 2017; Ma Ying et al., 2021; Piao Shilong et al., 2019; Wang et al., 2022; Wei et al., 2022; Yang et al., 2008). Numerous studies have shown that due to climate change and human activities, 19-60% of natural grasslands in the QTP are in varying degrees of degradation(Liu Ronggao et al., 2017; Ma Ying et al., 2021; Piao Shilong et al., 2019; Wang et al., 2022; Wei et al., 2022; Yang et al., 2008). It is of great importance to study the dynamics of soil carbon in grazed alpine meadows in order to improve the carbon function of the QTP soil ecosystem.
Given the key effects of grazing on soil carbon sequestration in ecologically fragile alpine areas, the effects of grazing on carbon storage and dynamics of QTP alpine meadow soil ecosystems have received considerable attention from the ecological research community. The results showed that 73% of the carbon in the terrestrial carbon pool was stored in the soil as soil organic matter, most of which was released as CO2. Soil microorganisms were directly involved in the soil carbon cycle through in vivo turnover and in vitro metabolism(Liang et al., 2017; Xun et al., 2018). Grazing mainly reduces the α-diversity of soil microorganisms and forms a community dominated by fast-growing trophic bacteria, which has a stimulating effect on soil microorganisms(Gou Yanni & Nan Zhibiao, 2015; Huang et al., 2022; Wang et al., 2022)and affects the carbon cycle of grassland ecosystems( Ding Chengxiang et al., 2020; Ma Ying et al., 2021). The so-called excitation effect refers to the process by which exogenous carbon inputs provide the carbon and energy required for the growth of heterotrophic microbial communities, thus changing the composition of the soil microbial community, accelerating the decomposition of soil organic matter by microorganisms and releasing CO2(Li et al., 2023). The excitation effect was mainly regulated by soil nutrient content, and microbial biomass was the most important factor influencing the intensity of the microbial response to grazing carbon input(Li et al., 2023; Xun et al., 2018). At the same time, grazing changes soil physical and chemical properties such as soil water content, nutrient acidity and alkalinity, and soil microorganisms are often restricted to varying degrees in the changing soil environment. Changes in environmental conditions also have important effects on the structure and function of microbial communities(Fierer, 2017). Clarifying the changes in soil nutrient content and soil microbial biomass in response to grazing is an important part of understanding changes in the soil carbon cycle led by soil microorganisms. However, the grassland ecosystem is a whole composed of the biological community and its environment, and the abiotic environment and the biological part are interrelated and mutually constrained. As a life-support system, the environment simultaneously supports vegetation and soil microorganisms, and the quality of environmental nutrient conditions directly affects the ecological functions of vegetation and microorganisms, as well as the resilience and recovery of the ecosystem(Jiang Jing & Song Minghua, 2010; Xie et al., 2013). Most of the existing studies on the grazing response of soil microorganisms are concentrated in temperate grasslands or research stations with stable environmental conditions(Ma Ying et al., 2021). However, the geography of QTP is very extremely complex(Liu Ronggao et al., 2017), environmentally restricted grasslands are usually an important component of the actual grassland area of QTP, and it is difficult to observe the influence of environmental conditions on the changes in soil microbial response under stable environmental conditions. Therefore, it is necessary to investigate the changes in soil ecosystem response of alpine meadows to grazing under environmental constraints.
In this study, we selected typical alpine meadows on the Qinghai-Tibet Plateau with poor environmental conditions for growth to conduct grazing experiments of different intensities to test the following hypotheses: 1) Due to the limited environmental nutrient conditions and the fragile ecosystem, grazing intensity may be linearly negatively correlated with soil nutrient content; 2) Soil microorganisms were affected by grazing stimulation, and microbial biomass nutrients showed an opposite response trend to soil nutrient content.