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.