关键词: 高寒草甸, 放牧干扰, 生态过程, 微生物量碳氮, 养分失衡

Abstract:

Alpine grassland represents a major grassland type in China. It accounts for about one-third of the country's total grassland area. Alpine grassland possesses a range of critical ecological functions, including water conservation, carbon sequestration, biodiversity preservation, climate regulation, and soil conservation. As an irreplaceable and vital ecosystem, it supports the development of regional animal husbandry. It also plays an indispensable role in maintaining ecological balance at both regional and global levels. However, due to the combined effects of climate change and human activities, alpine grasslands are facing severe degradation. During the degradation of alpine Kobresia grasslands, selective grazing by livestock leads to the transition from a healthy Gramineae-Kobresia humilis community to vegetation with a simpler structure. Examples of such vegetation include the K. humilis community and the K. pygmaea community. During this process, plant root systems become overdeveloped, leading to an increased root-to-soil ratio, while available soil nutrients decline markedly and total nutrients accumulate. These coordinated shifts ultimately induce an imbalance between soil nutrient supply and demand. Meanwhile, thickening of the mattic layer and the trend toward shorter, finer vegetation reduce soil water infiltration, causing the surface mat layer to die and crack, which further accelerates degradation. Additionally, the invasion and frequent activities of rodents lead to the loss of native vegetation and the spread of poisonous weeds, eventually transforming the grassland into a forb-black soil type secondary bare land. Soil provides the foundation for grassland vegetation growth, and grassland degradation inevitably leads to soil degradation. On the one hand, vegetation thinning exposes the topsoil, increasing water evaporation and making it more vulnerable to wind and water erosion. On the other hand, reduced plant litter and root exudates directly reduce the inputs of carbon and nitrogen, leading to soil impoverishment. The decline in soil nutrients further inhibits vegetation recovery, forming a vicious cycle. Soil microbial biomass carbon and nitrogen play a key role in this process, acting as a central link in the transformation between organic and inorganic nutrients and participating in almost all soil biochemical processes. Their dynamics directly regulate changes in soil structure and fertility. In the alpine grassland ecosystem of the Qinghai-Tibet Plateau, plants, soil, and microorganisms form a closely linked ecological complex. Vegetation degradation alters the soil environment, which in turn affects soil microbial biomass. Numerous studies have shown that alpine grassland degradation significantly reduces soil microbial biomass, which is closely related to changes in plant communities and soil environments. The degradation of alpine grassland reduces vegetation coverage and productivity, significantly decreasing plant litter and root exudates. This directly cuts off the primary energy and carbon sources for soil microorganisms. Subsequently, the loss of surface cover exacerbates soil water evaporation and erosion, resulting in increased soil aridity, compaction, and temperature fluctuations. Concurrently, the deterioration of soil aggregate structure and the decrease in organic matter and available nutrient content further degrade the habitat for microbial communities. These changes collectively inhibit microbial activity, hinder key processes such as organic matter decomposition and nitrogen mineralization, and ultimately lead to a marked reduction in soil microbial biomass. Degradation of alpine grasslands caused by grazing disturbance is essentially a process of imbalance between plants and soil nutrients, affecting the virtuous cycle and sustainable development of grasslands. In recent years, artificial grass planting has been an important measure for rapid restoration of degraded grasslands. However, the low species diversity of artificial grasslands weakens biodiversity, and their establishment must be strictly limited to suitable areas. Near-natural restoration techniques have become a focus of research and application. These methods include fencing, no-till reseeding, and pest control. They cause less disturbance to natural ecosystems compared to conventional approaches. Studying near-natural restoration requires analysis of the degradation process and underlying mechanisms of grassland ecosystems. However, research on the nutrient imbalance process and the plant-soil-microorganism interactions in degraded Kobresia grasslands in specific regions still needs to be further supplemented. Therefore, this study investigated different degradation successional stages of grazed alpine Kobresia grassland: Gramineae-K. humilis community (GK), K. humilis community (K), K. pygmaea community at thickened stage (KT), K. pygmaea community at cracked stage (KC), and forb-black soil type secondary bare land (FB). We used a space-for-time substitution approach to conduct field surveys and laboratory analyses determine vegetation characteristics (coverage, aboveground biomass, belowground biomass,plant functional group biomass), soil physicochemical properties (total carbon, total nitrogen, organic carbon, nitrate nitrogen, ammonium nitrogen, available phosphorus, water content, pH), and microbial biomass (microbial biomass carbon and nitrogen). One-way ANOVA was used to compare differences among stages, and Pearson correlation analysis and path analysis were applied to explore the interactions among plants, soil, and microorganisms. The aims were to: 1) clarify the characteristics of plant biomass and soil nutrients during the degradation of alpine Kobresia grassland; 2) analyze changes in soil microbial biomass carbon and nitrogen across different degradation successional stages; 3) explore the plant-soil-microorganism interactions and the nutrient imbalance process in degraded alpine grasslands. The results showed that: 1) Soil microbial biomass carbon and nitrogen decreased progressively with increasing degradation, from 1494.94 mg·kg⁻¹ and 640.74 mg·kg⁻¹ in GK to lower levels in K. pygmaea communities, reaching the lowest values of 523.50 mg·kg⁻¹ and 101.48 mg·kg⁻¹ in FB. 2) As degradation proceeded, vegetation coverage, total aboveground biomass, and biomass of Gramineae and Cyperaceae showed declining trends. Soil total carbon, total nitrogen, organic carbon, nitrate nitrogen, and water content decreased stepwise from GK to FB. 3) Microbial biomass carbon and nitrogen were positively correlated (p<0.05) with aboveground biomass, belowground biomass, and soil total nitrogen, total carbon, organic carbon, water content, nitrate nitrogen, ammonium nitrogen, and available phosphorus. This indicates that the decline in plant biomass and the loss of soil nutrients jointly affected the accumulation of microbial biomass carbon and nitrogen. Therefore, the reduction in plant community biomass and the imbalance of soil carbon and nitrogen nutrients caused by grazing are important factors leading to changes in soil microbial biomass carbon and nitrogen in alpine grasslands.

Key words: alpine meadow, disturbance of grazing, ecological process, microbial biomass carbon and nitrogen, nutrient imbalance