International Journal of Molecular Ecology and Conservation, 2025, Vol.15, No.6, 267-276 http://ecoevopublisher.com/index.php/ijmec 272 4.5 Driving forces of grazing, fire and land use change Grazing alters the structure of plant communities and the physical environment of soil through trampling, feeding and manure input. Light grazing may promote grassland productivity and enhance the efficiency of nutrient redistribution. Heavy grazing reduces the coverage of litter and lowers soil moisture, thereby decreasing the decomposition rate. Fire rapidly oxidizes the fallen materials and surface organic matter through high temperatures, while simultaneously introducing ash rich in mineral elements into the soil. The availability of nutrients increases in the short term after fire, but frequent and long-term burning will reduce the storage of soil organic matter. Land use changes (such as tillage and establishment of artificial grasslands) can disrupt aggregate structures, reduce soil animal diversity, and significantly lower system stability (Hou et al., 2022; Zhang et al., 2025). 5 The Coupling Relationship Between Grassland Nutrient Cycling and Decomposition Processes 5.1 The role of decomposition processes in the release of nutrients Decomposition is the main way for elements such as nitrogen, phosphorus and sulfur in grasslands to transform from organic to inorganic states. The nitrogen and phosphorus released from the early decomposition of litter can be directly absorbed by plants. The mineralization of refractory substances in the later stage is usually controlled by the oxidase system of microorganisms. When rainfall pulses occur, microbial activity rises rapidly, forming short-term mineralization peaks that cause soluble nitrogen to accumulate rapidly. This process is particularly important for the early spring regeneration of grassland plants (Cao et al., 2025). 5.2 Bidirectional feedback mechanism among plants, microorganisms and soil The grassland system has a significant plant-microbe-soil feedback mechanism: plants influence microorganisms and change the substrate supply of microorganisms through root secretions; Different plant functional groups have different litter masses, which also determine the structure of microbial communities. Microorganisms influence plants, providing inorganic nutrients to them through mineralization and nitrogen fixation, while simultaneously altering the availability of nitrogen and phosphorus to plants through competitive and symbiotic relationships. Soil affects both. Soil texture, aggregate structure, and the degree of pH and water coordination influence microbial activities, thereby indirectly determining the nutritional status of plants. This interaction forms a stable cyclic mechanism. However, in the case of intensified human interference and climate change, the feedback may be disrupted, leading to a decline in system function. 5.3 Soil Enzyme activity and nutrient cycling efficiency Soil enzymes are important explicit indicators of microbial metabolism and core parameters for predicting decomposition rate and nutrient turnover efficiency. Enzymes often of concern in grasslands include: β -glucanase, which decomposes cellulose and reflects carbon cycle activity; Urease and protease control nitrogen mineralization; Phosphatase promotes the transformation of phosphorus from organic to inorganic states. The grassland experiences frequent alternations between dry and wet conditions. Changes in moisture often lead to rapid adjustments in enzyme activity, resulting in a typical pulse-like response. Furthermore, the increase in nitrogen deposition can inhibit the activities of certain carbon-decomposing enzymes, thereby affecting soil carbon stability. 5.4 Regulation of nutrient cycling by food web structure The grassland soil food web is composed of microorganisms, protozoa, nematodes, arthropods and large soil animals, presenting a multi-level energy flow relationship. The complexity of the food web structure enhances the stability of the system: predation can regulate the number of microorganisms and prevent the excessive expansion of a single functional group. Soil animal excreta can serve as a "hot spot" to promote local nutrient release. A highly diverse microbial community helps enhance the resilience of the decomposition process under environmental disturbances. An increase in grazing intensity usually reduces the complexity of the food web and lowers the efficiency of the nutrient cycle.
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