International Journal of Molecular Ecology and Conservation, 2025, Vol.15, No.6, 267-276 http://ecoevopublisher.com/index.php/ijmec 273 6 The Impact of Human Activities on the Nutrient Cycling and Decomposition of Grasslands 6.1 Grazing intensity and grassland degradation Grazing is the most common utilization method of grasslands worldwide, and its intensity directly determines the direction and rate of grassland nutrient cycling. Light to moderate grazing and appropriate grazing can enhance grassland productivity, accelerate plant renewal and speed up the turnover of litter. Livestock excrement increases local nitrogen and phosphorus supply, enhances microbial activity, and sometimes even promotes the accumulation of soil organic matter. High-intensity grazing and degradation: Heavy grazing leads to a decline in vegetation coverage, shallower root systems, and a reduction in community uniformity. The reduction of litter input and the increase of soil exposure significantly intensified water evaporation and wind erosion (Figure 2) (Hao et al., 2024). Figure 2 Influence mechanisms of grazing intensity, grassland type and soil type on the formation process of soil organic carbon (SOC) (Adopted from Hao et al., 2024) At this point, microbial activity declines due to dryness and substrate scarcity, decomposition rates decrease, carbon mineralization is insufficient, soil animal diversity significantly drops, organic matter reduces, and soil structure becomes fragile and prone to desertification. In the northern grasslands of China and the arid regions of Central Asia, the phenomenon of weakened nutrient cycling and soil degradation caused by excessive grazing intensity is particularly typical (Rui et al., 2025). 6.2 Management measures such as irrigation and fertilization In semi-arid grasslands, irrigation is often used to enhance productivity. Fertilization is a common measure in artificial grasslands or improved grasslands. When irrigation increases water supply, adequate water can significantly enhance microbial activity and strengthen carbon and nitrogen mineralization. However, excessive irrigation can easily lead to the "explosive" growth of microorganisms, increase soil respiration, reduce carbon storage, some herbaceous plants are replaced by high-water-dominant species, the quality of litter changes, soil compaction, and affect the activities of soil animals (Heyburn et al., 2017). Fertilization, especially nitrogen fertilizer input, can significantly enhance initial productivity and increase the nitrogen content of plants. However, long-term nitrogen application can inhibit cellulose decomposition and lignin degradation, reduce the proportion of fungi and increase the proportion of bacteria, cause soil acidification, drive changes in microbial community structure, and weaken soil carbon stability. The stimulating effect of phosphorus fertilizer on decomposition is usually less than that of nitrogen fertilizer. However, in phosphorus-deficient grasslands, plant and microbial activities may be significantly enhanced. 6.3 Global climate change: warming and aridification trends Global warming is continuously altering the temperature and precipitation patterns of grasslands, which has an extremely profound impact on decomposition and nutrient cycling. Warming generally increases the decomposition rate, but there is a "carbon pool decay" effect over time: early soil respiration significantly increases, but the stable carbon pool gradually returns to a low level due to substrate depletion and microbial
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