International Journal of Molecular Ecology and Conservation, 2025, Vol.15, No.6, 267-276 http://ecoevopublisher.com/index.php/ijmec 270 deposition and nitrogen leaching. Among them, biological nitrogen fixation and soil nitrogen mineralization are the most important ways to provide inorganic nitrogen for plants. Common leguminous plants in grasslands form a symbiotic system with nitrogen-fixing microorganisms, which can significantly increase soil nitrogen input. With the increase of global nitrogen deposition, some grassland areas have begun to witness a transformation of plant communities from leguminous plants to highly productive graminogenic plants, leading to a reorganization of community structure and nutrient requirements. Furthermore, changes in soil temperature and humidity strongly affect the rates of mineralization and nitrification. In semi-arid grasslands with frequent alternations of dry and wet conditions, short-term moist pulses can rapidly stimulate nitrogen mineralization, forming the "nitrogen burst" phenomenon (Bicharanloo et al., 2022; Bilotto et al., 2022). There is a strong coupling relationship between the nitrogen cycle and the carbon cycle: the alleviation of nitrogen limitation can enhance the efficiency of photosynthesis, accelerate carbon input, and thereby affect the kinetics of soil organic matter formation and decomposition. 3.3 Phosphorus cycling: a restrictive process dominated by soil properties Unlike carbon and nitrogen, phosphorus in grassland ecosystems mainly comes from the weathering of parent material, with less external input. Therefore, it is a highly limiting nutrient in many grasslands. Phosphorus in soil mostly exists in inorganic adsorbed form and organic combined form, and it needs to be converted into usable forms through chemical weathering, microbial decomposition or enzymatic action in the rhizosphere. Disturbances such as drought, wind erosion and fire can alter the soil particle structure and mineral composition, thereby affecting the availability of phosphorus. For instance, frequent burning may lead to enhanced organic phosphorus mineralization in the soil, while long-term grazing may alter the spatial distribution of phosphorus through trampling and manure input. With the change of soil pH, the degree of combination of phosphorus with iron and aluminum oxides or calcium varies, thereby changing its availability. This is also one of the important reasons for the productivity differences among different types of grasslands. 3.4 Multi-scale coupling mechanism of element cycling Grassland nutrient cycling is not an isolated process of a single element, but a multi-element coupling network driven by the interaction among plants, microorganisms and soil. Recent studies have shown that the carbon, nitrogen and phosphorus cycles are closely related, and any change in the input or conversion rate of any one element may trigger a chain reaction. For instance, the increase in nitrogen deposition can alter the utilization efficiency of carbon substrates by microorganisms, thereby affecting the carbon stability of the soil. An increase in carbon input may raise the demand for phosphorus by microorganisms, leading to an intensification of phosphorus limitation. These interactive influences make the grassland nutrient cycle highly dynamic and complex (Figure 1)(Bilotto et al., 2022). Figure 1 Modelled Phosphorus (P), carbon (C) and nitrogen (N) processes and cycling depicted by nutrient transfer dynamics in hill-country pastoral landscapes to a 300 mm depth (Adopted from Bilotto et al., 2022) Image caption: TSP total soil phosphorus, SOC soil organic carbon, TSN total soil nitrogen (Adopted from Bilotto et al., 2022)
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