International Journal of Molecular Ecology and Conservation, 2025, Vol.15, No.5, 206-216 http://ecoevopublisher.com/index.php/ijmec 2 11 from rapid to extremely slow decomposition and the transformation from degradable substances to humus. Through this stage, the ecosystem transforms the short-term turnover of fallen soil into a long-term accumulation of soil organic matter, achieving a partial "retention" of carbon and nutrients. 5 Decomposition of Fallen Leaves and Circulation of Nutrients 5.1 Key roles in the carbon cycle Leaf decomposition, as an important link in the carbon cycle of the ecosystem, undertakes the dual function of returning the carbon fixed by plants to the atmosphere and soil (Friedlingstein et al., 2020). On the one hand, during the decomposition of fallen leaves, the respiration of microorganisms oxidizes organic carbon into carbon dioxide (CO₂) and releases it into the atmosphere, which is a major component of heterotrophic respiration in terrestrial ecosystems. On the other hand, the decomposition of fallen leaves transfers a portion of carbon to the soil and retains it for a long time, forming a soil organic carbon pool, which is an important mechanism for carbon sequestration in terrestrial ecosystems (Prescott and Vesterdal, 2021). The decomposition of fallen leaves returns a large amount of organic carbon to the atmosphere through heterotrophic respiration, which is an important carbon source process in the global carbon cycle. At the same time, it also stores part of the carbon in the soil through the formation of humus, which is an important process for maintaining terrestrial carbon sinks. This "dual identity" highlights the indispensable position and complex role of leaf decomposition in the carbon cycle. 5.2 Release and reuse of nutrients such as nitrogen and phosphorus The process of leaf decomposition plays a core role in the cycling of essential nutrient elements such as nitrogen and phosphorus in terrestrial ecosystems. Its main contribution lies in the decomposition and release of organic nutrients into inorganic forms for plants to reuse. Take the nitrogen cycle as an example: Plants absorb inorganic nitrogen (NH₄⁺, NO₃⁻) from the soil and assimilate it into organic tissues. After the leaves fall off, this nitrogen exists in the form of organic nitrogen in the fallen leaves (such as proteins, nucleic acids, etc.) (De Carvalho et al., 2024). Leaf decomposition is the engine of the nitrogen and phosphorus cycles in terrestrial ecosystems: it breaks the closed state of organic nutrients, reconverts nutrients into forms that plants can utilize, and regulates the slow-release and preservation of nutrients through humus (Chen et al., 2020). Without adequate decomposition of fallen leaves, soil nutrients will soon be locked up as the fallen leaves accumulate, and plant growth will be restricted due to the lack of effective nutrients. 5.3 Decomposition and plant nutrient feedback The process of leaf decomposition is not only a one-way nutrient release process, but also forms a feedback loop with the nutrient strategies of plants and community succession. Plants affect the decomposition rate and nutrient release by generating leaves with different characteristics, thereby altering the soil fertility environment. This environment, in turn, influences the growth and competitiveness of the plants themselves and their offspring. This phenomenon is often referred to as "plant-soil feedback" in ecology, where leaf decomposition and nutrient cycling are key links (Dinesha and Dey, 2023). There is a close feedback relationship between the decomposition of fallen leaves and the utilization of plant nutrients: plants affect decomposition, and decomposition in turn affects plants. This feedback ensures that most natural ecosystems can internally circulate nutrients and maintain the dynamic balance of the plant-soil system, and it is also one of the important driving forces for community succession and species replacement (Xu et al., 2020; Casanova-Lugo et al., 2024). 6 Six Cases of Regional and Ecosystem Differences 6.1 Characteristics of leaf decomposition in temperate forests The deciduous decomposition of temperate forests has a moderate rate and a significant seasonal rhythm, which is related to the mild climate and obvious seasonal variations in temperate regions (Zhang et al., 2021). In temperate regions, the litter decomposition rate of arbuloidal mycorrhizal (AM) tree species is faster than that of ectomycorrhizal (ECM) tree species. Among them, the nitrogen content of litter and phylogeny are key predictors (Figure 2) (Keller and Phillips, 2018). Take a typical temperate deciduous broad-leaved forest as an example. Every autumn, a large number of leaves fall and accumulate on the surface of the forest land, and the peak of decomposition usually occurs in the following summer. However, as spring and summer come and temperatures
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