Temporal dynamics of microbial-soil particulate organic matter in response to nitrogen addition in an alpine steppe

In alpine regions, particulate organic matter (POM) dominates topsoil organic carbon and is more sensitive to nitrogen (N) enrichment than mineral-associated organic matter. However, the mechanisms and temporal dynamics that govern N-induced POM reduction, particularly in relation to its interaction with the soil microbial community, are still not well understood in alpine ecosystems. To address this knowledge gap, we conducted a multi-level N addition experiment (0, 10, 20, 40, 80, and 160 kg N ha −1 yr −1 ) in an alpine grassland to investigate these mechanisms over time. Significant POM reduction (8.11–39.92%) was observed only at high N addition dose (≥80 kg N ha −1 yr −1 ) in the 2nd year. In contrast, all N addition doses led to a significant decrease in POM components (63.27–78.70%), including chitin (56.31–81.91%), alkyl compounds (61.37–81.97%), microbial N compounds (56.01–74.09%), and polysaccharides (60.66–75.81%) in the 5th year. Prolonged N addition for 5 years fostered microbial communities with higher taxonomic diversity (Shannon index), increased phylogenetic clustering (nearest taxon index), and greater dynamic turnover (species replacement). These communities showed greater sensitivity to POM variation and responded even to relatively low levels of N input. The sustained reduction in POM over five years coincided with markedly strengthened microbe–POM interactions. Co-occurrence network analysis further revealed the emergence of more complex and cooperative microbial networks. The simultaneous increase in microbial-POM connectivity and decline in POM content suggest a co-evolutionary response to prolonged nitrogen enrichment. Overall, under extended N addition (≥5 years), the dominant drivers of microbial community shifts appear to transition from abiotic factors, such as inorganic N availability, to biotic feedbacks mediated by POM quality. This transition further tightens microbe–POM coupling, accelerates soil carbon turnover, and reinforces a feedback loop that may have implications for the long-term stability of soil carbon in alpine grasslands. • The N addition effect was stronger after 5 years than after 2 years. • Prolonged N addition increased microbial diversity and phylogenetic clustering. • N input significantly reduced particulate organic matter (POM). • POM loss was linked to intensified microbe–organic matter interactions. • Microbe–POM feedback regulates carbon storage in alpine soils.