Impact of nitrogen addition on soil organic carbon across ecosystems: Microbial roles and environmental regulation

The influence of ecosystem type on the response of soil organic carbon to nitrogen addition remains a critical, yet understudied, facet of carbon cycling research. In this study, to address the persisting knowledge gap, the complex interplay between nitrogen addition and soil organic carbon dynamics across diverse ecosystems was systematically investigated, revealing a context-dependent relationship. Ecosystem type significantly influenced the primary pathways through which nitrogen addition affected soil carbon storage, largely mediated by shifts in microbial roles. In forest ecosystems, nitrogen addition positively influenced soil organic carbon accumulation by stimulating microbial biomass carbon (R2 = 0.460, P < 0.05), suggesting that alleviating the nitrogen limitation promoted microbial growth and subsequent carbon incorporation into soil. Conversely, in grasslands, nitrogen addition increased carbon accumulation by suppressing microbial respiration (R2 = 0.725, P < 0.05), indicating a reduction in decomposition rates potentially driven by changes in microbial community composition or activity. Despite these ecosystem-specific pathways, soil organic carbon was enhanced in both systems, linked to total nitrogen dynamics (forests: R2 = 0.637, P < 0.001; grasslands: R2 = 0.624, P < 0.001). The results highlight nitrogen’s dual role in soil organic carbon accrual. The varying influence of microbes explained the ecosystem-specific regulation of soil organic carbon depending on soil properties and nitrogen management strategies. For instance, prolonged nitrogen addition negatively impacted microbial growth in forests but inhibited microbial respiration in grasslands. Recognizing the nuanced impact of nitrogen addition and microbial activity within different ecological contexts will support environmentally specific strategies that optimize soil carbon sequestration practices.