Microbial mechanisms drive divergent soil nitrogen cycling in subtropical forests with contrasting carbon stocks

Long-term management has created pronounced spatial heterogeneity in carbon stocks in subtropical forests. However, how these differences in carbon storage are linked to soil nitrogen cycling and its underlying microbial mechanisms remains poorly understood. Here, we compared key soil nitrogen cycling processes—including net ammonification and nitrification—and the associated microbial functional genes across 33 high-carbon and 27 low-carbon forest sites. Results showed that although total nitrogen, ammonium, and nitrate pools did not differ significantly, nitrogen transformation pathways varied markedly between forest types. Specifically, forests with higher carbon stocks showed higher net ammonification but lower net nitrification, suggesting a greater potential for ammonium retention. Metagenomic analysis revealed a corresponding shift in microbial genetic potential, with ammonification genes (e.g., ureA / ureC ) enriched in high-carbon forests, and nitrification genes (e.g., amoA/amoB ) more abundant in low-carbon forests. Random forest models identified nitrogen assimilation, dissimilatory nitrate reduction to ammonium (DNRA) and ammonification pathways as key pathways driving net ammonification and nitrification in high-carbon forests. Furthermore, microbial co-occurrence networks were more complex, interconnected, and stable in high-carbon forests, indicating more integrated and stable microbial associations related to N cycling. These results suggest that high-carbon forests may support a conservative, retention-oriented nitrogen cycling mediated by specific microbial traits and network properties, whereas low-carbon forests may be associated with a more open, leaky nitrogen cycling. Our findings provide a microbial-level mechanistic understanding of nitrogen cycling regulation linked to forest carbon storage, offering insights for management strategies aimed at enhancing nutrient retention and long-term ecosystem sustainability.