Microbial phosphorus demand affects carbon-degrading potential under long-term nitrogen addition in a subtropical forest

Tropical and subtropical forests store over one-third of global soil carbon (C), substantially influencing global C-climate feedback. However, whether low phosphorus (P) availability and microbial P deficiency in these ecosystems limit soil organic C (SOC) sequestration remains poorly understood. Here, we investigated the effects of 8–9 years of nitrogen (N) addition on SOC content and its microbial mechanisms in a subtropical Moso bamboo forest. We observed that long-term N addition reduced both microbial necromass C (MNC) and SOC contents, while it increased microbial P demand as indicated by the changes in P-cycling genes and phosphatase activity. Genes and enzymes related to P-cycling were identified as the important drivers of SOC content. A structural equation model further showed that N addition-induced higher microbial P demand negatively affected SOC content. This effect was both direct and indirect, the latter via promoting microbial C demand, which subsequently negatively affected MNC and SOC contents. Altogether, our findings suggest that microbial P deficiency is detrimental to SOC storage, offering novel perspectives on the coupling of soil C and P cycling via microbial mechanisms.