Microbial life-history strategies and nutrient limitation modulate soil organic carbon stability under nitrogen addition in subtropical plantations

Continuous atmospheric nitrogen (N) deposition can substantially affect the stability of soil organic carbon (SOC) in forests. However, from the perspective of SOC fractions, the effects of different forms of N addition on SOC stability and the underlying microbial mechanisms remain unclear. In this study, an N addition experiment was conducted in subtropical plantations to investigate the effects of inorganic N (NH4Cl) and organic N (urea and glycine) on SOC fractions as well as their microbial drivers and pathways. The stable fraction was represented by microbial necromass carbon (MNC) and the labile fraction by cumulative CO2 emission (CCE). The results showed that organic and inorganic N additions reduced MNC by 20.7% and 18.8%, respectively, and increased CCE by 1.3% and 4.9%, while total SOC content remained unchanged. N addition increased microbial biomass, enhanced microbial carbon and phosphorus limitations, and shifted microbial communities from r- to K-strategists. Although different forms of N addition had consistent directions and pathways of effect on MNC and CCE, organic N exerted a stronger effect on microbial phosphorus limitation and a greater promotion of CCE. Furthermore, N addition significantly decreased the ratios of MNC:SOC and MNC:CCE. The shift of microbial communities toward K-strategists combined with intensified nutrient limitations represented the key regulatory pathway underlying the decline in MNC and increase in CCE, thereby weakening SOC stability. Taken together, short-term N addition markedly reshaped SOC fractions and weakened soil carbon stability, with microbial life-history strategies and nutrient limitations acting as central mediators. These findings highlight the necessity of integrating N forms and SOC fractions to more accurately assess the regulatory effects of atmospheric N deposition on critical carbon cycling processes in subtropical forests.