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.持续大气氮（N）沉降可显著影响森林土壤有机碳（soil organic carbon, SOC）的稳定性。然而，从SOC组分视角出发，不同形态氮添加对SOC稳定性的影响及其潜在微生物机制仍未明晰。本研究在亚热带人工林中开展氮添加实验，旨在探究无机氮（NH₄Cl）与有机氮（尿素、甘氨酸）对SOC组分的调控效应，及其微生物驱动因子与作用路径。本研究以微生物残体碳（microbial necromass carbon, MNC）表征土壤有机碳稳定组分，以累积CO₂排放（cumulative CO₂ emission, CCE）表征活性组分。结果显示，有机氮与无机氮添加分别使MNC降低20.7%与18.8%，同时使CCE提升1.3%与4.9%，但总SOC含量未发生显著变化。氮添加可提升微生物生物量，加剧微生物碳限制与磷限制，并推动微生物群落从r-策略型向K-策略型转变。尽管不同形态氮添加对MNC与CCE的影响方向与作用路径保持一致，但有机氮对微生物磷限制的强化作用更强，且对CCE的促进效果更为显著。此外，氮添加显著降低了MNC:SOC与MNC:CCE的比值。微生物群落向K-策略型的转变，叠加养分限制的加剧，是MNC下降与CCE上升的核心调控路径，进而削弱了SOC稳定性。综上，短期氮添加显著重塑了SOC组分并降低了土壤碳稳定性，微生物生活史策略与养分限制发挥了核心介导作用。本研究结果凸显了整合氮形态与SOC组分的必要性，以更精准地评估大气氮沉降对亚热带森林关键碳循环过程的调控效应。