Nitrogen-Fixing Trees Shift Soil Organic Carbon Toward Mineral-Associated Stability by Enhancing Microbial-Derived Carbon in Subtropical Plantations.

Introducing nitrogen (N)-fixing trees into plantations is a promising strategy to increase soil organic carbon (SOC) storage. However, whether this practice enhances SOC stability, a key determinant of long-term carbon sequestration, and through which specific microbial pathways, remains poorly understood. Here, we compared pure Eucalyptus plantations with mixed plantations containing the N-fixing tree Erythrophleum fordii (E. fordii) in subtropical China by measuring SOC fractions, microbial communities, extracellular enzymes, and microbial-derived carbon (MNC) content. After five years of plantation establishment, mixed plantations significantly increased total SOC by 22.41% compared to pure eucalypt stands. More importantly, this increase was accompanied by a pronounced shift in the carbon pool toward more stable mineral-associated organic carbon (MAOC), which increased by 28.39% in the surface layer. This increase was associated with microbial community restructuring characterized by a greater fungal contribution. The fungal-derived carbon (FNC) rose by 51.20%, and structural equation modeling showed that MNC contributed directly to MAOC formation, a stronger effect than the indirect enzyme-mediated pathway. Our findings provide field-based evidence that mixing Eucalyptus with N-fixing trees enhances SOC stability, primarily through the association between FNC accumulation and MAOC formation. This microbial mechanism provides a process-based perspective for evaluating mixed-species N-fixing tree plantations as a carbon-friendly management strategy in subtropical forestry.