Mechanisms of soil carbon accumulation under structure-based forest management: Coupled dynamics of active organic carbon fractions and nitrogen–phosphorus nutrients

Soil active organic carbon (AOC) is a sensitive indicator of soil quality and health, responding rapidly to early changes in the soil environment induced by forest management. Forest management practices, particularly those involving individual tree interventions, regulate soil organic carbon (SOC) dynamics by modifying stand composition and structural attributes. However, the mechanisms by which different management modes influence AOC fractions remain unclear. A field experiment was conducted in a Pinus tabulaeformis plantation under four forest management modes: structure-based forest management (SBFM), randomized forest management (RFM), close-to-nature forest management (CNFM), and an unmanaged control (CK). Topsoil (0-10 cm) AOC components were analyzed and compared among treatments. The results indicated that all three management modes improved soil properties, including soil organic matter (SOM), total nitrogen (TN), alkali-hydrolyzable nitrogen (AN), total phosphorus (TP), and available potassium (AK). Among these, SBFM produced the most pronounced enhancements in SOM, TN, TP, and AN. In addition, SOC and AOC contents, including dissolved organic carbon (DOC), readily oxidizable organic carbon (ROC), and microbial biomass carbon (MBC), were significantly increased by management, particularly under SBFM and RFM. Notably, SBFM exhibited the strongest promotion of AOC accumulation, with DOC, ROC, and MBC contents reaching 3.56, 2.36, and 2.74 times those of the control, respectively. Although management effects on the proportional distribution of AOC fractions were generally limited, SBFM significantly reduced both ROC/SOC and MBC/SOC ratios. Correlation analysis indicated that SOM, TN, TP, and AN were significantly and positively associated with SOC and most AOC components. Partial least squares path modeling (PLS-PM) revealed that management positively affected chemical properties, which largely mediated the effect on carbon components. Management's indirect effect on carbon ratio outweighed its direct effect, supporting a possible sequential pathway from management to carbon lability via chemical properties and carbon components, but these results are exploratory and require validation. These findings suggest that forest management may improve soil chemical properties by reducing stand density and optimizing stand structure, thereby increasing carbon inputs while moderately stimulating carbon outputs. This balance ultimately promotes the accumulation of soil AOC and alters the proportional distribution of labile carbon fractions, indicating that SBFM is a promising strategy for increasing labile carbon pools in the studied plantation.