Resource Quality and Fungal Richness Are Associated with Soil Organic Matter Dynamics in a Temperate Forest in South Korea

Forest soils play a critical role in carbon sequestration and nutrient cycling by storing large amounts of soil organic matter (SOM). However, the integrated associations linking aboveground resource properties and belowground microbial processes influencing SOM dynamics remain poorly understood. In this observational, cross-sectional study, we analyzed 84 plots across six forest types at Mt. Gariwang, South Korea, to evaluate the relative importance of resource quality and resource quantity in relation to SOM dynamics. Total organic carbon (TOC), total nitrogen (TN), and the C:N ratio were used as indicators of SOM dynamics. Resource quality was represented by species richness (SR) and functional dispersion (FDis) as indirect proxies, whereas resource quantity was represented by aboveground biomass (AGB) and diameter at breast height diversity (DBH diversity). We also evaluated whether fungal and bacterial ASV richness were statistically linked to the relationships between aboveground resource properties and SOM dynamics. Functional dispersion (FDis) was negatively associated with TOC (β = −0.28) and the C:N ratio (β = −0.51), whereas AGB showed no significant relationship with any SOM indicator. In contrast, SR was positively associated with Fungal ASV richness (β = 0.37), which was in turn positively linked to TOC (β = 0.43) and TN (β = 0.56) accumulation, suggesting an indirect statistical pathway consistent with fungi acting as a biological link between aboveground biodiversity and belowground carbon and nitrogen storage. These findings indicate that resource quality indicators showed divergent associations with SOM dynamics: a negative direct pathway via FDis and a positive indirect pathway via SR and fungal ASV richness, and that, overall, resource quality was more consistently associated with SOM dynamics than resource quantity. Our study provides new insights into the associations linking aboveground vegetation and belowground microbial communities and offers a scientific basis for biodiversity-based forest management to enhance long-term soil carbon sequestration.