Abstract: Modification of forest structure and species diversity is an effective strategy for enhancing forest carbon sequestration. However, the relative contributions of tree spatial arrangement, tree size, and species diversity to soil organic carbon (SOC) and its fractions remain poorly understood. To address this, we established 720 plots in Northeast China to investigate the influences of these structural characteristics on soil mineral-associated organic carbon (MAOC), particulate organic carbon (POC), and total SOC. We found that: (1) tree spatial arrangement and size produced more effects on SOC, POC, and MAOC dynamics than species diversity. Plots dominated by larger trees (with greater diameter and height) had 10.8–26.2% lower SOC, POC, and MAOC compared to plots with smaller trees. In contrast, plots with more neighborhood species mixing, spatial clumping arrangement, and more even species distribution had 1.11–1.31 folds higher SOC, POC, and MAOC than those with lower mingling, clustering, and evenness trees, especially more favoring the POC accrual. Compared with tree size and spatial arrangement parameters, species diversity showed marginal effects on these carbon parameters. (2) Soil properties were important mediators of the plant-soil interactions. Both redundancy analysis and structural equation model showed that soil pH and available P counteracted the negative effect of tree size on MAOC, while soil nitrogen (N) directly increased POC by over sevenfold higher than the influence of spatial arrangement (tree mixing and clustering distribution). (3) Microbes were closely associated with variations in POC and MAOC, and our data identified the most probable functional genes and dominant phyla. Our findings underscore the critical roles of soil properties and microbial communities in SOC dynamics and highlight the potential for optimizing forest spatial structure and tree size aimed at coping with climate change.

Key words: Tree size, Spatial arrangement, SOC, POC, MAOC