Depth-dependent patterns of soil organic carbon fractions in Sphagnum and herbaceous wetlands

Wetlands are global hotspots for soil organic carbon (SOC) storage, comprising particulate organic carbon (POC) and mineral-associated organic carbon (MAOC) fractions. However, these two SOC fractions are not typically considered along soil depth when assessing SOC persistence in wetland systems. Here, based on soil sampling at two depths (surface: 0–10 cm; subsurface: 20–30 cm) across three representative wetlands spanning subtropical, mid-temperate, and cold temperate regions, including Sphagnum - and herbaceous-dominated wetland systems, the distribution of SOC fractions was delineated, alongside soil properties and microbial characteristics. Results showed that Sphagnum wetlands stored more SOC in the POC fraction, with a higher surface soil POC/MAOC ratio relative to subsurface soils. By contrast, SOC in herbaceous wetlands was dominated by MAOC, with a lower surface soil POC/MAOC ratio than in subsurface soils. The POC/MAOC ratio was positively correlated with the ratio of particulate nitrogen to mineral-associated nitrogen in both two horizons. C/N imbalance destabilized surface SOC, whereas microbial diversity and abundance promoted subsurface SOC stabilization. Bacterial and fungal carbon pump efficacies emerged as dominant controls on POC/MAOC ratios in surface and subsurface soils, respectively. Collectively, these findings illuminated divergences of SOC functionality and potential persistence between surface and subsurface soils in Sphagnum - and herbaceous-dominated wetlands, with implications for SOC dynamics under anthropogenic perturbations and climatic stress.