Vegetation-driven soil organic carbon regulates mercury accumulation in karst soil profiles

Vegetation regulates soil mercury (Hg) sequestration primarily through the formation and transformation of soil organic carbon (SOC) pools, yet the mechanisms underlying this control remain poorly quantified, especially in ecologically fragile karst regions. We examined Hg distribution and speciation across cropland, grassland, and forestland soil profiles in a typical karst catchment, combining measurements of total Hg (THg), dissolved Hg (DHg), mineral-associated organic carbon (MAOC), particulate organic carbon (POC), iron-associated organic carbon (Fe-OC), and stable carbon and nitrogen isotopes with partial least squares structural equation modeling (PLS-SEM) and multivariate nonlinear regression analysis (MNRA). This study shows that forestland soils accumulate the most Hg, with topsoil concentrations exceeding 110 μg kg^-1 and the profile enrichment intensity of Hg (PEI _Hg) surpassing 154.5%, due to the accumulation of MAOC (M-Hg/THg: 84.2 ± 6.1%). The role of Fe-OC in Hg retention is strengthened markedly during vegetation succession from cropland to forestland, underscoring mineral adsorption as a key sequestration pathway in karst systems. Vegetation covers, soil depth, pH, and free iron oxides (Fed) collectively modulate Hg accumulation by regulating SOC composition: stable SOC promotes the sustained sequestration of Hg, whereas labile SOC may release the immobilized Hg and induce its migration. Global literature data show that the coefficient of determination (R²) between soil Hg and SOC in non-polluted areas (0.49 ± 0.32) is much higher than that in highly polluted areas (0.14 ± 0.19), which indicates that the Hg-C coupling mechanism in polluted areas may be influenced by additional factors such as geological background and exogenous inputs. This study confirms that in ecologically fragile karst regions with SOC deficiency, the vegetation restoration process can effectively sequester Hg in soils and inhibit its migration to the atmosphere or water bodies by promoting the formation of SOC, especially MAOC. This finding provides crucial scientific evidence for formulating vegetation restoration-oriented land use regulation and ecological restoration strategies in karst areas.