Formation mechanism of the geological high background of soil selenium during the weathering of Paleozoic carbonate rocks in the Karst area of Guangxi, southeastern China

The global distribution of soil selenium (Se) is highly heterogeneous, with 500–1000 million people facing low Se intake. In China, 72% of soils are Se-deficient. Interestingly, Guangxi, where karst covers 41.57% of the land, exhibits distinctly high geological background levels of soil Se, though the mechanism remains unclear. To clarify this, we analyzed 5670 topsoil and 50 rock samples from Tiandeng County, Guangxi, determining major elements, Se, Mn, SOC, and pH, along with rock acid-insoluble residues and soil Fe-Mn nodules. Results show topsoil Se averages 0.16–8.34 mg/kg, with a background value of 0.55 mg/kg—2.75 times the national average. Among the samples, 4621 (81.50%) are classified as Se-rich. Spatial distribution of soil Se displays clear zonation, correlating with stratigraphy and fold structures, indicating a bedrock origin. While black rock series and bauxite-rich formations show high Se in both rock and soil, carbonate areas exhibit low rock Se but high soil Se, contrary to clastic areas. Although Se leaching is higher during carbonate weathering, the low soil formation rate of carbonates leads to significant Se enrichment in their acid-insoluble residues, providing a richer Se source for subsequent pedogenesis. This is the primary reason for the high Se background in karst soils, with Fe-Mn oxides acting as key adsorbents for secondary Se enrichment. Our findings help explain Se accumulation in Guangxi's karst regions and Se heterogeneity in Chinese soils, supporting human Se intake management. • This study resolves the paradox of Se-rich soils forming over Se-poor carbonate bedrock in the Guangxi karst region, elucidating the systemic mechanism responsible for this regional geological high-background phenomenon. • We propose a two-stage enrichment model: Se is first highly concentrated in the acid-insoluble residues during intense carbonate weathering, which then serves as a potent source for subsequent soil formation. • Secondary Fe-Mn (oxyhydr)oxides are identified as the critical carrier for adsorption and retention, completing the key sequestration pathway for Se’s secondary enrichment and stabilization in karst soils. • A comprehensive evidence chain, integrating spatial distribution, weathering simulations, and soil analysis, systematically traces Se from parent rock to soil enrichment, robustly validating the proposed genetic model.