Impact of land use changes on selenium distribution and bioavailability in Mollisol of Sanjiang Plain

Recent changes in land use in the Mollisol region of the Sanjiang Plain have significantly impacted speciation and bioavailability of selenium (Se) in soils. This study examines Se species and distribution across various land use types, focusing on the interactions between iron oxides and organic matter in Se migration and transformation. The total Se content in woodland, dryland, and paddy soils is 0.79 mg/kg, 0.80 mg/kg, and 0.82 mg/kg, respectively, showing a uniform distribution. Soil organic carbon (SOC) is positively correlated with total Se, with complex effects on Se species. In paddy soils, SOC is positively associated with organic-bound Se and residual Se; Se content decreases with increasing soil depth in both profiles, following a typical surface accumulation pattern. Additionally, water management in paddy fields plays a critical role: prolonged water retention at the surface promotes Se enrichment in the topsoil and enhances dissolved Se leaching from deeper layers. This research indicated that coupling iron oxides and organic matter is crucial in Se fixation, particularly in paddy soils. Bioaccumulation factor (BCF) analysis shows a significant correlation between Se content in crops and soil Eh and pH, with higher Eh enhancing Se uptake. This study highlights the influence of land use and soil properties on Se bioavailability.