Enrichment mechanisms of selenium in topsoil and subsoil of alluvial and lacustrine basins in Northeast China.

Different hydrodynamic sedimentary conditions may cause significant differences in the enrichment behavior of selenium (Se) in soils with the same provenance. To elucidate basin-scale Se enrichment mechanisms, adjacent Songhua River (alluvial) and Naoli River (lacustrine) Basins with the same bedrocks but differing sedimentary environments were investigated, and 5369 topsoils (0-20 cm depth) and 1380 subsoils (at 1.5 m depth) were collected and analyzed. The Geographical Detector method was employed to quantify the explanatory power (%) of factors influencing Se distribution. The results demonstrate that topsoils exhibit elevated Se contents (0.20-0.28 mg kg) compared to subsoils (0.08-0.10 mg kg) in both river basins attributed to distinct geochemical processes. In the high-hydrodynamic alluvial plain from the Songhua River Basin, subsoil Se is primarily governed by pH (34% explanatory power), followed by weathering and organic carbon (Org C) stabilization (21%), whereas Se in the topsoils is predominantly influenced by weathering (32%) and Org C fixation (18%). Conversely, in the low-hydrodynamic and reductive lacustrine environment from the Naoli River Basin, the Se distribution is affected by soil properties, with variations in Se contents attributed to pH and clay thickness in subsoils (explaining 36% and 30% of the variability, respectively) and in topsoils (accounting for 30% and 21%, respectively). In addition, atmospheric deposition accounts for 18% of Se variation in the topsoils of Naoli River Basin, inferred from a significant positive Se-iodine correlation. These findings will help to understand the basin-scale environmental behavior of Se and better utilize Se resources in farmlands.