The origin and geochemical cycle of soil selenium in a Se-rich area of China

Mianyang City, located in the Fujiang River Basin, Sichuan Province, is a Se-rich area of China. The distribution of Selenium (Se) in the Mianyang area was studied based on assay data obtained from soil, irrigation water, fertilizer, and rice (grain and stem) samples. The ratio between natural and anthropogenic sources in the area was derived by analyzing the concentrations and spatial distributions of multiple elements (such as Se, cadmium, arsenic, and mercury) in the soil. The controlling factors affecting Se concentration in the soil were also investigated. We established a geochemical model of the Se cycle among the different media (i.e., the atmosphere, water, soils, and plants). We then calculated the annual Se flux caused by various inputs' (such as precipitation, fertilization, and irrigation) and outputs' (such as infiltration, crop harvest, removal of straws from cropland, and volatilization) pathways in the topsoil. We discuss the contribution of different pathways to the Se cycle and provide evidence for exploring Se-rich land in the study area.