Selenium (Se) is of great environmental and geochemical concern as an essential trace element for humans and a powerful proxy for paleo-ocean (Lenz & Lens 2009, Qin et al. 2012, 2013, 2017a). In the soil environment, Se naturally exists in a variety of oxidation states (VI, IV, 0, and -II) and organic forms. The study on Se speciation is critically important for understanding the mobility, bioavailability, and toxicity of Se in soils. However, the distribution, speciation, and coordination environment of Se in soils at the molecular level are still largely unknown.