Selenium (Se) biofortification depends on Se content in the soil or exogenous applications of Se to the plant. The transformation of Se in soil determines its availability for plant absorption. Exogenous organic material will experience a series of biochemical transformations and stimulate fractionation and speciation transformations of Se in soil. Straw amendment will enhance soil-dissolved organic matter (DOM) and affect Se availability through the formation of DOM-Se, which involves hydrophilic acid-bound Se (Hy-Se), fulvic acid-bound Se (FA-Se), humic acid-bound Se (HA-Se) and hydrophobic organic matter bound-Se (HON-Se). The Hy-Se and FA-Se are the bioavailable forms for plant uptake in the environment (Ren et al. 2015). When selenite is applied into the soil, it is rapidly converted into soil solid-phase with much faster aging rate than that of selenate (Li et al. 2016). However, whether straw derived DOM affects selenite aging is still unknown. Hence, it is important to understand the dynamic transformation of DOM-Se during selenite aging to illustrate (1) the binding capacity of different DOM fractions with selenite; (2) the transformation of different DOM-bound Se species; and