Technical Abstract: The majority of the SeMet additions to two soils were recovered as volatile species (50 to 80%). In contrast to SeMet, SeCys additions were rapidly nonextractable (<6 h) from both soils with little to no volatile Se detected. The released Se from SeCys additions was initially recovered as phosphate-soluble selenite and selenide after 6 h of incubation. These results suggest that Se present in seleniferous plant tissue as SeMet will not accumulate in soil due to extensive volatilization. In contrast, additions of seleniferous plant residues rich in SeCys will result in organic Se mineralization to inorganic Se forms in soil. An exponential relationship (R2 = 0.96) was determined for the Se content and organic C contents for the nine seleniferous soil materials, suggesting that the Se was highly associated with the soil organic matter fraction. An additional 12 Se values and organic C contents from Se contaminated semi-arid soils or irrigation drainage water evaporation basins obtained from the literature closely fit the exponential function established for the nine soil materials. The results suggest that an initial rapid release of organic-associated Se, along with a long-term release of Se would be expected with C oxidation, due to the reversion of the former Kesterson wetland evaporation pond ecosystem to a native semi-arid grassland.