Technical Abstract: Selenium (Se) toxicity has been noted in several areas of the western U.S. arising from disposal of agricultural irrigation drainage waters containing inorganic Se. Since microbial synthesis of toxic Se-amino acids is limited and toxicity has been limited to ponds with extensive plant growth, studies were conducted to produce Se-enriched plant material and determine the form and speciation of Se in soil following decomposition. Decomposition of Se-enriched residue resulted in an enrichment of nonamino acid organic Se compounds. Chemical analyses showed soils exhibiting extensive dimethylselenide (DMSe) volatilization from SeMet addition had volatilization pathway intermediates methylSeMet and dimethylselenopropionate while the intermediates were not found in soils showing little DMSe volatilization, suggesting that microbial methylation and demethylation pathways were present in different soils. Synthesis of additional pathway intermediates confirmed the different Se oxidation-detoxification pathways and may explain why areas of expectant selenosis have not yielded terata consistent with total Se contamination.