Mineralization of Selenium‐Containing Amino Acids in Two California Soils

Organic forms of Se as the selenoamino acids, selenomethionine (SeMet) and selenocystine (SeCys), are detected in plants grown on seleniferous soil, yet little is known about the speciation and distribution of Se upon decomposition of SeMet and SeCys in soil. To determine the mineralization rate of selenoamino acids in soil, three concentrations of SeMet, SeCys, methionine (Met), or cystine (Cys) were added to samples of a Panoche (fine‐loamy, mixed [calcareous], thermic Typic Torriorthent) or a Panhill (fine‐silty, mixed, thermic Typic Haplargid) soil and aerobically incubated at 22°C for up to 168 h. The amino acid concentrations were analyzed by high performance anion chromatography. Methionine additions closely followed first‐order reaction kinetics and SeMet additions followed pseudo‐first‐order kinetics, with order dependent on SeMet concentration. The time required for 50% mineralization of SeMet additions in the Panhill and Panoche soils was 23.5 and 3.2 h (5 mg Se kg ‐1 ), 41.6 and 15.5 h (25 mg Se kg ‐1 ), and 47.1 and 36.0 h (50 mg Se kg ‐1 ), respectively. The majority of the SeMet and Met additions were recovered as volatile species (50–80%). In contrast to SeMet, SeCys was rapidly nonextractable (<6 h) from both soils, with little to no volatile Se detected and 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.