Selenate Reduction in an Alluvial Soil

Recent studies of the mobility and solubility of Se in western San Joaquín Valley soils suggest that this potentially hazardous element can be managed by controlling its oxidation‐reduction reactions. The soluble species, SeO 4 , which is highly mobile and toxic, can, in principle, be reduced to SeO 3 , which is strongly adsorbed, or to organoselenium species, which may volatilize under suitable conditions. Chemical thermodynamics predicts that the reduction sequence in soils should be: NO 3 → SeO 4 → MnO 2 at pH >5. The objective of this study was to establish the position of SeO 4 in the kinetic reduction sequence for a representative western San Joaquín Valley soil incubated in suspension with its own saturation extract. In a series of replications of an incubation experiment, it was observed that native NO 3 (plus NO 2 ) concentrations became undetectable after 100 h in the soil suspension without O 2 supply. Soluble Se, either added as Na 2 SeO 4 or indigenous to the soil, disappeared after 50 to 200 h. Native soluble Mn began to rise after 50 h and showed a sharp increase after 100 h of incubation. Retardation of SeO 4 reduction in the presence of added NO 3 was noted. The results indicated that, at native levels of NO 3 , effective microbial catalysis of SeO 4 reduction occurred in the soil under the conditions of the experiments, in agreement with the recent isolation of bacterial species that can respire SeO 4 while oxidizing organic acids typical of suboxic soil environments.