Effects of Elevated Selenium Concentration on Selenium Accumulation and Nitrogen Fixation Symbiotic Activity of Melilotus indica L

Abstract

Biological and soil factors which contribute to the successful colonization of an annual legume species, Melilotus indica L., in soils with elevated selenium (Se) were studied. This species was introduced into the Kesterson Reservoir in the fresh top soil that was brought in under the Kesterson Cleanup Action to fill lowering pond sites and prevent the formation of ephemeral pools containing hazardous levels of Se. In 4 years since its introduction, it has expanded its range of colonization from the fresh soil fill sites to the native soil sites and contributed 10 to 50% of biomass to the grassland communities. The plant and nodule tissue Se concentrations of the field grown plants were found to be negatively correlated with the soil sulfate concentration. Nutrient solution culture studies discovered that M. indica was able to accumulate 500 μg Se g −1 dry weight without a reduction of growth rate. Plants without nodulation were found to accumulate a greater amount of Se and more vulnerable to Se toxicity. Acetylene reduction rate measurements indicate that the nitrogen fixation symbiotic activity appears to be more susceptible to an elevated Se concentration than its host plant. M. indica is a winter weed, and it occurs naturally in the Se-rich soils. It grows actively over the winter and spring and complete its life cycle in May. If the root nodules and root tissues are incorporated into the soil, the rate of soil Se volatilization may be accelerated over the warm summer months. For disposal of the Se-rich plant materials the plant shoot tissues may be harvested for Se-deficient forage supplementation. Therefore, this species may be useful for field management and reclamation of Se-contaminated soils.