The Study On Bioavailability And Valence State Changes Of Selenium In Calcarelous Soil

In this study, pot experiments by adding different concentrations of selenite and selenate were carried out under planting pakchoi (Brassica rapa L. Chinensis Group) in a greenhouse. The objective of the study was to identify selenium valence state changes in calcarelous soil and their bioavaibility to pakchoi when applied as selenite and selenate. Meanwile, growth and physiological effects of selenite and selenate pollution on pakchoi were measured to verify the influence of selenium on pakchoi growth, and effects of different concentrations of selenite and selenate on soil catalase, dehydrogenase, urease and alkaline phosphatase activities in calcarelous soil were also studied to evaluate bioecological toxicity of selenium. The study also attempted to provide information for the research of selenium translocation of selenium in the system of soil-plant and it's regulation in food-chain. The main results were as follows:1. Selenite and selenate ranged from 0 to 20 mg·kg-1 inhibited leaf growth of pakchoi. Low concentration of selenite and selenate stimulate the elongation of the roots of pakchoi. While the inhibitory effects of selenite and selenate increased with the prolonged growth period of pakchoi when high concentration selenium was applied to the soil. Shoot and root DW biomass of pakchoi was not affected by selenium when selenite were added in small quantities.The inhibitory effects of high concentration selenite on shoot and root DW biomass of pakchoi and selenate on shoot DW biomass of pakchoi was enhanced with the prolonged growth period.On the contrary, the inhibitory effect of high concentration selenate on root DW biomass of pakchoi was decrease with the prolonged growth periods of pakchoi. The results of our study showed that roots were the dominant sites for selenium accumulation in selenite-supplied pakchoi, whereas shoots were the major sinks for selenate accumulation.Compared to the treatments with selenite, significantly lower values of translocation efficiencies were found for all selenate-supplied treatments.The inhibitory effects of selenium on the growth of pakchoi was enhanced with the increasing of shoots and roots selenium concentration in pakchoi and selenate treatments showed a stronger effects compared to selenite. 2. Se(IV) and Se(VI) was transformed to Se(0) in soil with the prolonged growth periods of pakchoi when selenite and selenate was applied to the soil.A greater decrease of Se(VI) concentreation in soil applied with selenate than Se(IV) concentreation in soil treated with selenite was observed. The total selenium concentration and Se(VI) concentration of soil under planting pakchoi were significantly lower than that in soil without pakchoi, while no remarkable changes of Se(IV) and Se(0) concentration of soil were found in vegetable soils and soils without plants.The absorbency of pakchoi to different valence state of selenium was Se(VI)>Se(IV)>Se(0).3. Soil catalase, dehydrogenase, alkaline phosphatase and urease activities increased after decreasing with the increasing of the concentration of selenite added to the soil and represented"resistive enzyme activity"phenomenon. The inhibitory action was observed in soil catalase, dehydrogenase activity in soil amended with selenate. Low concentration of selenate stimulated urease activity whereasa selenate inhibited the activity of urease in high concentration.Soil catalase and alkaline phosphatase activities increased first and decreased with prolongation of time when different concentration of selenite and selenate were added to soil. With time prolonging, soil dehydrogenase and urease activities increased in selenite treatments while soil dehydrogenase activity of selenate treatments decreased after increasing. Urease activity of selenate-treated soil increased at low concentration selenium but first decreased and increased in the high concentration of selente treatments. Catalase and dehydrogenase activities of soil applied with selenite were both higher than that of the same concentration selenate treatments.Soil urease activity of selenate treatments was higher than that of selenate at high selenium concentration.Soil urease activity was suggested to be the perfect enzymology indexe which to evaluate soil selenium pollution.4. At low concentrations, both selenite and selenate enhanced the antioxidantive effects which was associated with an increase in glutathione peroxidase(GSH-Px)activity and decrease in lipid peroxidation products MDA content. However, superoxide dismutase (SOD) activity, catalase (CAT) activity and proline content were not affected at low selenium concentrations.Whereas at high concertrations, selenite and selenate exerted pro-oxidantive effect, enhancing the accumulation of MDA and proline, but GSH-Px acitivity decreased.CAT activity decreased but SOD activity increased at high selenium concentration. Peroxidase (POD) activity decreased when both selenium forms was added to soil. The antioxidantive effects of selenate at low selenium concentration and pro-oxidantive effects at high selenium concentration were both stronger than selenite. Selenite and selenate can obviously increase soluble sugar and Vc content of pakchoi which implied that selenium improved the qualities of pakchoi. Shoot DW was most sensitive to selenium in all growth and physiological indexes and SOD activity was more sensitive to selenium than other pro-oxidantive enzymes.