Effects Of Phosphate On Bioavailability Of Selenate And Selenite

Selenium is one of the essential microelements for humans, and either deficient or excessselenium is harmful to human beings. Therefore, both Se supplement in deficiency areas andphytoremediation in selenium-contaminated soils have became the focus in environmentalchemistry. Phosphate fertilizer, as the crop essential abundance element, is widely used inagricultural practices to improve crop yields. However, what cannot be ignored is that theinteraction between phosphorus and selenium will directly affect the transport, transformation,uptake and accumulation of selenium in environmental media. In this paper, a sand cultureand pot experiment were carried out to study the combined effects of phosphate and seleniumon the growth, physiology, absorption of phosphorus and selenium and the transformationbetween various selenium fractions, intended to provide theoretical basis for safely utilizingselenium and effectively phytoremediated contaminated soils. The main results were shown asfollows:1. Application of selenite or selenate alone inhibited the growth of pakchoi, butphosphorus could relieve the toxic effect in the sand culture. Low concentrations of seleniteand selenate could enhance the oxidation resistance of pakchoi by increasing the glutathioneperoxidase(GSH-Px) activity resulting in the decline of activity of the superoxide dismutases(SOD), catalase (CAT) enzyme, ascorbic acid content and malondialdehyde(MDA) contentwere reduced too; but high concentrations of selenite and selenate could produce peroxidation,resulting in the increase of antioxidant enzymes (SOD, CAT, GSH-Px) activity andanti-oxidants (ascorbic acid), while the MDA content also increased in the selenite treatmentwhen phosphorus applied. After the phosphate was added, selenite and selenate were suitablefor the growth of pakchoi when the concentrations of selenite and selenate were1.0mg Se·L-1.The physiology indexs in low concentrations of phosphorus were lower than the highconcentrations, which demonstrated that applied phosphate could enhanced the antioxidant ofpakchoi. So, phosphorus could relieve the toxic effect of selenite and selenate thoughenhancing the antioxidant of pakchoi.2. Selenite could inhibit the uptake of phosphate and decline the acceleration ofphosphate to the root of pakchoi in the pot experiment. The promotion and suppression arecoexistent in the effect of phosphate on the uptake of selenite. At a high amount of selenite(≥2.5mg·kg-1), phosphate could promote the uptake of selenite compared with the no phosphate treatment, but the high mount of phosphate could reduce the root concentration ofselenium in pakchoi significantly. It showed that phosphate would promote the phosphorusuptake of pakchoi in a suitable concentration and inhibit the uptake in high concentrations.Pakchoi had a greater preference for phosphate than selenite, and phosphate inhibited thetransfer of selenium from the root to shoot in high selenite treatment.3. The results showed that the addition of selenate had a toxic action to pakchoi, andselenate could significantly reduced the growth index. Amended phosphate in soil couldincrease the biomass and significantly decrease the concentrations of Se of root and shoot inpackchoi, due to the biological dilution effect took place and relatively lower toxicity.However, the selenium content of packchoi was increased with the application of phosphatebecause of the increased biomass. However, selenium use efficiency was bigger in lowphosphate treatments than high phosphate treatments, which demonstrated reasonableapplying phosphate could increase selenium use efficiency. Added selenate in soil couldreduce the phosphorus concentration when no phosphate applied and the phosphorus contentof packchoi in all phosphate treatments due to the toxic effect of selenate, but no influence onthe shoot phosphorus concentration was founded when phosphate was applied.4. The main selenium specie in no selenium added soil was residual Se (F5), while afterapplication of Se, exchangeable Se and carbonate-bound Se (F2) and Soluble Se (F1) were themain species. Phosphate could lead to the transformation of selenium by the unstable state tothe steady state, because of the increase of selenium content and percentage of stable state, aswell as the increase of IR. And the soil Se treated with selenate was more susceptible to beinfluenced by phosphate compared with selenite. The soil selenium would take more time toreturn to the equilibrium system when the concentrations of selenite and selenateincreased.Phosphate could shorten the time required to return to the equilibrium system in theselenate treatment, but it extended the time in the selenite treatment. At the same time,phosphate could influence the bonding strength and the transformation of Se in the seleniteand selenate treatment though the promotion of selenium accumulation.