Transformation Of Sulfur And Its Effect On Bioavailability Of Arsenic And Cadmium In Rhizosphere Of Rice

A pot experiment was carried out by adding different concentrations and different forms of sulfur fertilizers into unpolluted and heavy metal contaminated paddy soil.The effects of sulphur and heavy metal on p H、Eh and pe+p H of paddy were studied.The chemical morphological alteration process and the migration mechanism of sulphur both in rhizosphere and in non rhizosphere soil of rice were comprehensively researched.And the effects of sulphur on dry matter weight of different parts of rice in different growth stage,concentration of Fe and Mn plaque in rice in different growth stages,concentration of S in different parts of rice in different growth stage,concentration of root surface adsorption As in rice,concentration of As and Cd in different parts in rice growth stage as well as the morphology change and the migration mechanism of As、Cd both in rhizosphere and in non rhizosphere soil of rice in different growth period were discussed as well.The results demonstrated that:(1)During whole growth period of rice,the redox potential of rhizosphere soil and non rhizosphere soil of rice was between 71 ~ 285 m V and 85 ~ 255 m V,respectively.What is more,the overall redox potential in non rhizosphere soil was lower than that in rhizosphere soil.The variation of p H and pe+p H value in rhizosphere and non rhizosphere soil of rice was 7.6 ~ 8.4,7.4~ 8.4 and 9.5 ~ 13.4,9.4 ~ 12.8,respectively.(2)In rhizosphere soil,the existence of sulphur can significantly increase the mass fraction of water-soluble sulfur and adsorptive sulfur.The mass fraction of unpolluted paddy soil was significantly higher than that of paddy soil polluted by heavy metals.Inorganic silfur existed in rhizosphere mainly by water-soluble sulfur,followed by adsorptive sulfur and hydrochloric acid soluble sulfur,concentration of hydrochloric acid volatile sulfur is the least.In non rhizosphere soil,at tillering stage and flowering stage,the mass fraction of water-soluble sulfur and adsorptive sulfur increased observably by utilizing gypsum instead of elemental sulfur.During the ripening,the mass fraction of adsorptive sulfur decreased from 0.5 to 1.6 mg.kg-1.Inorganic silfur existed in non rhizosphere mainly by water-soluble sulfur,followed by and hydrochloric acid soluble sulfur and hydrochloric acid volatile sulfur,concentration of adsorptive sulfur is the least.(3)In rhizosphere soil,the organic and inorganic sulfur of the total sulfur accounted for 20% ~ 40% and 60% ~ 80%,respectively;In non rhizosphere soil,the organic and inorganic sulfur of the total sulfur accounted for 18% ~ 46% and 54% ~ 82%,respectively.The mass fraction of organic sulfur,water soluble sulfur,sulfur and soluble sulfur in rhizosphere soil was significantly higher than that in non rhizosphere soil,while non rhizosphere soil possessed higher hydrochloric acid volatile.(4)According to the dry weight of rice stems,leaves,roots and grains at different growth stages,the application of sulfur fertilizer could alleviate the toxicity of heavy metals to rice and increase the dry weight at mature stage,while gypsum performed better in this respect.(5)The absorption of heavy metal by rice mainly occurred in root,the mass fraction was relatively lower in upper surface while lowest in grains.The application of sulfur,especially low concentration of sulfer,could significantly reduce the content of heavy metals in roots,shoots as well as grains of rice,while gypsum performed better in this respect.(6)In rhizosphere soil,the mass fraction of residual As increased gradually with the extension of growth period in each processes;The nonspecific adsorption state,the specific adsorption state and the crystalline Fe-Mn oxide bound of As decreased with the extension of the growth period.In non rhizosphere soil,the nonspecific adsorption state of As decreased gradually with the extension of growth period,while the residual As performed an opposite trend.The specific adsorption state,amorphous Fe-Mn oxide binding state,crystalline iron manganese oxide binding state of As maintained the same.In the rhizosphere and non rhizosphere soil,where As existed mainly in specific adsorption state and amorphous Fe-Mn oxide binding state,followed by crystalline Fe-Mn oxides,and the residual state and the nonspecific adsorption state are the least.(7)In rhizosphere soil,the overall mass fraction of total Cd,carbonate combined state Cd,strongly organic state Cd and residual state Cd increased gradually with the extension of growth period in each processes;While the mass fraction of weakly organic state Cd and Fe-Mn oxide bound state Cd decreased first and then increased with the extension of growth period;And the mass fraction of ion exchange state Cd decreased totally.In non rhizosphere soil,the mass fraction of carbonate combined state Cd,Fe-Mn oxide bound state Cd increased gradually with the extension of growth period in each processes;The mass fraction of total Cd,ion exchange state Cd,and weakly organic state Cd decreased gradually with the extension of growth period;while the mass fraction of strongly organic state Cd and residual state Cd had no significant change.In rhizosphere and non rhizosphere soil,where Cd existed mainly in ion exchange state and carbonate combined state,followed by Fe-Mn oxide bound state and weakly organic state,while residual state and strongly organic state accounted for the least.