Effect Of Sulfur On Labile Cadmium At Microinterface In Rice Root Zone And Its Mechanism

The effective form of cadmium(Cd)determines the activity,migration,transformation and environmental risk of Cd.How to evaluate the heavy metal pollution in farmland soil effectively,especially the environmental behavior of Cd in paddy soil,regulate bioavailability and ensure the safety of agricultural products has become a hot and difficult issue in the field of environmental risk control.Sulfur(S)is one of the essential nutrient elements for plants.Sulfur deficiency exists in some paddy soils in China,and more and more attention has been paid to the agronomic measures of sulfur.Sulfur can affect the physical and chemical properties,microorganism,enzyme activity and redox process of soil,thus affecting the morphological transformation of heavy metals.At present,the research on the bioavailability of sulfur on Cd is mainly focused on the migration,transformation and accumulation of plants,but there are few reports on the transformation of environmental behavior at the micro-interface of rhizosphere soil.Therefore,it has important theoretical basis and scientific value to study the environmental behavior of sulfur in Cd at the micro-interface of rice root zone,so as to guide agricultural production and accurately evaluate the ecological risk management of Cd and soil quality and safety.In this study,the sulfur deficiency paddy soil that moderately polluted by Cd in Hunan was used as the research object.The gradient diffusion thin film technique(Diffusive gradient in thin-films,DGT)was used to characterize the effective form of Cd and the synergistic change of Fe(?)and S(-?)at the micro-interface of rice root zone under the condition of water-sulfur coupling.Combined with anaerobic soil culture experiment,pot experiment and root box experiment,the flooding condition was studied.The effects of three forms of sulfur(S0,Na2SO4,MP)on the environmental chemical behavior of Cd at the micro-interface of rice root zone and its influencing factors were discussed,and its possible mechanism was discussed.The main conclusions of this paper are as follows:(1)In anaerobic soil culture experiments,Dissolved organic carbon(DOC).S(-?),and Fe(?)were the important factors affecting sulfur reduction of soil available Cd concentration.With the extension of culture time,the available state Cd decreased rapidly and then tended to be stable,while Fe(?)and S(-?)increased rapidly and then tended to be stable.Sulfur application reduced soil Eh and Dissolved oxygen(DO),and promoted the increase of DOC,which drove the reduction of sulfate and iron.leading to the increase of S(-?)and Fe(?)contents.Stepwise regression and path analysis showed that DOC,S(-?)and Fe(?)were the important factors affecting the effective state of Cd.(2)Through pot experiment and root box experiment,the effect of sulfur on available Cd in rhizosphere soil micro interface at rice growth stage(tillering stage,heading stage and matnrity stage)was studied,and the direct contribution of S(-?)and Fe(?)to available Cd and the indirect effect of DOC on available Cd were further confirmed.During the dynamic change process of growth period,the concentration of available Cd in rhizospliere soil increased first and then decreased,and the activity of available Cd in soil was the highest at heading stage.The application of sulfur changed the rhizosphere soil environment decreased the rhizosphere soil pH,increased Eh,Do,increased the contents of DOC(17%46%),S(-?)(1-3.5 times increase).Fe(?)(1-4 times increase),and promoted the formation of secondary pig iron oxide to reduce the content of available Cd(reduced by 19%?30%)in the rhizosphere soil.Fe(?)had the greatest direct contribution to Cd in rhizosphere soil at tillering stage and maturity stage.At heading stage.S(-?)had the greatest direct contribution to Cd in rhizosphere soil.MP treatment group mainly decreased the activity of Cd(reduced by 62%?83%)by promoting the increase of S(-?),Fe(?)and DOC.Na2SO4 and S0 treatment groups mainly reduced the activity of Cd(reduced by 19%?48%)by affecting the contents of S(-?)and Fe(?).The water-sulfur coupling reduces the available state of Cd by regulating the transition from available state of Cd to oxidized state of Fe and Mn(increased by 10%?45%)in soil.The water-suifur coupling reduces the recharging ability of Cd from solid phase to liquid phase.(3)Through the study of enzyme activity and microorganism,the mechanism of sulfur reducing Cd activity was further explored by promoting enzyme activity and bacterial functional community to drive the redox reaction of Fe.S and C.promoting the increase of DOC,S(-?)and Fe(?).The application of sulfur promoted the activities of urease(UA),catalase(CAT)and dehydrogenase(DHA)in rhizosphere soil.There was a significant negative correlation between enzyme activity and available Cd.The application of sulfur reduced the diversity of rhizosphere soil microorganisms and changed the community composition and function.The Fermentation(Lysobacter,Gemmatimonadaceae)and Chemoheterotroph(Geobacter,SBR1031)of rhizosphere soil bacteria were increased.The mechanisms by which enzyme activity and functional bacteria regulate the redox reactions of C.Fe and S to reduce the available state Cd mainly include:(1)increasing soil DOC as the electron donor of soil redox reaction;(2)enhancing the dissimilation and reduction of Fe,promoting the formation of FeCO3,FeS.FeS2 and coprecipitation with Cd;(3)driving the oxidation of Fe and adsorbing Cd on the surface of iron oxides;(4)accelerating the reduction of S and increasing the formation of CdS precipitation.(4)Using chemical analysis and in situ characterization techniques such as ESEM-EDX,it was confirmed that sulfiur can also reduce the bioavailability of Cd by increasing the content of DO in the rhizosphere and promoting the formation of irou film.A negative correlation was observed of the Cd content between in iron film and in plant tissue.Different forms of sulfur have an effect on the formation of iron film.which is caused by the difference of oxygen secretion in the rhizosphere caused.The application of sulfur can promote the growth and development of rice and increase the biomass and yield of rice.Sulfur promoted the migration of Cd from root to shoot.The determination of available Cd in rhizosphere soil by LDHs-DGT can accurately reflect the bioavailability of Cd.