| Recent decades,the contamination of heavy metal in agricultural soil has raised people’s great concern on the safety of our country’s food products.Controlling the contamination of heavy metal and improving the quality of food products has become research hotspots and also a great challenge in environmental management.Sulfur(S)is one of the six macronutrients in plants,however,it has been widely reported that sulfur deficiency has occurred in many areas in our country.In order to meet the demands of food,sulfur fertilizer application in agricultural soil has obtained great attention.The speciation transformation of S in soil system is closely related to the biogeochemical behavior of heavy metal.Exploring the influence of S fertilization on the transporting and speciation transformation of heavy metal in paddy soil-rice system can be of great help for agricultural activities and assessing the environmental risk of heavy metal.In this study,paddy soil was selected as our analytical object.By conducting pot cultivating experiment in greenhouse and applying both traditional chemical analysis methods and advanced synchrotron radiation techniques(including synchrontron radiation X-ray fluorescence spectroscopy,X-ray absorption fine structure spectroscopy),the influence of two forms of sulfur fertilizer(S0 and Na2SO4)applied in paddy soil on the transportation and speciation transformation of copper(Cu)in paddy soil and rice plants system has been comprehensively studied.Primary conclusions were summarized as follows:(1)The influence of sulfur fertilization on the migration and transformation of Cu in soil solution of rice rhizosphere was revealed,sulfur fertilization in paddy soil decreased the migration of Cu in the rice rhizosphere soil solution.The zeta potential of soil colloid fell into the range of 2.4～7.8 mV and the particle size of soil colloid was 440～740 nm.Iron oxides and organic matter were the main components of soil colloid.Sulfur fertilization reduced the proportion of iron in soil colloid,meanwhile the proportion of C and O increased.The concentration of Cu in soil solution and colloid was reduced by S fertilization,which may due to the changed composition of soil colloid caused by sulfur fertilization.The main speciation of Cu in soil colloid were Cu2S,Cu-Cysteine,Cu-Citrate and Cu-Humic,and S fertilization increased the proportion of Cu-Cysteine,meanwhile Cu2S decreased.The reduction of iron oxides in soil colloid caused by S fertilization decreased the adsorption of Cu on soil colloid,leading to higher proportion of organic bound Cu in soil colloid.(2)The influence of S fertilization on Cu bioavailability and speciation transformation in rice rhizosphere soil during the whole life cycle of rice plants was revealed,which mechanism could be related to the different growth stages of rice plants.In the vegetative stages(seedling and tillering stages)of rice plants,sulfur fertilization increased the concentration of bioavailable-Cu(CaCl2 and DTPA extractable Cu)as well as the DOC concentration in rhizosphere soil.Higher proportion of Cu-Humic was found in the +S treatment than that in the control.However,in the heading stage,long time’s flooding condition led to the reduction of S fertilizer to sulfide,which offered substrate for the formation of Cu2S.Higher proportion of Cu2S and lower concentration of DOC in +S treatments led to lower concentration of bioavailable-Cu in rice rhizosphere soil in the heading stage when compared to the control.It can be concluded that both DOC and redox potential changed by S fertilization in paddy soil are critical factors that controlled Cu speciation in rice rhizosphere soil.(3)Impact of S fertilization in paddy soils on iron plaque formation on rice root surface and Cu accumulation in rice plants under flooding conditions was explored,we found the influence of S fertilizer on iron plaque formation was related to the form and application rate of S fertilizer.Low amount of S fertilization(50 mg/kg,100 mg/kg)promoted the formation of iron plaque.However,high amount of S fertilization(500 mg/kg,1000 mg/kg)caused toxicity to rice root,leading to reduced iron plaque formation.S0 showed higher efficiency of iron plaque formation promotion when compared to Na2SO4.Iron plaque was found within and on root epidermal cells and was composed mainly of Fe,O,Al and Si,reducing Cu penetrated into root cells.Sulfur fertilization decreased the bioavailability of Cu by inducing transformation of Cu bioavailable fractions to Cu bound to organic matter.Besides,the percentage of Cu2S and Cu-Cysteine species in rice rhizosphere soil was increased by S fertilization,thus reducing Cu mobility.Sulfur fertilization in paddy soil increased formation of iron plaque and decreased Cu bioavailability,leading to the decreased Cu concentration in rice plants.(4)The mechanism of S fertilization mediation Cu translocation and biotransformation in rice plants was revealed,we found that sulfur fertilization in paddy soil increased the accumulation of Cu in rice grain.Fertilization of S increased the biomass and yield of rice plants,as well as the translocation factor of Cu from root to shoot and shoot to grain,resulting in enhanced Cu in grain.Cu accumulated mainly in the aleurone layer of rice grain,while the concentration in endosperm was relative low.Sulfur fertilization promoted Cu accumulating in the alenrone of rice grain.Sulfur increased the proportion of Cu(Ⅰ)species in rice grain,husk and leaf,suggesting the inducing of Cu(Ⅱ)reduction in rice tissues by S fertilization.Our results suggested that fertilization of S in paddy soils increased the accumulation of Cu in rice grain,possibly due to the reduction of Cu(Ⅱ)to Cu(Ⅰ)by enhancing glutathione synthesis and increasing the translocation of Cu(Ⅰ)from shoot to grain. |
