Effects Of Exogenous Sulfate On Bioavailability And Transformation Of As And Cd In Soil-rice(Oryza Sativa L.) System

Arsenic（As） and cadmium（Cd） are two heavy metal elements that have a strong toxic effect on the human body and coexist in the soil around the mining area.With surface runoff and underground seepage,these toxic substances enter the farmland ecosystem,threatening agricultural production safety and human health.Rice is one of the important food crops in China and even in Asia.Because of its alternate flooding and drainage cultivation mode and its unique physiological characteristics,rice is easier to accumulate As and Cd than other crops.The pollution of As and Cd in rice has attracted widespread attention.Sulfur（S）,as an essential element for plant growth,also affects the migration and transformation of As and Cd in soil.Flooding reduces the soil redox potential and stimulates the microbial sulfate reduction,which in turn affects the solubility of As and Cd.Exploring the mechanisms of sulfur fertilizer on the migration and transformation of As and Cd in the soil-rice system is of great significance to rational fertilization and guarantee the quality and safety of agricultural products.We selected Shangyu（SY）,Tongling（TL）and Ma’anshan（MA）soil co-contaminated by As and Cd as research objects.The change of Fe,S,As,Cd concentrations and microbial diversity in soil after Na₂SO₄addition were monitored in situ.Meanwhile,the effects of sulfate application on rice growth and accumulation of As and Cd were systematically studied.The main results are as follows:1.The study found that the exogenous sulfate was exhausted on the 22nd day,and the process of microbial sulfate reduction promoted the further decline of Eh.The As concentrations in the porewater of SY and TL soils reached 3.5 mg/L and 1.9 mg/L on the40th day,while it maintained at 1.5 mg/L and 0.9 mg/L with sulfate treatment.The results of solid phase extraction showed that the proportion of crystalline iron（hydroxide）oxide co-precipitated As and orpiment increased.The Cd concentrations in SY and TL porewater quickly dropped below the detection line regardless of whether sulfate was added or not,but the soil sequential extraction found that the proportion of Ex-Cd decreased by 25.6%and 18.6%,respectively,after sulfate application（p<0.05）,mainly transformed to Fe-Mn-Cd.High Eh in MA soil limited the dissolution of As,and the concentration of As in porewater was lower than 0.3 mg/L before and after sulfate addition.The Cd concentrations have declined faster in MA+100S treatment,but there was no significant change of Cd species in soil.The above results showed that addition of exogenous sulfate can significantly reduce the dissolution and release of As and Cd through forming sulfide or binding to iron and manganese oxides,thereby reducing their availability,and the degree of influence is related to the soil background.2.There was no significant difference in the number of OTUs and Alpha diversity index in the soil between with and without the sulfate addition.Compared with MA soil,the microbial richness in SY and TL soil was elevated.Moreover,the abundance of Desulfobacterota was significantly higher than that in MA soil,which explained the strong microbial sulfate reduction process in SY and TL soil.Sulfate addition decreased the abundance of Desulfitobacterium in SY and TL soils,but significantly increased the abundance of Desulfotomaculum and Desulfurispora（p<0.05）.Exogenous sulfate also significantly increased the copy number of dsr gene in SY soils（p<0.05）,but had no significant effect on TL and MA soils.The above results indicated sulfate added to the soil did not affect the overall microbial diversity in the soil,but changed the abundance of dominant SRBs,increased the copies of SRB functional genes,and stimulated the microbial sulfate reduction process.3.the Fe concentrations in the rhizosphere solution of SY and TL decreased by 151.733.6%and 43.2%,respectively,after sulfate addition for 45 d,compared with the treatment without sulfur fertilizer.Almost all Fe in porewater existed in the form of Fe²⁺.Meanwhile,DCB-Fe content in the iron plaque was also significantly decreased by59.4%and 28.0%（p<0.05）,and the adsorbed As and Cd in the iron plaque also decreased dramatically.This means that the reduced Fe²⁺in the rhizosphere was not oxidized to more iron plaque on the root surface due to root oxygen loss.Combined with the increase in the proportion of pyrite in soil sequential extraction,it can be speculated that the addition of sulfate may promote the combination of Fe²⁺and S^2-to form secondary iron-sulfide minerals through the microbial sulfate reduction process.The results showed that the formation of iron plaque was inhibited by sulfate addition,and the effect of hindering As and Cd was declined,but the forming secondary iron-sulfide minerals might participate in the immobilization of As and Cd.4.The application of sulfate significantly increased the fresh weight and plant height of rice seedlings,and promoted rice tillering.Compared with no Na₂SO₄treatment,the electrolyte leakage levels of rice roots in SY+100S and TL+100S were reduced by 52.7%and 52.8%（p<0.05）,the chlorophyll content of leaves increased by 165.1%and 48.2%（p<0.05）.In addition,sulfate also increased the content of thiol-containing polypeptides such as GSH and PCs in rice leaves.These results indicated that the sulfate addition alleviated the As and Cd stress on rice seedlings and improved the self-detoxification ability of rice.As accumulation in roots and shoots of SY+100S decreased by 38.8%and60.2%,and Cd accumulation decreased by 46.8%and 40.8%,respectively,compared with control soil treatments.In TL+100S,As accumulation in roots and shoots decreased by 42.9%and 39.6%,respectively,but Cd did not decrease significantly.For the sulfate addition in MA soil,only the accumulation of Cd in shoots decreased by 23.0%.In grains,Cd accumulation in TL and MA rice decreased by 73.7%and 37.4%after Na₂SO₄addition,while there was no significant difference in As accumulation.The results showed that the application of Na₂SO₄increased the self-detoxification ability in rice seedlings,alleviated the toxic effects of As and Cd on plant.The application of Na₂SO₄early can reduce the accumulation of As and Cd in rice tissues under the As and Cd co-contaminated soil,and the inhibiting effect on Cd will last until maturity stage.