Controlling Effects Of Nitrate,Sulfate And Water-soluble Organic Fertilizer On Arsenic Pollution In Paddy Soils

As a"hometown of non-ferrous metals"and"land of fish and rice",Hunan has widespread pollution of arsenic?As?in paddy soil.Rice can accumulate more than 10 times the concentration of As.As can enter the human body through the food chain and cause irreversible damage.Therefore,how to reduce the accumulation of rice As has become a research hotspot.The first research content of this paper is to explore the effect of nitrate on the As pollution of paddy soil.Different concentrations of nitrate would be added to As-contaminated paddy soil for long-term flooding and rice seedling cultivation experiments.The second part is to study the effect of water-soluble organic fertilizer and sulphate on the bioavailability of As in paddy soil.The water-soluble organicfertilizer-soil,water-solubleorganicfertilizer-sulfate-soil co-cultivation experiment would be set up in As-contaminated paddy soil.At the same time,rice would be cultivated in the whole growth period.The main findings are as follows:?1?The addition of high-concentration nitrate(25 mmol NO₃^-kg^-1soil)reduced the porewater As???concentration to 0 after 20 days of flooding,which was in sharp contrast to the control porewater 122.5?g l^-1As???.The high concentration of nitrate treatment increased the As???concentration by 1.2-16 times and the Fe concentration by 99.7%.The five-step extraction of soil solid phase As showed that the unstable As in the soil was 49%lower than that of the control soil under nitrate treatment.At the same time,the nitrate-treated group retained up to 1.6times more Fe oxide crystals and up to 4 times the aioA gene abundance.This indicated that nitrate can provide an effective way to reduce the solubility of As in rice soil by inhibiting the reduction and dissolution of Fe oxide and enhancing the oxidation of microorganism As???under flooding conditions.This directly leaded to a 85%reduction in the bioavailability and accumulation of As in rice seedlings,highlighting the potential of nitrates to alleviate the accumulation of As in rice.?2?Water-soluble organic fertilizer reduced As???of soil porewater by 64%and increased As???content after 4 weeks of flooding.The results of five-step extraction of soil As showed that soil As bound to Fe/Al oxide state under water-soluble organic fertilizer treatment was reduced by 6%,and the residual state As was increased by 4%,indicating that organic matter was beneficial to promote the conversion of soil As to a more stable residue state.At the stage of rice seedling stage,the water-soluble organic fertilizer had a significant effect on the reduction of As bioavailability,reducing the accumulation of As in the aerial part by44%and the accumulation of As in roots of 47%.However,water-soluble organic fertilizer significantly increased the accumulation of As in the root surface of rice and enhanced the transport efficiency of rice As from the root to the ground.Therefore,the effect of water-soluble organic fertilizer on the bioavailability of As was not obvious in rice maturity.?3?The combined application of water-soluble organic fertilizer and sulphate had a more significant As-effectiveness of As bioavailability than the application of water-soluble organic fertilizer alone,which reduced the soil porewater As???after flooding for 4 weeks by 80%.The results of five-step extraction of soil As showed that the sulfated and water-soluble organic fertilizer treatment group increased the soil As bound to stable crystalline Fe/Al oxide by 1%,and the porewater Fe was also reduced correspondingly.This indicated that sulfate as an electron acceptor can inhibit the dissolution and reductive dissolution of soil Fe to a certain extent,and its reduction product H₂S may reduce As fluidity by forming As₂S₃ precipitation with As???.Therefore,the combined use of water-soluble organic fertilizer and sulphate significantly reduced the accumulation of 56%rice root As and 47%aboveground part As in rice seedlings and the accumulation of 10%As in rice brown rice at maturity.