Effect Of Sulfur Fertilization And Pig Manure On As Migration And Transformation In Soil And Paddy Rice(Orvza Sativa L.)System

Arsenic(As)is one of the typical pollutants in paddy soils of southern China.As a staple crop in China,rice is considered to be a major route of As ingestion for people.Thus,it has been a hot spot to find a way to reduce As bioavailability,reduce As uptake by rice and regulate As pollution in paddy soil.Sulfur(S)and organic matter are important factors that affect the migration and transformation of As in soil.Sulfur deficiency in soil in our country has become increasingly serious.The application of S fertilization in agriculture has been paid more attention.Application of pig manure as an organic material to farmland can realize resource utilization of waste and increase soil fertility.However,it contains As,which may increase the environmental risk of As.Therefore,exploring the effect of S fertilization and pig manure on the migration and transformation of As in soil-rice system is of great significance for rational fertilization and the safety of agricultural products.In this paper,As in paddy soil was selected as the research object.Indoor simulation and field experiments were conducted and traditional chemical analysis methods combined with high-throughput sequencing technology and fluorescence quantitative PCR were applied to study the influence of S fertilization and pig manure on As speciation transformation,bioavailability and rice uptake.The impact of the two fertilizers on microbial communities and functional genes and the microbiological mechanism of As uptake by rice plant were revealed.The main results of this work were as follows:(1)We demonstrated the effects of sulfur fertilization on As speciation transformation,bioavailability and rice uptake in soil-rice system.Pot experiment was conducted and found that the factors affecting the mobility of As and Fe(Ⅱ)in soil solution of different soils were different.In exogenous As-contaminated soil,both microbial-mediated reductive dissolution and non-biologically-mediated reductive dissolution existed,whereas in actual As-contaminated soils,only microbial mediated reductive dissolution existed.The addition of S fertilization reduced Fe(Ⅱ)and As concentrations in the rhizosphere and non-rhizosphere soil solution,which reduced the mobility of As.As(Ⅲ)and As(V)concentrations in soil solution were also reduced to a certain degree.With the same sulfur content,Na2SO4 was more effective than S0.The addition of S fertilization promoted the growth of rice,increased the yield and to some extent reduced the accumulation of As in rice shoots.(2)The microbial mechanisms of S fertilization on As speciation transformation,bioavailability and rice uptake was clarified.Miseq high-throughput sequencing and qPCR technology was applied in the study and found that the microbial diversity of non-rhizosphere soil was slightly lower than that of the rhizosphere soil.The rhizosphere and non-rhizosphere soil had similar community composition.The addition of S fertilization had little effect on genes involved in As metabolism.The addition of Na2SO4 significantly increased the microorganisms associated with Fe(Ⅲ)and sulfate reduction in the rhizosphere soil,which was consistent with the increase expression of the Fe(Ⅲ)reducing bacteria and the sulfate reducing gene.In the flooded condition,the presence of As(Ⅲ)and the increase of Fe(Ⅱ)and sulfide simultaneously promoted the formation of As sulfide or As-Fe sulfide minerals.Therefore,the concentrations of As(Ⅲ)and Fe(Ⅱ)in the soil solution was reduced,and eventually rice uptake was reduced.(3)The effect of pig manure on soil As bioavailability and rice uptake and the microbial mechanisms were revealed.Field experiment was conducted and found that although containing As,the application of pig manure did not affect the total As in soil.However,more stable As fractions in soil turned to non-stable As fractions.Long-term pig manure application increased the accumulation of As in rice tissues.As in the root was mainly distributed in the epidermis and exodermis,while As distribution in rice mainly distributed in the episperm,aleurone layer and endosperm near aleurone layer.There was no significant change in total As in rice,but the addition of pig manure significantly increased As(Ⅲ)in rice.Long-term pig manure application did not significantly affect the soil microbial diversity,but significantly increased the relative content of Anaeromyxobacter(As resistance with ars/ARC)and Bradyrhizobium(rhizobium)which might promote As(Ⅲ)uptake and yield.It was consistent with the increase of arsC.The addition of pig manure promotes As reduction.(4)We confirmed that microorganisms in root plaque played an important role in influencing As uptake.The root plaque was extracted and we found that the diversity of microbial communities in the root palque was lower than that in the rhizosphere soil,and there was a certain transformation in the composition of microbial communities.The addition of pig manure significantly affected the changes of the microbial community structure and diversity of root plaque,among which the relative contents of genus associated with As,Fe and S cycles like Bradyrhizobium,Burkholderia,Rals tonia,etc.significantly changed.It was consistent with the change of As,Fe and S related functional genes.The addition of pig manure promoted As reduction in root plaque,and inhibited Fe and S reduction.Microorganisms in the root plaque,especially those associated with As,Fe and S are more likely to affect the transformation of As species and rice uptake than those of rhizosphere soils.(5)An agronomic practice based on fertilization and water management was developed.The addition of FeSO4 reduced the As bioavailability under non-flooded condition.The addition of FeSO4 and non-flooded condition significantly accelerated rice growth while ensuring rice survival and significantly decreased As uptake by rice shoots.With the addition of FeSO4,iron plaques were significantly promoted and most of the As were sequestered in iron plaques.The addition FeSO4 and non-flooding condition significantly reduced the translocation of As from roots to above-ground parts which might have been aided by the reduction of As availability in soil,the preventing effect of rice roots,and the formation of more reduced glutathione(GSH).The addition of FeSO4 and non-flooded condition might be feasible for rice cultivation in As-contaminated paddy soil which could improve the growth of rice and reduce As uptake.