Effects Of Fe/S Cycling On Arsenic Behavior In Soil-rice System In Wetland Reclamation Areas Of The Yangtze Estuary

In recent years,environmental problems caused by land reclamation in coastal areas have aroused wide attention.As an important wetland reclamation area in the Yangtze Estuary,Chongming Island has become the largest area of agricultural production and green agricultural development in Shanghai.The concept of green development has put forward higher quality requirements for food security.Arsenic（As）pollution in paddy fields would affect rice production and food safety.How to effectively prevent As contamination in paddy soil and ensure the safety of agricultural products has become one of the research hotspots in the environmental field.Many studies focused on the migration and transformation of As in farmland soils.However,few studies considered the effects of Fe/S cycling characteristics on As in the soil-rice system at different reclamation stages in cosatal wetland reclamation areas.Therefore,this study took the soil-rice system in the natural wetland reclamation areas of the Yangtze Estuary as the research object,and judged the distribution and source of heavy metals by one-way analysis of variance,correlation analysis,principal component analysis（PCA）and positive matrix factorization（PMF）model.This study analyzed the influence of Fe/S species on the behavior of As in soil solution with changes in redox conditions by short-term redox simulation experiments.In addition,this research explored the migration and transformation of As-Fe-S between soil and pore water,and the absorption,transport and enrichment of As in rice by rice pot simulation experiments.This study revealed the influence mechanism of Fe/S dynamic redox cycle on As behavior in the natural wetland reclamation areas of the Yangtze Estuary,and provided data support and theoretical basis for the agricultural utilization of reclaimed soil and the safety of agricultural products.The main findings are as follows:（1）The speciation of Fe/S,the distribution and sources of heavy metals of sediment/soil at different reclamation stages in the wetland reclamation areas of the Yangtze Estuary were clarified.The concentrations of heavy metals including Zn,Cu,Ni,Cr,Hg and As in the soil of reclamation area with long-term cultivation history were significantly higher than those of other areas.This may lead to the risk of heavy metal polluttion.Heavy metals in the study areas were mainly affected by soil moisture content,p H,total organic carbon,clay size fraction content and long-term human agricultural activities.The stability of Hg and As was controlled by soil Fe/S species.The oxidation conditions in the enclosure area were stronger,and the adsorption of Fe（III）（hydroxy）oxide was the main factor to improve the stability of Hg.In natural wetland and rice-culitivation areas,the iron sulfide had a stronger fixation effect on As.Through the source analysis of heavy metals,it was found that the sources of Zn,Cu,Ni and As in the soil of study areas were mainly from agricultural activities and natural sources,while Hg and Cr mainly came from surrounding industrial activities.（2）The effect of Fe/S migration and transformation on As behavior in soil solution under short-term redox conditions was revealed.In the simulation experiment,soil As was dominated by residual As.Solution As was dominated by inorganic As and had a significant positive correlation with the concentration of dissolved Fe（III）（p<0.05）.There was an inhibitory effect of SO₄^2-on the release of As in soil.When the soil reducing conditions were the strongest,the concentration of oxidizable As was higher.The soil had the strongest ability to release and reduce iron,thus promoting the release of soil As and the formation of As（III）.Under oxidative conditions,Fe（III）in solution precipitated in the speciation of oxides in soil.Fe（III）（hydroxy）oxide had adsorption and immobilization effects on the reducible As in soil.Exogenous sulfate could promote the methylation of soil As in the period of the reduction-oxidation experiment.In addition to the redox-sensitive elements Fe and S,p H was also an important factor controlling the migration and transformation of As in soil solution.The total As concentration in soil solution was positively correlated with p H（r=0.95,p<0.05）.The release and migration capacity of soil As（III）in the enclosure areas was higher than that in the long-time rice cultivation areas,indicating potential toxic effects.（3）The effects of exogenous sulfur addition and Fe/S variations with rice growth period on the migration,transformation and bioavailability of As in the soil-rice system were elucidated.During the rice growth period,As and Fe showed similar characteristics of speciation transformations in the soil-rice system.The translation of Fe（Ⅱ）to Fe（III）was accompanied by the transformation of As（III）to As（V）with the extension of rice growth time.The heading stage and the tillering stage was the key period for the release of soil Fe and As.From the heading stage to the mature stage,the As（III）concentration in pore water decreased by 50.33%-91.20%,which was related to the absorption of roots iron oxide in soil.The mobility of As in the enclosed soil was higher than that in the long-term paddy soil.Exogenous sulfur input could reduce the migration of As in paddy soil,and reduce the transport,enrichment and accumulation of As in rice.Whatsmore,exogenous sulfur could increase the yield of rice grains.（4）The influence of rhizosphere effects on the migration,transformation and bioavailability of As in the soil-rice system were discussed.The concentration of Fe（II）released from the rice rhizosphere soil was larger than that of the non-rhizosphere soil.This was beneficial to the formation of root iron plaque and the adsorption of As.The concentration of dissolved As in rhizosphere soil was higher than that in non-rhizosphere soil.At the heading stage of rice,As（Ⅲ）in rhizosphere soil solution was most easily absorbed by rice.In the condition of adding sulfate,the concentration of dissolved SO₄^2-in rhizosphere pore water was 5.47 times that in non-rhizosphere pore water.The inhibitory effect of sulfate on the dissolved As in the rhizosphere soil was stronger than that in the non-rhizosphere soil.These results indicated that the key process of reducing As accumulation by exogenous sulfur input mainly occurred in the rice rhizosphere.Therefore,improving the rhizosphere environment may be one of the effective measures to reduce As accumulation in rice.