| Arsenic（As）in soil is harmful to plant growth and can also hazard human health through the food chain.Arsenic is easy to accumulate in rice,because the continuous flooding environment during rice growth increases the mobility and bioavailability of As in paddy soil.As can be adsorbed and immobilized by iron（hydro）oxides.It is of great significance to study the influencing factors and mechanisms of iron（hydro）oxides on root surface inhibiting the uptake of As by rice.Therefore,the rice pot experiments and incubation experiments were designed to study the influencing factors and mechanisms of water management,phosphate fertilizer application,organic acid on root surface iron（hydro）oxides inhibiting As uptake by rice.The main results are as follows:（1）Iron（hydro）oxides on rice root surface are an important factor to control the accumulation of As in brown rice.According to the experiment,in alkaline paddy soil,the As content on root surface iron（hydro）oxides was significantly positively correlated with Fe content（P<0.05）,and most of As（88-97%）was sequestered by poorly crystalline and crystalline Fe（hydro）oxides.The As immobilization index by iron（hydro）oxides on root surface（IASroot）was 16.8-25.0 mg As/（g Fe）),which was significantly higher than that in rhizosphere（IASrhizo 1.4-2.0 mg As/（g Fe））.The content of As in brown rice was negatively correlated with the As content of iron（hydro）oxides on root surface,but the correlation was not significant（P=0.20）.There was a significant positive correlation between the As content in brown rice and the partition ratio(PRAs=IASroot/IASrhizo)of As immobilization on root surface and in rhizosphere（P<0.05）.Therefore,the As immobilization effect of iron（hydro）oxides on rice root surface is stronger than that of rhizosphere soil,which is an important factor to control the accumulation of As in brown rice.（2）Alternate wetting and drying（AWD）treatment promotes the formation of iron（hydro）oxides on root surface,thus enhances the inhibition effect of iron（hydro）oxides on the uptake of As by rice and reduces the accumulation of As in brown rice.The contents of Fe and As on root surface iron（hydro）oxides in AWD treatments were 32.1 g/kg-39.0 g/kg and 423.4 mg/kg-471.4 mg/kg,which were 9.3%-32.9%and 4.3%-15.9%higher than that in continuous flooding（CF）treatment,respectively.The As content on root surface iron（hydro）oxides was significantly positively correlated with the Fe content of root surface iron（hydro）oxides（P<0.05）,and the As content in brown rice was significantly negatively correlated with the Fe content（P<0.01）.The As content in brown rice in AWD treatments（0.03-0.17 mg/kg）was 43.3%-85.0%lower than that in CF treatment.In addition,AWD treatment promoted the transformation of As in rhizosphere soil from H2O and HCl-extractable As to oxalate and DCB-extractable As,which decreased the release and availability of As in rhizosphere soil.In general,AWD treatment can promote the formation of iron（hydro）oxides on root surface,decrease the bioavailability of As in soil,reduce the transfer of As from the root surface to root,and thus reduce the accumulation of As in brown rice.（3）The application of phosphorus fertilizer is not conducive to the formation of iron（hydro）oxides on root surface,inhibits As immobilization by iron（hydro）oxides on root surface,and increases the accumulation of As in brown rice.The contents of Fe and As on root surface iron（hydro）oxides in phosphorus free and low phosphorus treatments（P0,no phosphorus applied;P75,applied 75 mg P2O5/kg soil）were 33.0 g/kg-34.1g/kg and 387.3mg/kg-401.2mg/kg,which were 15.4%-19.2%and 14.6%-18.7%higher than that in normal phosphorus treatment（P150,applied 150 mg P2O5/kg soil）,respectively.The relative proportions of As（V）on root surface iron（hydro）oxides in P0 and P75 treatments were 89.1%and 82.9%,which were 11.0%and 3.2%higher than that in P150 treatment,respectively.The As content on root surface iron（hydro）oxides was significantly positively correlated with Fe content of root surface iron（hydro）oxides（P<0.01）,and there was a negatively correlation between the As contents in brown rice and As content on root surface iron（hydro）oxides（P>0.05）.The As content in brown rice in P0 and P75 treatments were14.3-28.6%lower than that in P150 treatment.Moreover,the activities of catalase,superoxide dismutase and peroxidase in rice roots in P0 treatment were significantly higher than that in P150 treatment.The abundance of aio A gene in P0 treatment was 73.5%higher than that in P150 treatment,and Rhizobium and Rhodoferax were the dominant aio A at genus level.In general,P0 treatment can increase the root oxidation activity and promote the formation of iron（hydro）oxides on root surface.In addition,P0 treatment can increase the abundance of the aio A gene in the rhizosphere soil,promote the oxidation of As,and enhance the As immobilization by iron（hydro）oxides on root surface,and that in turn can reduce As accumulation in brown rice.（4）Gallic acid,an organic acid in paddy soil,inhibits the immobilization effect of root surface iron（hydro）oxides（ferrihydrite,goethite and hematite）on As.When the p H was 7,the As adsorption amounts by ferrihydrite,goethite and hematite in the presence of gallic acid were 41.7 g/kg,4.4 g/kg and 3.4 g/kg,which were 45.1%,25.5%and 30.2%lower than that in the absence of gallic acid,respectively.The adsorption amount of As（V）by ferrihydrite was 49.8%lower than that of As（III）,while the adsorption amount of As（V）by goethite and hematite were 9.3%and 27.1%higher than that of As（III）,respectively.The As adsorption amount by ferrihydrite,goethite and hematite in the treatment of adding gallic acid before adding As were 9.6%,3.4%and 10.6%lower than that in the treatment of simultaneously adding gallic acid and As.The As amount adsorbed on ferrihydrite,goethite and hematite all decreased with the p H rising from 5 to 9 in the presence of gallic acid.The oxidation efficiency of As（III）by gallic acid increased with the increase of p H,and the highest oxidation efficiency of As（III）at p H 9 was 90.3%.Hydrogen peroxide（H2O2）and semiquinone radicals（SQ-）contributed about 52.6%and 27.1%to the As（III）oxidation at p H 9.The oxidation and competition adsorption of As by gallic acid inhibited the As immobilization by ferrihydrite,and the competition adsorption of As by gallic acid inhibited the As immobilization by goethite and hematite.In conclusion,the iron（hydro）oxides on root surface of rice are an important factor to control the accumulation of As in brown rice.Alternate drying and wetting treatment and low phosphorus treatment can promote the formation of iron（hydro）oxides on root surface of rice,and enhance the resistance of iron（hydro）oxides on the uptake of As by rice.Gallic acid in soil is not conducive to the adsorption and immobilization of As by the iron（hydro）oxides on root surface. |
