Effects Of Manganese On Formation Of Iron And Manganese Oxides In Soiland Accumulation Of Cadmium And Arsenic In Rice

The prevention and control of soil cadmium（Cd）-arsenic（As）composite pollution are urgent environmental problems currently,and the iron（Fe）and manganese（Mn）oxides in the soil are important factors affecting the availability of Cd and As in soil and their accumulation in rice.In this research,pot experiments were conducted to explore the effect of exogenous Mn on the formation of Fe-Mn oxides in paddy soil and their fixation of Cd and As and further explore the mechanism.The treatments setting as follow,different amounts of manganese sulfate（Mn SO₄）were added to form different Mn levels:200 mg·kg^-1,400 mg·kg^-1,600 mg·kg^-1,800 mg·kg^-1（Treatment numbers:CK1、CK2、Mn200、Mn400、Mn600、Mn800）.By analyzing the potting soil p H value;Eh value;the content of Fe,Mn oxide;the content and form of Cd,As;and the change of pore water-related ion content,the effect of Mn on the formation of Fe-Mn oxide and the influence of arsenic and Cd on soil were researched.Furthermore,the effects of manganese application on the REDOX process of Fe and Mn in the rhizosphere and the accumulation of Cd and As in rice were studied through analyzed the content of Fe,Mn,Cd and As in various parts of rice and Fe-Mn film.The results show that:1.The application of manganese sulfate（Mn200、Mn400、Mn600、Mn800）reduced the soil p H value,increased the Eh value,increased the content of various forms of manganese oxides and amorphous iron oxides in the soil,and reduced the content of complexed iron,meanwhile,increased the content of Fe²⁺,Fe³⁺and Mn²⁺in the soil pore water at the early stage of rice growth.Compared with the control（CK2）,Mn400 treatment promoted the formation of iron and manganese oxides in soil.The above results indicate that manganese application promotes the formation of soil iron and manganese oxides.2.Mn SO₄treatment increased the content of exchangeable Cd at the seedling stage;the content of Fe and Mn oxide binding Cd at the heading stage;grout stage and maturation stage;and the content of residual Cd at grout stage,however,decreased exchangeable Cd at the tillering stage;the content of carbonate binding Cd at the maturity stage.With the application of Mn SO₄,the content of As in the specific adsorbed state and the binding state of the crystal iron oxides at the tillering stage,heading stage,filling stage and maturing stage was decreased.While the content of As in the amorphous Fe-Mn oxide binding state at the seedling stage,heading stage,filling stage,and maturing stage and the content of residue As at the seedling stage,filling stage and maturing stage was increased by Mn treatment.Besides,the application of Mn increased the content of Cd and As in amorphous Fe and Mn oxides and decreased the content of As³⁺and As⁵⁺ions in pore water.Compared with CK2,Mn400 treatment,the content of organic matter-sulfide-bound Cd and crystalline Fe oxide-bound As in soil were significantly reduced,although the residual Cd and As had no differences.However,the content of residual Cd and As in the soil treated with CK1 decreased significantly.These results indicate that Mn treatments changed the combined form of Cd and As and reduced their availability in soil.Among them,Mn promoted the formation of Fe-Mn-bound Cd and As,and sulfate promoted the formation of organic-sulfide-bound Cd,crystalline Fe oxide-bound As,and residual Cd and As.3.The application of Mn promoted the content of Fe and Mn but decreased the content of Cd and As in root surface Fe-Mn film.At the mature stage,Fe content in root Fe-Mn films under Mn400,Mn600,and Mn800 treatments increased by 80.76%,80.47%,and 107.99%;Cd content in root Fe-Mn films under Mn400,Mn600,and Mn800 treatments decreased by 48.02%,41.85%,and 51.56%;and As content in the root Fe-Mn films under Mn400,Mn600 and Mn800 treatments decreased by 43.88%,62.50%,and 56.25%,respectively.Compared with CK2,the content of Fe and Mn in Fe-Mn films under Mn400treatment significantly increased,while the Cd content decreased significantly.It showed that the application of manganese promoted the formation of the root surface Fe-Mn film.4.Mn SO₄treatments increased the content of Fe,Mn,As,Cd in each part of the rice,and reduces Cd content in the shoot and As content in both root and shoot.Among them,brown rice Cd content of Mn200,Mn400,Mn600 and Mn800 treatments was significantly decreased by 30.01%,36.27%,36.27%,and46.11%,and brown rice As content of Mn400,Mn600,and Mn800 treatments was reduced by 30.14%,34.25%,and 46.58%respectively.Compared with CK2,the content of Cd and As in brown rice in Mn400 treatment was significantly reduced.Indicated that manganese application could reduce the accumulation of Cd and As in rice.5.Mn SO₄treatments increased plant height at different stages but had no significant effect on biomass and grain yield at different stages.