Mechanism Of Iron And Manganese Regulating Cadmium Immobilization By Oyster Shell Powder In Soil-rice System

Cd pollution in paddy soil can affects the absorption and accumulation of Cd in rice.Calcareous amendments can reduce the availability of Cd in soil.However,the absorption and transport of Cd in rice can be affected by rice varieties,water conditions and availability of soil mineral elements under the field conditions.In this paper,pot experiment or hydroponic experiment was conducted to study the immobilization effect of some typical calcareous amendments on Cd in soil-rice system and applying Fe and Mn fertilizer on the immobilization effect and its mechanisms.The main results of this study are as follows:（1）In unplanted soil,the addition of lime sharply increased the p H,DOC,and Ca content;decreased the Eh,Fe content,and Mn content;and gradually decreased the water-soluble Cd content in the soil profile.Hydroxyapatite showed no obvious effects in reducing Cd concentrations in different soil zones.The watersoluble Fe,Mn,Ca,and DOC content did not differ significantly between soil zones over time and a non-significant correlation with water-soluble Cd was shown.In planted soil,the p H increased while the Eh value decreased with an increase in the distance from the roots,regardless of the soil amendments used during the rice growth period.The concentration of water-soluble Cd in the rhizosphere was higher than that of the other soil zones during rice growth.Hydroxyapatite gradually increased but lime decreased the water-soluble Cd in the rhizosphere.Compared with hydroxyapatite,lime treatment could significantly reduce the water-soble Cd contents in the rhizosphere soil,the Cd in iron plaque of root surface and reduce the content of Cd in rice tissues.In conclusion,the application of lime can significantly reduce the cumulative risk of Cd uptake by rice under flooding conditions.（2）Oyster shell powder application in Cd contaminated paddy soils could significantly increase the p H,Ca and Mg concentrations of soil pore water,and decrease soil Eh,but could not effectively reduce the Cd content in soil pore water.The contents of Ca Cl₂-Cd and DTPA-Cd in soil were significantly decreased at tillering stage and maturity stage.The accumulation of Cd of the two type rices（Donglian 5 and Zheyou 18）showed an opposite trend.For Donglian 5,increasing the addition of oyster shell powder significantly reduced the content of available Cd in soil and the content of Cd in the iron plaque of root surface,and decreased the absorption of Cd by root and its transport to shoot,and ultimately reduced the accumulation of Cd in brown rice.The contents of Cd,Fe and Mn in rice tissues for Zheyou 18 increased with the increase of adding amount of oyster shell powder.The excessive addition of oyster shell powder（1.2%w/w）could promote the Cd,Fe,and Mn accumulation of in the iron plaque of root surface,which promote the synergistic absorption of Cd,Fe and Mn in roots and brown rice for Zheyou 18.Therefore,it is necessary to consider the bioavailability of Fe and Mn and choose the appropriate amount of oyster shell and rice varieties to reduce the Cd content in rice grains for the remediation of Cd contaminated soil used by oyster shell powder.（3）Adding Fe could increase the contents of Fe and Cd in iron plaque on the rice root surface,and decrease the contents of Cd in stems and leaves under different Cd concentrations in a hydroponic experiment.With the increase of Cd concentration,the amounts of iron plaque on root surface decreased,while the contents of Cd in iron plaque increased,which increased the contents of Cd in rice roots.Cd exposure leads to damage of cell wall structure of rice root,which lead Cd to transport to the root cells,and then increased the root Cd contents.Fe supplementation significantly increased root Fe content,down-regulated the relative expression levels of Os Nramp1 and Os IRT1 in rice roots,which reduced the transfer factors of Cd from root to stem and leaves,and then reduced the accumulation of Cd in shoot and leaves.（4）Under continuous flooding conditions,compared with oyster shell powder application,oyster shell powder supplemented with Fe fertilizer increased the contents of Fe and Cd in root iron plaque,up-regulated the relative expression levels of Os Nramp1 and Os IRT1 in rice roots,promoted the transfer factors of Cd from iron membrane to root,and increased the absorption of Fe and Cd by roots and straw,while Cd content in brown rice could not effectively be reduced.Under alternate flooding and drying conditions,compared with oyster shell powder application,oyster shell powder supplemented with Fe fertilizer promoted the formation of iron plaque on the rice root surface,increased the ability of iron plaque on the root surface to immobilize Cd,down-regulated the relative expression levels of Os IRT1 in rice root,and decreased the transfer coefficients of Cd from iron plaque to root,root to straw and straw to brown rice.Finally,the absorption and accumulation of Cd in straw and brown rice were significantly reduced.Compared with continuous flooding conditions,oyster shell powder supplemented with Fe fertilizer could effectively reduce the content of Cd in brown rice under alternate flooding and drying conditions.（5）Under continuous flooding conditions,compared with oyster shell powder application,oyster shell powder supplemented with 0.005%（w/w）Mn fertilizer increased the accumulation of Mn and Cd in root iron plaque,up-regulated the relative expression level of Os Nramp5 in rice root,promoted the absorption of Mn and Cd by roots and straw,but significantly decreased Cd content in brown rice.However,the transfer of Cd to brown rice and the content of Cd in grains increased with the increase of Mn fertilizer.Under alternate flooding and drying conditions,compared with oyster shell powder application,oyster shell powder supplemented with Mn fertilizer up-regulated the relative expression level of Os Nramp5 gene in rice root,and increased Mn contents in iron plaque,root,straw,and brown rice.Adding 0.005%（w/w）Mn fertilizer showed no significant effect on Cd accumulation in brown rice,while adding 0.01%（w/w）Mn fertilizer increased the transfer of Cd to brown rice,and then significantly increased Cd accumulation in brown rice.Compared with alternate flooding and drying conditions,oyster shell powder supplemented with Mn fertilizer could effectively reduce the content of Cd in brown rice under continuous flooding.