Effects Of Microbial Iron Oxidation On Soil Cadmium Morphology And Its Transport In Rice

In recent years,the phenomenon of excessive heavy metals in paddy soil and rice in China has become increasingly serious.Cadmium is one of the most harmful heavy metal elements in soils.Iron,the fourth most abundant element in the Earth’s crust,is extremely abundant in the soil.The changes in the form of soil iron oxides play a crucial role in regulating the availability of cadmium.The purpose of this study was to investigate the effect of microbial iron oxidation on cadmium activity in paddy soil and its transfer in rice.Firstly,a soil incubation experiment was conducted with paddy soil with relatively low iron content.By adding biochar,Chlorella vulgaris,and cadmium,the effects of different forms of iron content and cadmium availability were studied,and the treatment with optimal effect was selected.Zhenjiang paddy soil with relatively high iron content was used to further investigate the influences of iron oxide formation on soil cadmium activity(at the upper soil layer with iron oxide present and lower layer without obviousiron oxide formation),by comparing the contents of different forms of iron,soil available cadmium and the shift of soil bacterial community.Finally,a rice pot experiment was set up to study the formation of iron oxides on rice roots and cadmium transfer in rice plant.Soil samples were collected during the tillering and ripening stages for analyses of basic physical and chemical properties,iron oxide and cadmium contents,as well as soil bacterial community structure.The cadmium transfer in rice was determined at harvest.The main results are as follows:1.The growth of three strains of iron oxidizing bacteria screened from rice paddy soil in Zhenjiang River varied greatly on different media.The iron in the media affected the color of the bacterial colonies on solid plates,appearing white in iron deficient media and light yellow or grayish brown in iron containing media;Compared with the blank,all three strains of bacteria promoted iron oxidation,with the most significant effect within 0-12 hours and then slowing down.Within 36 hours,the iron oxidation efficiency followed the sequence of 2#>1#>3#;2.After incubation,a rusty soil layer with a high concentration of iron oxide was formed on the surface.This layer contained higher concentration cadmium than the lower part.Adding 5%biochar to a simulated cadmium soil with a concentration of 3 mg·kg-1 had the best immobilization effect on soil cadmium;3.In the incubation experiment,the content of available cadmium in the soil was significantly negatively correlated with the content of Fe2+,Fe3+,Feo and Fed.The iron oxide content of Zhenjiang soil was higher,with better immobilization effect of than Yangzhou soil with lower iron content.The addition of 5%biochar increased the immobilization efficiency of iron oxide on cadmium;4.In soil incubation experiments,the relative abundance of Firmicutes in cadmium contaminated soil was significantly higher than that in original soil.After adding 5%biochar to cadmium contaminated soil,this difference would decrease.Adding biochar alone to the original soil also lead an increase in the relative abundance of Firmicutes;5.Cadmium had a significant inhibitory effect on rice growth,along with decrease in plant height,biomass,tiller number,and chlorophyll content.Biochar addition could reduce the toxic effect of cadmium on rice to some extents,and significantly increased the content of soil organic matter.It also improved soil nutrient content,promoted plant nutrient absorption,and provided favorable conditions for plant growth and development;6.The cadmium content in rice ranged from high to low,followed by roots,stems,leaves,and grains.Compared with the control group,the addition of biochar significantly increased the content of iron plaque on the root surface of rice.The content of iron film on the root surface was significantly positively correlated with the cadmium content in the iron film and the cadmium content in the roots;The increase in iron plaque content hindered the transportation of cadmium from rice roots to stems,leaves,and grains.The iron plaque content on the root surface was significantly positively correlated with Proteobacteria and Bacillus,and with norank_f_Anaerolinaceae showed a significant negative correlation;In rice pot experiment,adding biochar improved the bacterial diversity of the soil,and the dominant bacteria in the soil are Chloroflexi,Actinobacteriota,and Acidobacteria.