Mechanisms Of Rice Cadmium Reduction With Organic Fertilizer,Lime And Soil Conditioner Input:A Preliminary Study

Recently,cadmium（Cd）pollution in cultivated land is increasingly serious.Agronomic measures characterized by low cost,easy application,and effective in short-term are widely adopted in the remediation practice of heavy metal-contaminated soils.In this study,totally 12 double rice sites were selected in Zhuzhou County of eastern Hunan Province.Each paddy field site was divided into two plots,one half was the control,and the other half was applied with organic fertilizer,lime or soil inorganic conditioner（N+P₂O₅+K₂O=8%、Ca O≥20%、Mg O≥12%、Si O₂≥12%）.This study aimed at exploring the key mechanisms of rice Cd reduction following the application of organic fertilizer,lime and soil conditioner in double rice fields.During different rice growing stages（seedling,tillering,booting,heading,and yellow maturity stage）,customed bamboo soil sampler was used to collect soil samples at 0-15 cm tillage layer,and the soil p H and soil available Cd content were also measured.Soil chemical and microbiological properties were also analysis in the laboratory to explore soil controls of rice Cd.The results show that:（1）During the 4-5 years（2018-2019）of continuous application of organic fertilizer,lime and soil conditioner for double-season rice,the Cd content of early rice brown rice decreased by 40%-57%,51%-64%and 56%-84%;the Cd content of late rice brown rice decreased by 43%-60%,70%-72%and 53%-62%.Applying organic fertilizer,lime and soil conditioner,the double-season rice brown rice Cd content is less than 0.2 mg kg^-1（GB2762-2017）for lime treatment（2018 late rice-2019 early rice and late rice）and soil conditioner treatment（2018-2019）.Compared with the control,applying organic fertilizer,lime and soil conditioner,the available Cd content of the double-season rice decreased by8%,12%,and 14%on average at all growth stages;overall,the available Cd content of the double-season rice soil was tillering stage（decrease by 8%,18%and 14%）and heading stage（decrease by 10%,13%and 16%）,the decrease was most obvious;however,the available Cd content of the soil showed an upward trend during the yellow ripening period.Regression analysis showed that the Cd content of brown rice in double-season rice had a significant positive correlation with the available Cd content in the soil at the tillering stage and the full panicle stage.The application of organic fertilizer,lime and soil conditioner all reduced the Cd content of various parts of rice（root system,stem and leaves,rice husk and brown rice）.In comparison,when organic fertilizer,lime and soil conditioner were applied,Cd content decreased by 29%,26%and 48%in root,46%,40%and 66%in stem and leaf,64%,69%and 60%in rice husk,and 60%,71%and 68%in brown rice.（2）The analysis of the chemical and biological properties of the double-rice rhizosphere and non-rhizosphere soils showed that,compared with the control,the application of organic fertilizers,lime and soil conditioners can reduce the effective Cd content in the rhizosphere and non-rhizosphere soils.The available Cd content in the soil decreased by 18%,9%and 23%,and the available Cd content in the non-rhizosphere soil decreased by 9%,14%and 5%;the application of organic fertilizer caused the p H value of the rhizosphere soil to decrease by 0.17 units.In addition,the p H values of the rhizosphere and non-rhizosphere soils increased to different degrees;in general,the total mineral nutrients and available nutrients（N,P,K）in the rhizosphere soil were higher than those in the non-rhizosphere;The systematic soil microbial community analysis showed that compared with the control soil,the total amount of microbial PLFA in the rhizosphere and non-rhizosphere soils of the three treatments and their control were very different,and the absolute content of PLFA in the rhizosphere soil(between 109.41-228.36 nmol g^-1)Compared with non-rhizosphere soil microorganisms,the absolute content of PLFA(between 80.57-138.37 nmol g^-1)is more abundant;in the rhizosphere and non-rhizosphere soil microbial communities,the bacterial content(between 40.91-89.19 nmol g^-1)highest,P of methane-oxidizing bacteria The absolute content of LFA(between 1.44-12.78 nmol g^-1)is higher than that of sulfate-reducing bacteria PLFA(between 0.39-6.39 nmol g^-1)However,after applying lime and soil conditioner,the absolute content of soil microbial groups PLFA decreased.（3）The effect of the soil properties of the double-season rice field ecosystem on soil Cd-rice Cd traits showed that the application of organic fertilizers,lime and soil conditioners can improve soil soluble organic carbon,particulate organic carbon,light group organic carbon,and easy Oxidized organic carbon and microbial biomass carbon content;of which,when applying organic fertilizer,the dissolved organic carbon content increased by21%,the coarse particulate organic carbon content increased by 107%,the fine particulate organic carbon content increased by 77%,and the light group organic carbon content Increased by 53%,iron（aluminum）bonded organic carbon content increased by 19%,and calcium bonded organic carbon content decreased by 27%;coarse lime organic carbon content increased by 136%,fine organic carbon content increased by lime application 27%,light group organic carbon content increased by 60%,iron（aluminum）bonded organic carbon content decreased by 13%;the application of soil conditioner,coarse particulate organic carbon content increased by 57%,fine particulate organic carbon increased by 68%,light Group organic carbon content increased by 51%,calcium bonded organic carbon content increased by 34%,and soil microbial biomass carbon content increased by 20%.Applying organic fertilizer application,soil urease and soil phosphatase activities increased by 29%and 8%,and catalase activity decreased by 14%;when lime was applied,soil urease activity increased by 23%,soil catalase activity decreased by 11%,and soil phosphate The enzyme activity has no significant effect.The application of soil conditioner increased soil phosphatase activity by 10%.Correlation analysis showed that the content of brown rice Cd and soil available Cd was negatively correlated with soil active organic matter components.With the application of organic fertilizer,the Cd content of brown rice decreased with the increase of the amorphous iron content of the soil,showing a negative correlation.With the application of lime and soil conditioner,the Cd content of brown rice decreased with the decrease of the complexed iron content,which was extremely significant Correlation;application of organic fertilizers,brown rice Cd and soil available Cd content were significantly positively correlated with catalase activity.