Effects Of Trichlorfon On Phosphorus Transformation At The Soil-Maize Rhizosphere Interface And The Structural Characteristics Of Its Degrading Bacteria

Trichlorfon（TCF）is a broad-spectrum organophosphorus pesticide widely used for its high efficiency and low toxicity to control various crop pests.As an organic compound containing phosphorus,and the phosphorus contained in TCF and its degradation products exists in the environment through geochemical processes,influencing the accumulation and cycling of phosphorus in soil and nearby surface water.Meanwhile,TCF in soil may also be absorbed by roots and accumulated in plants through a process of bioenrichment,causing accumulation of pesticides and affecting the food safety of crops.Current study mostly focused on degrading bacteria and biotoxicity of TCF,while its effects on soil phosphorus and its absorption and transport in crops are not clear at present.Therefore,this study investigated the effects of TCF on soil phosphorus fractions and soil bacterial community in the process of degradation,transport in soil-maize interface and the response variation of soil bacterial community to clarify the soil ecological effect of TCF,which help to provide theoretical guidance for the reasonable application and soil decontamination of TCF in agriculture.The main results are as follows:（1）TCF degraded rapidly in soil,and the main degradation products were dichlorvos,2-chloroethanol,dimethyl phosphate and dichloroacetone,TCF could be initially hydrolyzed to dichlorvos via C-O and C-Cl bond,and then dimethyl phosphate and other phosphorus-containing degradation products were generated through the cleavage P-C bond;the addition of TCF increased the content of soil available phosphorus from 2.76 mg/kg（control）to 3.23 mg/kg（TCF-50）,5.12 mg/kg（TCF-100）and 5.72 mg/kg（TCF-200）,respectively;inorganic CaCl₂-P could be easily converted to primary mineral inorganic phosphorus（HCl-P and citrate-P）,while the proportion of enzyme-P was not changed abruptly throughout TCF degradation.（2）There were significant changes in soil bacterial genera,among which the relative abundance of Arthrobacter,Streptomyces and Micromonspora belonging to genera of Actinobacteria,increased significantly during TCF degradation process;spearman correlation and redundancy analysis showed that the relative abundance of Arthrobacter and Streptomyces was significantly negatively correlated with enzyme-P（organic phosphorus）,while significantly positively correlated with HCl-P and citrate-P in stable inorganic phosphorus forms（p<0.05）;the results suggested that soil available phosphorus concentrations and inorganic phosphorus fractions had significant correlations with the relative abundance of Actinobacteria,Actinobacteria could use TCF as a source of phosphorus to increase its community richness,thus affecting the transformation of phosphorus fractions and regulating soil phosphorus cycle.（3）In the control soil without maize seedling,the degradation rates of TCF in 50,100 and 200 mg/kg treatments were 28.1%,50.1%and 43.6%,respectively,after 216h of incubation,while in planted soil with maize seedling,the degradation rates were73.2%,66.3%and 87.4%,respectively;residual TCF in soil was absorbed by maize seedling roots and transported upward to shoots and leaves,while TCF mainly accumulated in roots due to its weak upward transport ability;the accumulation and distribution of TCF in maize seedling showed that residual TCF in soil and maize seedling would not affect the growth of maize seedling in the short term;TCF spraying and maize seedling planting both affected the diversity of bacterial community in maize seedling rhizosphere and bulk soils,which caused the increase of Proteobacteria and the decrease of Actinobacteria;the principal component analysis,linear discriminant LEfSe analysis and correlation network analysis indicated that Kaistobacter,Burkholderia,Streptomyces,Sinomonas and other bacteria were enriched in maize seedling rhizosphere,whereas Chloroflexi were enriched in the bulk soil.The bacteria enriched in the rhizosphere belonging to plant growth-promoting bacteria and their enrichment characteristics would benefit the growth of maize seedling and the degradation of TCF in the maize seedling rhizosphere.