Vegetable Regulation Pathway Of Nitrogen Cycle In Enrofloxacin Contaminated Soil

Nutrient cycling in agroecosystems has always been of great concern,and the nitrogen cycle,as an important component,is concerned with the dynamic balance of soil ecosystems.Soil nitrogen transformation mainly depends on soil microorganisms,and antibiotics,as one of the new agricultural surface source pollution,will disturb the community structure and function of soil microorganisms after entering the soil,so the antibiotic residues in the soil will have an impact on soil nitrogen transformation.There is an interaction between plant cultivation and soil environment,and this mutualistic relationship further affects the migration transformation and ecotoxicity of antibiotics in soil.In this study,we selected small powdered soils from southern vegetable fields to study the effect of vegetable-enrofloxacin interactions on soil nitrogen cycling under pot test conditions,to clarify the effects of different concentrations of enrofloxacin stress on soil nitrogen transformation,and to analyze the changes in soil physicochemical and biological environment by different vegetable cultivation methods（no cultivation;growing radish;growing pakchoi）and their role in regulating the nitrogen cycle of enrofloxacin-contaminated soils.This will provide a reference for the environmental risk assessment of antibiotics in actual vegetable fields and provide a theoretical basis for phytoremediation of antibiotic contamination in soil.The main findings are as follows.（1）Dynamic monitoring of soil nitrogen morphology changes and nitrous oxide（N₂O）release patterns during the pot experiment.By comparing the N₂O release patterns between different concentrations of enrofloxacin treatments,we found that N₂O release increased with increasing enrofloxacin residual concentrations in the soil;compared with bare soil,soil N₂O release was lower in packchoi and radish cultivation,with radish cultivation playing a better role in reducing soil N₂O release than pakchoi cultivation.Increasing enrofloxacin concentration reduced the ammonium-nitrate ratio and promoted nitrate-nitrogen accumulation in bare soil,but this phenomenon was mitigated in radish and pakchoi soils.Overall,cultivation of radish and pakchoi helped to reduce N₂O release and nitrate-nitrogen accumulation in enrofloxacin-contaminated soils.（2）Comparing the degradation behavior of enrofloxacin in different cultivated soils,it is clear that pakchoi and radish cultivation promoted the degradation of enrofloxacin in the low concentration treatment soils,with the promotion effect of pakchoi being slightly better than that of radish in the low concentration treatment;while only radish cultivation achieved a similar promotion effect in the high concentration treatment.The alteration of basic soil physicochemical properties by enrofloxacin and vegetable cultivation was mainly manifested at the early stage of exposure（7 days）,e.g.,lowering of soil p H.Thus,vegetable cultivation may alter the soil physicochemical environment,including accelerating soil antibiotic removal.（3）High-throughput sequencing based on 16S rRNA fragments was used to analyze the role of vegetable cultivation and enrofloxacin contamination on the composition and function of soil bacterial communities.Differences in soil bacterial community structure between different enrofloxacin concentration treatments reached significant levels in bare soils,but changes in enrofloxacin concentration did not significantly affect the soil bacterial community structure of pakchoi and radish.Further analysis revealed that soil enrofloxacin-tolerant bacteria tended to be distributed in the Acidobacteria phylum（Acidobacteria）,but pakchoi and radish cultivation induced the emergence of more different types of bacterial phylum enrofloxacin-tolerant bacteria.FAPROTAX functional predictions subsequently indicated that the relative abundance of bacterial functional groups associated with nitrogen catabolism anabolism in bare soil increased with enrofloxacin concentration,whereas the same phenomenon was not observed in pakchoi and radish soils.（4）Using fluorescence quantitative PCR to analyze the relative abundance of genes related to the nitrogen cycle,we found that the addition of enrofloxacin significantly affected the composition of microbial nitrogen conversion genes in bare soil,and the increase of enrofloxacin concentration increased the copy number of hao,nxrA,narG,nirK,nirS and norB genes in bare soil and decreased the relative abundance of nos Z,an oxidative nitrite reductase gene that can reduce N₂O to N₂,Enrofloxacin did not significantly affect the nitrogen conversion gene composition of radish and pakchoi soils,but in terms of relative gene abundance,increased enrofloxacin concentration decreased narG,nxrA,nirK,and norB and increased nos Z relative abundance in pakchoi soil.（5）With the help of Spearman’s rank correlation analysis and structural equation modeling,we explored the influence mechanism of N₂O release in soil,and the results showed that vegetable cultivation,soil physicochemical properties,bacterial community structure and nitrogen cycle-related genes could directly and significantly influence N₂O release,with bacterial community structure having the greatest influence,followed by nitrogen cycle-related genes.N₂O release was mainly negatively correlated with Bacillus phylum（Gemmatimonadetes）,nos Z,etc.were negatively correlated.The results of the previous studies together suggest that vegetable cultivation mitigates enrofloxacin-induced N₂O release by reducing the effects of enrofloxacin on soil bacterial community structure and nitrogen cycle-related genes.