Effects Of Root Exudates Of Brassica Parachinensis With High/Low Accumulating Ciprofloxacin On Soil Environmental Behavior Of Ciprofloxacin

As a kind of emerging pollutants, antibiotics have been extensively studied in milk, eggsand meat, but very few studies have been focused on the environmental behavior and fate ofantibiotics in soil-plant system. Systematical study about the effects of root exudates on theenvironmental behavior of antibiotics in soils and its mechanism had important theoretical andpractical significance for promoting the development of agriculture and ensuring the safety andsecurity of agricultural products. In our previousstudy, two cultivars of Brassica parachinensis,namely Youqingsijiu and Youlvcutai, were screened with high-and low-accumulating quinolones,respectively. So, in this study, the two cultivars of Brassica parachinensis were grown usedhydroponic systems. Ciprofloxacin (CIP) was selected as a representative quinolone compounddue to their frequent and high detection in soil. Hydroponic experiment was used to investigatethe chemical compositions (including low molecular organic acids and organic compounds) andtheir contents of root exudates excreted from the two cultivars of Brassica parachinensis. Then,indoor experiments was conducted to study the effects and mechanism of low molecular organicacids on the sorption, desorption and degradation of CIP in soil. Thus, the mechanism of CIPhigh-accumulation and low-accumulation cultivars was explained from a perspective of the rootexudates. The main results are shown as follows:(1) The concentration of maleic acid (6.47~52.27mg/L) was significantly higher than otherkinds of low molecular organic acids, and that from cultivar Youqingsijiu (high-accumulatin CIP)was higher significantly than that from cultivar Youlvcutai (low-accumulating CIP).(2) The adsorption process of CIP was consistent with pseudo-second-order kinetics, andthe adsorption equilibrium time was8h. With adding maleic acid, Freundlich model was betterto describe adsorption isotherms of CIP on soil than Langmuir model. The CIP adsorption on soilwas physical adsorption. Moreover, low molecular organic acids could promote CIP adsorptionon soil at low concentrations but inhibit at high concentration. Furthermore, suppressioncapabilities of maleic acid were higher than mixed acid (ARE) for CIP adsorption on soil. Lowmolecular organic acids could influence the adsorption of CIP by changing pH value of thesolution and then changing the charge state of both soil surface and CIP molecule. Moreover,low molecular organic acids could change the concentrations of metal ions in the solution and then decrease the sorption of CIP on soil, especially for the ions of Cu2+and Zn2+(both bridgingion).(3) Low molecular organic acids at low concentrations could inhibit desorption of CIP fromsoil, but that at high concentration promote desorption of CIP. The mechanism of low molecularorganic acids to CIP desorption is similar to the process of sorption.(4) After incubating the soil added both CIP and differerent concentrations of maleic acid,the residual concentrations of CIP in soil significantly decreased over time, which met with thefirst-order kinetics equation. In the treatment with0.5g/L of maleic acid, the degradation rateconstant and half-time of CIP were0.0216and32.09days, respectively, and the degradation ratewas60.49%after incubation of40days, which were significantly different from thecorresponding values (rate constant:0.0161~0.0185; half-time:37.47~43.06d) of the othertreatments (with other concentrations of maleic acid). After incubation of40days, the residualweight of CIP was30.26mg/kg andWe could conclude that maleic acid had little effects on thedegradation of CIP in soil.The results of T-RFLP analysis showed that the dominant species as well as identifiedspecies of bacteria in the soil added maleic acid changed over time. After incubation for20days,no significant difference was observed among the dominant species of bacteria, identifiedspecies of bacteria and community diversity of bacteria from CIP polluted soil with differentmaleic acid concentration. Therefore, maleic acid had little influence on community diversity ofbacteria, and thus it could not change the degradation rate of CIP in the soil.Overall, maleic acid could decrease significantly the adsorption ability of CIP on soil. Theconcentration of maleic acid in root exudates from cultivar Youqingsijiu (high-accumulating CIP)is higher than that from cultivar Youlvcutai (low-accumulating CIP), thus the former improvedgreater the bioavailability of CIP in soil than the latter, which is more beneficial for uptake CIPby cultivar Youqingsijiu. Obviously, the concentration variation of maleic acid in root exudateswas an important factor of Brassica parachinensis with high/low accumulating CIP.