Occurrence and risk assessment of tetracycline antibiotics in soil from organic vegetable farms in a subtropical city, south China

This study investigated the occurrence of tetracycline antibiotics in soils from different organic vegetable farms in Guangzhou, a subtropical city, South China and evaluated their ecological risk. Four tetracycline compounds (oxytetracycline, tetracycline, chlortetracycline, and doxycycline) were extracted ultrasonically from soil samples (n = 69), with a solid-phase extraction cleanup, and were then measured by high-performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS). The results showed that four compounds were detected in all samples, with the concentrations of the individual compounds ranging from 0.04 to 184.8 mu g/kg (dry weight). The concentrations of tetracycline compounds in the soils from different vegetable farms varied greatly, but their patterns of distribution were similar. Doxycycline was the predominant compound with a mean of 21.87 mu g/kg, followed by chlortetracycline. The concentrations of doxycycline and chlortetracycline in 7.46 % of the samples were higher than the ecotoxic effect trigger value (100 mu g/kg) set by the Steering Committee of Veterinary International Committee on Harmonization. Additionally, the concentrations of tetracyclines in greenhouse soils were significantly lower than those in open-field soils. Risk assessment based on single compound exposure showed that doxycycline could pose medium or high risks. Compared with other studies, the levels of tetracyclines in this study were relatively low. The hypothesis that antibiotic residues in the soil of organic farms fertilized with manure are higher than in the soils of conventional farms was not supported in the area studied due to the high levels of moisture, temperature, and microbial activity.