Toxicity of OTC to Ipomoea aquatica Forsk. and to microorganisms in a long-term sewage-irrigated farmland soil

Water spinach (Ipomoea aquatic Forsk.) was selected to investigate the effects of oxytetracycline (OTC) on the toxicity of soil contaminated by long-term sewage irrigation. After acute toxicity test in petri dish at nine different OTC-spiked levels for 48 h, the germination rate was found to be generally inhibited in all treatments treated with OTC but the root elongation and activities of several antioxidant enzymes, superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) were either forward or backward stimulated to varying extent. During a 60-day sub-chronic toxicity test by means of a pot experiment, activities of SOD, POD and CAT in both the leaf and root tissue at 25 mg OTC per kg soil (dry weight) and in root tissue at 1 mg OTC per kg soil (dry weight) were significantly different than those in other treatments, which also indicated the higher sensitivity of the root. The foliar photosynthetic rate, stomatal conductance and transpiration rate were all gradually inhibited in spite of elevated water use efficiency under the pressure of the different OTC concentrations, which were highly significant different at 10 mg OTC per kg soil (dry weight). Indices of soil microbial diversity at 4 mg OTC kg⁻¹ soil were significantly different from those of the control, indicating the potential adverse effects of OTC to soil microorganisms. The results suggest that the introduction of OTC could damage both plants and soil microorganisms, and during sub-chronic incubation, the sensitivity of different indices generally followed the order of root tissue antioxidant enzyme activities, soil microbial diversity indices, leaf photosynthesis-related index and leaf tissue enzyme antioxidant activities. In addition, the application of livestock and poultry manure containing pollutants like OTC in farmland soil, especially if the soil has been contaminated before, should be taken more seriously in the context of the current pursuit of increased agricultural production.