Abstract
Oxytetracycline (OTC) and sulfamethoxazole (SMX) are two of most widely used antibiotics in livestock and poultry industry. After consumption of antibiotics, a major portion of these compounds is excreted through the feces and urine of animals. Land application of antibiotic-treated animal wastes has caused increasing concern about their adverse effects on ecosystem health. In this regard, inconsistent results have been reported regarding the effects of antibiotics on soil microbial activities. This study was conducted based on the completely randomized design to the measure microbial biomass carbon, cumulative respiration and iron (III) reduction bioassays. Concentrations of OTC and SMX including 0, 1, 10, 25, 50, and 100 mg/kg were spiked in triplicate to a sandy loam soil and incubated for 21 days at 25°C. Results showed that the effects of OTC and SMX antibiotics on cumulative respiration and microbial biomass carbon were different. SMX antibiotic significantly affected soil microbial biomass carbon and cumulative respiration at different treatments compared to control with increasing incubation time. OTC antibiotic, on the other hand, negatively affected cumulative respiration compared to control treatment throughout the incubation period. Although OTC antibiotic positively affected microbial biomass carbon at day one of incubation, there was no clear trend in microbial biomass carbon between different treatments of this antibiotic after that time period. Nevertheless, sulfamethoxazole and oxytetracycline antibiotics had similar effects on iron (III) reduction such that they considerably affected iron (III) reduction at 1 and 10 mg/kg, and iron (III) reduction was completely inhibited at concentrations above 10 mg/kg. Hence, according to our results, microbial biomass carbon and cumulative respiration experiments are not able alone to exhibit the effect of antibiotics on soil microbial activity, but combination of these two experiments with iron (III) reduction test could well display the effects of sulfamethoxazole (SMX) and oxytetracycline (OTC) antibiotics on soil biochemical activities.
Highlights
Application of antibiotics is increasingly widespread in the livestock industry to facilitate animal growth and production and for the prevention of infectious diseases
Different studies have shown that bacteriostatic antibiotics such as SMX have adverse effects on soil bacteria, while their adverse effects have not been observed on fungal activity [28]
Cumulative respiration, and iron (III) reduction tests were examined on soil treated with increasing concentrations of oxytetracycline (OTC) and sulfamethoxazole (SMX) antibiotics
Summary
Application of antibiotics is increasingly widespread in the livestock industry to facilitate animal growth and production and for the prevention of infectious diseases. Antibiotics may either get stabilized on soil matrix or be degraded by photohydrolysis or may undergo biotransformation [4]. Stabilization on soil matrix provides more stability to antibiotics by preventing degradation and result in longer persistence of these compounds in soil. It may impose environmental risks and be a potential hazard to soil microbiota which in turn may affect soil and human health [6]. Chander et al [7] found that tetracycline (TC) bonded to soil particles retained its antibiotic activity and inhibited the growth of Escherichia coli, and sensitive and resistant strains of Salmonella bacteria. Resistant Salmonella is inherently resistant; it showed lesser decline in growth compared to sensitive Salmonella and E. coli
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