Toxic effects of oxytetracycline and copper, separately or combined, on soil microbial biomasses

The production of commercial livestock and poultry often involves using with antibiotics and feed additives, such as oxytetracycline (OTC) and copper (Cu). These are often excreted into the soil by animal feces; hence, combined pollutants may contaminate the soil. To evaluate single and combined toxic effects of OTC and Cu on the soil ecology, changes in quantities of bacteria, fungi, and actinomycetes in the soil were studied over a 28-d incubation period by a plate count method, microbes numbers counted on days 7, 14, 21, and 28. Abundances of ammonia monooxygenase (amoA) gene expression by ammonia-oxidizing bacteria (AOB) and ammonia-oxidizing archaea (AOA) in soil samples also were tested by real-time polymerase chain reactions (RT-PCRs) on day 21. The results revealed that the numbers of bacteria, fungi and actinomycetes and amoA genes copies of AOA and AOB were reduced seriously by exposure to Cu (1.60 mmol/kg). Similarly, the combined pollution treatments (mole ratios of OTC: Cu was 1:2, 1:8, and 1:32) also had inhibitory effect on bacteria, fungi, and actinomycetes numbers and amoA gene copies of AOA and AOB; the inhibitory rate was on obvious growth trend with the increasing mole ratios. Effects from single OTC pollution were found on bacteria (days 7 and 14), fungi (days 7, 14, 21, and 28), and AOA-amoA gene copies (day 21), with promotion at a low concentration (0.05 mmol/kg) and suppression at higher concentrations (0.2 and 0.8 mmol/kg). Also, numbers of bacteria, fungi, and actinomycetes decreased with longer culture times. Combining OTC and Cu led to a higher inhibition of soil microbes than when either chemical was used alone. However, there was no significant relationship between single and combined toxic chemicals because of their complicated interactions, either antagonistic or synergistic. The results also indicated the sensitivity of bacteria, fungi, actinomycetes on toxic chemicals existed difference and that the AOA were more tolerant than the AOB to these chemicals.