Abstract: An incubation test was conducted to study the effects of antibiotics oxytetracycline on the microbial number and activity in three soils varied with f
ertility level under the conditions with or without pig manure application. The soils were injected with six concentrations (0, 0.1, 1, 10, 100, and 1000 mg·kg^-1) oxytetracycline, and incubated for 30 d at 25 ℃. Soil samples were collected on the 1st, 4th, and 30th day of incubation to measure the bacterial number, respiratory activity, and enzyme activity. Under no pig manure application, oxytetracycline had little effects on the soil bacterial number and activity. The EC₁₀ values of oxytetracycline based on a 10% decrement of bacterial number, respiratory activity, enzyme activity, and NO₃-N concentration in treatment without pig manure were 36-1000 mg·kg^-1, 20-1000 mg·kg^-1, and 4-1000 mg·kg^-1 for S₁, S₂ and S₃, while those in treat
ment with pig manure application were 2-56 mg·kg^-1, 2-1 mg·kg^-1, and 1-2 mg·kg^-1 for S₁, S₂ and S3, respectively. Soil fertility level had obvious effects on the dose-sponse relations between oxytetracycline and soil microbial number and activity. The effects of oxytetracycline on soil microbial number and activity increased with increasing soil fertility level, and the effects on microbial number and respiratory activity were higher than those on enzyme activity and NO₃-N concentration. The effects of oxytetracycline on soil microbial number and activity were timeepending, being the greatest on the 4th day of incubation. Overall speaking, oxytetracycline could exert temporary inhibition on soil microbes.

Key words: oxytetracycline, soil microbial activity, organic manure, oil fertility