Influence of two type of pharmaceutical antibiotics on some biological properties of clay and sandy loam soils

During the last few years, the misapplication and overdose of pharmaceutical antibiotics has become a major problem in relation to destruction and pollution of different ecosystems. Soil is one of the biomes that because of receiving animal manures, severely affected by these materials. As respects that soil microbial activities have a direct relationship with the health and soil quality, so in this research the effect of two pharmaceutical antibiotics gentamicin and trimethoprim on microbial activity of clay and sandy loam soils was studied.
Two soils with different texture that had received any manure fertilizer throughout the last decades were sampled (0-25 cm depth). The different concentrations of antibiotics (0.1, 1, 10 and 100 mg L-1) were prepared from the stock solution (1000 mg L-1) and were added to the soil samples. Basal respiration, substrate induced respiration and ferric iron reduction was measured in sampling time (2, 8, 18 and 30). Experiment was done in a completely randomized design with factorial arrangement and three replications. Treatments without antibiotics were considered as control.
in clay soil, respiratory activity in treatments that received gentamicin showed significant difference with control after 2 days. The respiratory activity after 8 days reduced in compared with second day. The basal respiration in 0.1, 1, 10 and 100 mg Kg-1 gentamicin reduced 52, 66, 68 and 84% respectively in compared to control on 8th day. Substrate induced respiration in different incubation times showed a similar trend to basal respiration. Basal respiration in sandy loam in the presence of gentamicin was the lowest after 2 days than any other time and after 8 days again increased. In clay soil receiving 0.1, 1, 10 and 100 mg Kg-1 trimethoprim, basal respiration reduced 51, 55, 59 and 89% respectively, in compared with control on 8th day. In sandy loam soil in the presence of trimethoprim after 2 days, basal and substrate induced respiration showed the lowest amount and all treatments showed the significant difference with control. In clay soil in different levels of both antibiotic, the ratio of Fe(II) of control was higher in second day than 8th day. In sandy loam soil, the most negative effect of antibiotics for ratio of Fe(II) was on second day. Generally, the ferric iron reducing bacteria in soils that receiving trimethoprim has been less affected by the negative impact due to difference in performance between the two antibiotics.
Influence of antibiotics on soil biological activity is dependent on the length of time that soil was exposed to antibiotics and also performance of antibiotic and soil properties. Soil microbial activity accepted negative impact even at low concentrations of antibiotics. In order to reduce the risk of environmental contamination, caution should be exercised regarding the use of antibiotics in human and livestock.