Divergent Effects of Antibiotics on Plants and Microbiota in Soils with Contrasting Humus Content

Despite the large number of scientific studies on the effects of antibiotics on soil microorganisms, little is known about the role played by soil organic matter (humus) in the interaction of antibiotics with microorganisms and plants, including the impacts on respiration and growth rate and the implications for nitrogen metabolism, which is an important factor in soil fertility The aim of this study was to analyze the effects of two widely used antibiotics, tetracycline and streptomycin, on microbiotic activity and plant growth in two soils with dissimilar organic carbon content, at the extremes of the fertility spectrum based on humus content. The study used humus-rich (Corg 5.4%) and humus-poor soils (Corg 1.5%) and measured basal respiration, substrate-induced respiration, nitric oxide emission, germination, and growth of white mustard 3 and 60 days after three progressively increasing doses of antibiotics were applied. Tetracycline was found to impair the ecological function of humus-rich soil by reducing denitrification and compromising soil microbial activity, while the effect of streptomycin on humus-poor soil was to reduce nitrification and soil fertility due to nitrogen escape. Both streptomycin and tetracycline increased the microbial biomass and suppressed the growth of white mustard seeds, which indicates an increase in the allelopathic activity of microorganisms in the soil conditions under the influence of antibiotics and their metabolites. Due to the low sorption of streptomycin in humus-poor soils, it poses a great danger to agricultural production, especially in areas of low fertility. In humus-rich soils, high concentrations of tetracycline caused numerous problems, including death of the crop plants. Thus, the effect of antibiotics as well as the more traditional soil pollutants, such as heavy metals, to a large extent, depends on the humus content of soils.