Antibiotics and Antibiotic Resistance Genes in Bulk and Rhizosphere Soils Subject to Manure Amendment and Vegetable Cultivation

There is interest in understanding effects of amending soil with manure in a cultivation setting and if composting can provide benefits. Raw or composted manure from cattle administered with and without sulfamethazine, chlortetracycline, and tylosin was amended to loamy sand and silty clay loam soils, where lettuce (Lactuca sativa L.), radish (Raphanus sativus L.), and broccoli (Brassica oleracea L. var. italica) were cultivated and compared with those grown in soil amended with fertilizer as a control. Upon plant maturation, rhizosphere and bulk soils were analyzed for antibiotics, and sul1, ermB, tet(W), and intI1 genes were quantified. Antibiotic concentrations in compost-amended soils were below detection limits. For soils amended with manure containing antibiotics, sulfamethazine ranged from 1.1 to 3.1 mu g kg(-1) in the bulk soils but was below detection limits in the rhizosphere soils. Chlortetracycline (2.8-9.3 mu g kg(-1)) was two times lower in the rhizosphere than in the bulk soil. Levels of tylosin in the rhizosphere soil were similar to the bulk soil. Soil texture or vegetable type did not have significant influence on antibiotic concentration differences between the bulk and rhizosphere soils. Relative abundances of tet(W) and intI1 in the fertilizer-amended soil were significantly lower than in those amended with manure or compost (p < 0.05), whereas ermB was not detected in any soils. Rhizosphere zone has no significant effect on the detected antibiotic resistance genes. It is suggested that plant roots may have a substantial effect on the fate of certain antibiotics in manure-amended fields, but less of an effect on antibiotic resistance and mobility genes.