Common and management-specific responses of grassland soil bacterial communities to manure and chlortetracycline amendments

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Manure application to soil affects the composition of soil bacterial communities as well as levels of antibiotic resistance genes in a manner dependent on various environmental and agricultural factors. These effects have been widely studied in arable soils, while the situation in grasslands that also receive inputs of fresh animal manure is largely unknown. Here, we focused on grassland soils under three different management types (pristine meadow, pasture, wintering pasture) from the same locality and compared the response of their bacterial communities to fresh cattle manure spiked or not with chlortetracycline. Soil microcosms were established either with soils (controls), + manure, + manure and chlortetracycline (CTC, 0.2 mg kg−1), and sampled at the same day (t0) and after 28 days (t28). Bacterial community composition was assessed by 454-pyrosequencing of 16S rRNA genes and correlations between the abundance of bacterial taxa and chosen tetracycline resistance genes (measured in our previous study) were calculated. Control soil bacterial communities at t0 differed significantly, reflecting differences in soil management history. Despite these initial differences, the responses of soil bacterial communities to cattle manure amendment showed similar trends in all three soils. Fresh cattle manure amendments significantly modified bacterial community structure in all soils at t0, increasing the relative abundance of OTUs affiliated to Clostridia and Bacilli. At 28d, all manure-amended soils showed a partial resilience, though they remained significantly different from controls. The effect of additional CTC was comparatively lower and only significant at t0 in all three soils. Several members of Firmicutes showed strong positive correlation with the abundance of tetracycline resistance genes tet(O), tet(Q) and tet(W), indicating that these taxa are potentially involved in dissemination of tetracycline resistance in manure-treated soils. In addition, this study indicated certain soil-management specific taxa, such as Geothrix and uncultured Burkholderiaceae, which could explain the previously observed differences in persistence of tetracycline resistance genes among soils.