Drying and rewetting events change the response pattern of nitrifiers but not of denitrifiers to the application of manure containing antibiotic in soil

• Effects of sulfadiazine on N turnover are minor when compared to those caused by the manure application.
• Variations on soil moisture drive the availability and intensify effects the of sulfadiazine on soil nitrification.

Application of manure for soil fertilization is a common practice in arable and pasture soils. As antibiotics are regularly used in animal husbandry, these compounds often enter the soil via manure application. The effects of antibiotics on microbial communities, however, might depend on soil moisture, as water availability may influence both the fate of the compound and the activity of the microbes. To test this hypothesis, we investigated the effects of the application of manure containing the antibiotic sulfadiazine (SDZ) on the abundance and activity of nitrifies and denitrifies in soil, based on the copy number of marker genes and their related potential activities, as affected by different moisture regimes. We observed significant effects of SDZ on potential denitrification activity, but those were not influenced by the soil moisture regime. Nevertheless, neither SDZ nor changes on moisture significantly affected the abundance of denitrifies. In contrast, both potential nitrification activity and abundance of ammonia oxidizing bacteria were significantly affected by the application of manure containing SDZ and moisture regime. Interestingly, no effects were observed for ammonia oxidizing archaea. Overall, our data show that soil moisture modulates the effects of antibiotics in soil microbial communities, and we recommend to include this parameter in the risk assessment of new chemicals.