Soil microbial community responses to sulfadiazine-contaminated manure in different soil microhabitats

Veterinary antibiotics such as sulfadiazine (SDZ) are applied with manure to agricultural soil. Antimicrobial effects of SDZ on soil microbial community structures and functions were reported for homogenized bulk soils. In contrast, field soil is structured. The resulting microhabitats are often hot spots that account for most of the microbial activity and contain strains of different antibiotic sensitivity or resilience. We therefore hypothesize that effects of SDZ are different in diverse soil microhabitats. We combined the results of laboratory and field experiments that evaluated the fate of SDZ and the response of the microbial community in rhizosphere, earthworm burrow, and soil macroaggregate microhabitats. Microbial communities were characterized by phenotypic phospholipid fatty acid (PLFA) and genotypic 16S rRNA gene patterns (DGGE) and other methods. Data was evaluated by principle component analyses followed by two-way ANOVA with post-hoc tests. Extractable SDZ concentrations in rhizosphere soil were not clearly different and varied by a factor 0.7-1.2 from those in bulk soil. In contrast to bulk soil, the extractable SDZ content was two-fold larger in earthworm burrows, which are characterized by a more hydrophobic organic matter along the burrow surface. Also, extractable SDZ was larger by up to factor 2.6 in the macroaggregate surface soil. The rhizosphere effect clearly increased the microbial biomass. Nonetheless, in the 10 mg SDZ kg−1 treatment, the biomass deceased by about 20% to the level of uncontaminated bulk soil. SDZ contamination lowered the total PLFA concentrations by 14% in the rhizosphere and 3% in bulk soil of the field experiment. Structural shifts represented by Pseudomonas DGGE data were larger in SDZ-contaminated earthworm burrows compared to bulk soils. In the laboratory experiment, a functional shift was indicated by a four-fold reduced acid phosphatase activity in SDZ-contaminated burrows compared to bulk soil. Structural and functional shifts after SDZ contamination were larger by a factor of 2.5 in the soil macroaggregate surface versus interior, but this relation reversed over the long-term under field conditions. Overall, the combined effects of soil microhabitat, microbial community composition, and exposure to SDZ influenced the microbial susceptibility towards antibiotics under laboratory and field conditions.