Effect of sulfadiazine-contaminated pig manure on the abundances of genes and transcripts involved in nitrogen transformation in the root-rhizosphere complexes of maize and clover

Abstract

The antibiotic sulfadiazine (SDZ) can enter the environment by application of manure from antibiotic-treated animals to arable soil. Because antibiotics are explicitly designed to target microorganisms, they likely affect microbes in the soil ecosystem, compromising important soil functions and disturbing processes in nutrient cycles. In a greenhouse experiment, we investigated the impact of sulfadiazine-contaminated pig manure on functional microbial communities involved in key processes of the nitrogen cycle in the root-rhizosphere complexes (RRCs) of maize (Zea mays) and clover (Trifolium alexandrinum). At both the gene and transcript level, we performed real-time PCR using nifH, amoA (in both ammonia-oxidizing bacteria and archaea), nirK, nirS, and nosZ as molecular markers for nitrogen fixation, nitrification, and denitrification. Sampling was performed 10, 20, and 30 days after the application. SDZ affected the abundance pattern of all investigated genes in the RRCs of both plant species (with stronger effects in the RRC of clover) 20 and 30 days after the addition. Surprisingly, effects on the transcript level were less pronounced, which might indicate that parts of the investigated functional groups were tolerant or resistant against SDZ or, as in the case of nifH and clover, have been protected by the nodules.

FIG. 1.

Ammonium and nitrate concentrations in the root-rhizosphere complex of clover (A) and maize (B) after the addition of pig manure (PM) or pig manure plus sulfadiazine (PMSDZ) at three different time points (10, 20, and 30 days) after application. Error bars represent standard deviations of means (n = 4). Abbreviations: RRC, root-rhizosphere complex; bld, below limit of detection.

FIG. 2.

Quantification of functional gene and transcript copies involved in nitrogen cycling (nifH, AOB amoA, AOA amoA, nirK, nirS, and nosZ) in the root-rhizosphere complex of clover (A) and maize (B) after the addition of pig manure (PM) or pig manure plus sulfadiazine (PMSDZ) at three different time points (10, 20, and 30 days) after application. Significant differences between the two treatments at a particular time point are indicated by solid black circles (P < 0.017). Error bars represent standard deviations of means (n = 4). Abbreviations: RRC, root-rhizosphere complex; bld, below limit of detection; AOA, ammonia-oxidizing archaea; AOB, ammonia-oxidizing bacteria.