Effects of veterinary medicines on the functional diversity of the microbial biomass in soils
Zusammenfassung der Projektergebnisse
Veterinary antibiotics are used in the animal husbandry for therapeutic purposes and for growth promotion. By the application of manure from treated animals as fertilizer in agriculture, excreted antibiotics and their associated metabolites enter the soil environment. In the present project, we investigated the effects of the antibiotics sulfadiazine, amoxicillin and difloxacin, used in veterinary medicine, on the activity and functional diversity of microbial communities. Thereby a special emphasis was given to nitrogen cycle, due to the relevance of this nutrient to plant growth, crop production and sustainable use of soils. The focus of the study was to test the hypothesis that the fate and effects of antibiotics is highly determined by the i) type of antibiotics, ii) the mode and number of application to soil as well as iii) the abiotic soil properties and the climatic conditions. The degree to which the antibiotics affected microbial functions was to a large extent controlled by the exposure of the soil microbial community to the antibiotics which in turn depended on the compound’s fate. Amoxicillin was so readily degraded that its residues hardly affected microbial community functions. Difloxacin in turn was strongly adsorbed by soil particles, so that it was not bioavailable. Both antibiotics had therefore only low potential to affect microbial processes assessed in the experiments. Larger and lasting effects were only reported for sulfadiazine (SDZ). Yet, since SDZ is a bacteriostatic agent, these effects were only observed following the stimulation of microbial activity by adding manure as a substrate. As expected, the measured effects greatly differed between the investigated soil compartments. While consequences of the application of manure containing antibiotics to microbial communities of bulk soils were relatively low, these were more pronounced in the rhizosphere of maize and clover. Also environmental conditions, like soil moisture, influenced the effect pattern of the applied antibiotics on microbial communities involved in nitrogen transformation. In general, the bacterial communities that carry on the process of nitrification were more sensitive to the application of manure containing antibiotic than those related to denitrification. This might be explained by the fact that denitrification is a facultative process carried out by very diverse group of microorganisms. Yet, nitrification potential was less affected by the antibiotic application due to the functional redundancy between ammonia oxidizing bacterial (AOB) and ammonia oxidizing archaeal (AOA) communities. Repeated applications of SDZ-manure led to progressing changes of the microbial community structure and to long-lasting (permanent) decrease of the soil microbial activity. Yet, as shown for AOB, after many applications the community begins to recover due to the selection of either intrinsically better adapted organisms or development of SDZ resistance. In conclusion, we observed that in order to evaluate the risk of the introduction of antibiotics to soil ecosystems many factors must be considered such as bioavailability of the compound, type of soil, concentration and frequency of application. Besides, the selection of general processes carried out by diverse group of microbes as indicator parameters might mask effects of antibiotics on key soil processes, such as nitrification.
