Abstract Background Urease inhibitors and nitrification inhibitors delay nitrogen fertilizer transformations in soil to reduce nitrogen losses and increase nitrogen use efficiency. While new inhibitor compounds are constantly being developed, little is known about non-target effects on the soil microbiome. This is the first study to investigate non-target effects of the urease inhibitor 2-NPT and the nitrification inhibitor MPA on the soil microbiome. In addition, the more established nitrification inhibitor DMPP was investigated. Target effects and potential non-target effects on the function and composition of microbial communities in three soils from Germany were assessed. Results Soil microcosms were treated with practically relevant doses of inhibited and non-inhibited fertilizer products. Effects on soil nutrients and enzyme activities from nutrient cycling were analyzed. Gene abundances of bacterial and archaeal amoA as well as 16S rRNA and ITS marker genes were quantified using the QIAcuity nanoplate digital PCR. The bacterial and fungal community compositions were analyzed via amplicon sequencing of 16S rRNA and ITS marker genes. Significant reductions of target enzyme activities were found for 2-NPT and DMPP but not MPA. Effect size of inhibition was soil-dependent. Ammonium and nitrate concentrations were significantly affected by the inhibitors in one of the three soils. The non-target enzyme activities of phosphatase, beta-glucosidase, and arylsulfatase were not affected by the inhibitors. Both nitrification inhibitors primarily targeted bacterial ammonia oxidizers, as bacterial but not archaeal amoA genes were reduced. Overall bacterial and fungal communities were not clearly affected. Observed abundance shifts of soil microorganisms were linked to indirect effects driven by nutrient availability rather than direct effects of the inhibitors. Conclusion Our study suggests that the newer inhibitors 2-NPT and MPA as well as DMPP do not directly affect the function and composition of the soil microbiome in the short-term. Significant target effects of the inhibitors change the availability of mineral N which causes indirect effects with minimal non-target microbial community shifts. Further tests involving other soil organisms and long-term field studies are required to further improve the environmental risk assessment of inhibitors.
Siedt et al. (Fri,) studied this question.