Effects of the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) on abundance and activity of ammonia oxidizers in soil

Recent evidence from several environments suggest that besides autotrophic ammonia-oxidizing bacteria (AOB), ammonia-oxidizing archaea (AOA) are also able to perform the oxidation of NH₄ ⁺ to NO₂ ⁻, although the relative importance of AOA in nitrification, compared to AOB, and their differential susceptibility to inhibitory compounds remains unclear. Experimental microcosms were set up to evaluate the effect of the addition of the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) combined with a cattle effluent as organic fertilizer on the abundance and expression of ammonia oxidizers, denitrifiers, and non-target microbial populations using reverse transcription–real-time PCR, as well as on the diversity of metabolically active soil bacterial and archaeal communities by terminal restriction fragment length polymorphism. While no significant changes in soil mineral N concentrations or amoA gene copies could be detected between treatments, short-term changes in transcriptional activity revealed that DMPP impaired both bacterial and archaeal amoA mRNA, being significant at every time point for AOB and at one time point for AOA. Our findings revealed that, despite the different cellular biochemistry and metabolism existing between bacteria and archaea domains, DMPP exerts its inhibitory effect against both soil bacterial and archaeal ammonia-oxidizing transcriptional activity.