On-target and off-target effects of the nitrification inhibitor 3,4-dimethylpyrazole phosphate (DMPP) on the soil microbial community under contrasting soil pH

DMPP (3,4-dimethylpyrazole-phosphate) is a leading nitrification inhibitor (NI). Recent in vitro studies suggested its selective activity against soil ammonia-oxidizing bacteria (AOB). Beyond on-target activity, the effects of DMPP on other functionally related microbial groups, and on the soil prokaryotic and fungal community remain unknown. We assessed the impact of DMPP, applied at two doses (low: recommended for agronomic use and high: >100x the recommended), on the function, diversity, and dynamics of target microorganisms (ammonia-oxidizing microorganisms, AOM), functionally associated microorganisms (nitrite-oxidizing bacteria (NOB) and denitrifiers), and on the prokaryotic and fungal microbial community. Effects were assessed in two agricultural soils of contrasting pH (acidic vs. alkaline), known to drive AOM niche differentiation and possibly DMPP efficiency. DMPP showed a dose-dependent dissipation pattern and higher persistence in the acidic soil. The low dose of DMPP successfully inhibited nitrification in the alkaline but not in the acidic soil, where effective inhibition was observed only at the high dose. This was attributed to its consistently higher activity towards AOB prevailing in the alkaline soil, as verified by q-PCR and RT-q-PCR, unlike AOA whose abundance and transcriptional activity was reduced only by the high DMPP dose. DMPP reduced the abundance of Nitrobacter NOB, but not Nitrospira, only at the high dose, while no consistent dose-dependent effects on denitrifying bacteria were observed. Amplicon sequencing revealed a significant effect of DMPP on the diversity of prokaryotic, fungal and AOB communities in both soils unlike AOA which, in line with functional data, were less responsive. DMPP induced dose-dependent changes in the abundance of bacteria and fungi known to control key soil functions implying possible effects for the soil ecosystem homeostasis. Our study paves the way for a more comprehensive analysis of the effects of NIs on the soil microbial community, beyond the current focus on target AOM.