Dicyandiamide (DCD) and 3,4-dimethylpyrazole phosphate (DMPP) demonstrate distinct inhibitory activity, dissipation patterns and off-target effects in soils under repeated exposure regimes

Abstract Nitrification inhibitors (NIs) such as dicyandiamide (DCD) and 3,4-dimethylpyrazole phosphate (DMPP) are routinely applied alongside ammonium-based fertilizers. This frequent exposure of agricultural soils to NIs raises concerns about their environmental behavior, health impacts, and long-term efficacy. We hypothesized that extensive soil exposure to DCD and DMPP will (i) favor microbial communities with enhanced catabolic capacities for NIs or sub-populations of ammonia-oxidizing microorganisms (AOM) with increasing tolerance to NIs; both cases challenging NI performance or (ii) lead to deceleration of NIs dissipation with reciprocal effects on functional microbial groups involved in nitrogen cycling (nitrite-oxidizing bacteria (NOB), denitrifiers), other biogeochemical cycles and microbial diversity. We employed a long-term microcosm experiment where agricultural soils, with or without field history of exposure to NIs (“History’ versus “No History”), were repeatedly treated in the laboratory with DCD and DMPP. We observed a deceleration of DMPP dissipation and consistent inhibitory activity on nitrification and AOB. Conversely, DCD showed accelerated dissipation in the “No History” soil and a gradual reduction in its inhibitory activity in the “History” soil, signifying the involvement of microbial adaptation mechanisms potentially compromising its agronomic performance. Beyond on-target effects, NIs significantly affected the abundance of NOB and denitrifiers, as well as marker genes associated with carbon, phosphorus, and sulfur cycling and affected the diversity of AOM, bacterial and fungal communities. These findings highlight the broader impacts of NIs on microbial networks and biogeochemical processes, emphasizing the importance of soil-specific and adaptive management strategies to optimize nitrogen use efficiency while preserving ecosystem health.