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

Nitrification inhibitors (NIs) like Dicyandiamide (DCD) and 3,4-Dimethylpyrazole phosphate (DMPP) are commonly incorporated into ammonium-based fertilizers in agriculture. However, their repeated use raises concerns about their environmental behavior, health implications, and long-term efficacy. We hypothesized that long-term exposure to DCD and DMPP would promote the development of microbial communities with enhanced catabolic capabilities for NIs or increase the tolerance of ammonia-oxidizing microorganisms (AOM) to NIs, thus challenging the performance of the inhibitors, or slow the dissipation of the NIs, potentially influencing functional microbial groups involved in nitrogen cycling, other biogeochemical cycles (e.g., carbon, phosphorus, and sulfur), and overall microbial diversity. To test this, we conducted a long-term microcosm experiment using agricultural soils with and without prior exposure to NIs, which were repeatedly treated with DCD and DMPP. We observed a gradual deceleration in DMPP dissipation, with sustained inhibitory effects on nitrification and ammonia-oxidizing bacteria (AOB). In contrast, DCD showed accelerated dissipation in the No History soil and a gradual reduction in its inhibitory effects in the History soil, suggesting microbial adaptation mechanisms that could undermine its effectiveness over time. Beyond these direct effects on nitrification, NIs also triggered significant changes in the abundance of nitrite-oxidizing bacteria (NOB) and denitrifiers, as well as in marker genes related to carbon, phosphorus, and sulfur cycling. Furthermore, both DCD and DMPP altered the diversity of AOM, bacterial, and fungal communities. These findings underscore the broader impacts of NIs on microbial networks and biogeochemical processes, highlighting the importance of adopting soil-specific and adaptive management strategies to optimize nitrogen use efficiency (NUE) while preserving microbial diversity and ecosystem health.