Degradation of the Nitrification Inhibitor 3,4-Dimethylpyrazole Phosphate in Soils: Indication of Chemical Pathways

Nitrogen fertilizers amended with nitrification inhibitors (NIs) are used to increase nitrogen use efficiencies in agricultural systems. 3,4-Dimethylpyrazole phosphate (DMPP) is the most successful commercial NI to date but has a highly variable efficacy. To explore whether degradation could contribute to its inconsistent performance, incubation studies were performed with DMPP and 3,4-dimethylpyrazole glycolate (DMPG) in two alkaline clay soils that were treated with the fertilizer ammonium sulfate ((NH₄)₂SO₄). Analysis of the soil extracts revealed a qualitative correlation between the amount of NI present in the soil and inhibition efficiency as well as several degradation products resulting from the oxidation of a methyl side chain and dimerization. A similar outcome was obtained for the degradation in sterilized soil and in accelerated weathering studies in the absence of soil. Our data suggest that chemical and not microbiological pathways are primarily responsible for the degradation of this inhibitor, which could potentially be initiated by reactive oxygen species (ROS) resulting from both biotic and abiotic processes in soils.