Fate of the nitrification inhibitor 3,4-dimethylpyrazole in sandy arable soils: Low leachability mainly due to sorption in topsoil

Despite its widespread usage in agriculture, the environmental fate of 3,4-dimethylpyrazole (DMP), the active part of the nitrification inhibitor DMPP, is relatively little investigated, especially with regards to the risk of leaching. We therefore used a combination of batch, column and lab studies to determine the fate of DMP and its degradation product 3-methylpyrazole-4-carboxylic acid (3-MPA) on sandy soils at conditions representative for a temperate climate such as that found in Northern Europe. DMP was degraded fastest in non-sterilized topsoil and slowest in sterilized subsoil. Manure increased degradation rates, but the degradation leveled off over time, leaving at least 5% after 5-12 months in both lab and field experiments. At environmentally realistic concentrations, DMP showed a relatively high sorption to topsoil, which increased by aging. Sorption cannot, however, be expected to retard the leaching of DMP much once it has reached the subsoil where degradation was also slow. The leaching potential was confirmed by the detection of low concentrations of DMP in column leachates and in pore water just above the groundwater table in the field, nonetheless, most of the non-degraded DMP remained in the upper 10 cm. The degradation product 3-MPA was quickly formed after application of DMPP, but leached in very low amounts, probably due to degradation in the topsoil. All in all, leaching of DMP and its transformation product 3-MPA seems to be low in sandy arable soil with plug flow conditions, while additional studies in other soil types are needed to evaluate if DMP can leach in soils where for example preferential flow conditions are more prevailing.