Veni, inhibui, disparui - The Journey of Nitrification and Urease Inhibitors in Soil

Nitrification inhibitors (NI) and urease inhibitors (UI) are substances which are useful to delay nitrification in soil. This delay can have several advantages: nitrate leaching reduction, longer availability of ammonium for plants and slower conversion of urea to ammonia. But everything comes with a price. The release of artificial substances can have negative impacts on the environment such as observed with PFAS or some pesticides and their transformation products. Therefore, it is crucial to know their environmental fate before using it on a large scale.

To learn about the fate of NI and UI, we conducted two soil column studies. In the first study, we applied three fertilizers with different inhibitors on two different topsoils at a concentration of  15 g/m²: ENTEC 26 (3,4-dimethylpyrazole phosphate [DMPP]), ENSIN PLUS (Dicyandiamide [DCD] and 4-amino-1,2,4-triazole [ATC]), Alzon Neo-N (reaction mass of N-((5-Methyl-1H-pyrazol-1-yl)methyl)acetamide, N-((3-Methyl-1H-pyrazol-1-yl)methyl)acetamide [MPA] and N-(2-nitrophenyl)phosphoric triamide [2-NPT]. In the second study we applied the same inhibitors without fertilizer on two different subsoils.

Both studies were conducted under two different temperatures to learn about its impact on transformation rates. In the first study, mass balances after 40 weeks showed that 0.36-1.26% DMPP, 0.03-0.22% DCD and 4.09-9.22% ATC were recovered from soils and cumulated percolates. Temperature effects were especially visible for the less transformed ATC, but with differences between both soils. In one soil, more ATC dissipated at 20 °C than at 10 °C, in the second soil it was the other way around. There are several possible explanations for those temperature differences such as different soil properties influencing adsorption and formation of non-extractable residues, the composition of microorganisms and their available nutrients.

No masses of MPA and 2-NPT were detected in either soil or percolate, indicating complete transformation. These results are consistent with reported DT50 values in the literature, implying that both substances undergo 50% biotransformation in soil within less than 10 days. 1,2,4-Triazole masses in one of the soil and related percolate increased with a factor of 4-14, compared to background concentrations which were analyzed at the beginning of study. These results will be compared with those gained by the second study, in which the subsoils are less adsorptive.