The Environmental Afterlife of Nitrification and Urease Inhibitors: A meta-analysis of Transformation and Fate

After decades of using nitrification and urease inhibitors (NI and UI) in agriculture to delay the rapid conversion of urea into ammonia (UI) as well as the nitrification, to reduce nitrate leaching and to maintain plant-available ammonium in the soil for a longer period, substantial knowledge gaps about their environmental fate still exist.

Synthetic compounds such as NI and UI can pose considerable risks, as their true environmental impacts are sometimes only revealed after extensive use, as demonstrated by historical cases of PFAS, DDT and PCBs. This underlines the necessity of studying the fate of synthetic chemicals as well as their potential transformation products before the damage is done.

For this purpose, an extensive meta-analysis of publications published after 1990, as well as databases such as the registration dossiers provided by the European Chemicals Agency (ECHA), was performed, focusing on transformation behavior, potential transformation paths and products, adsorption behavior in soils as well as physicochemical properties such as water solubility. This analysis included eight NIs and three UIs currently used in commercially available agricultural fertilizers: 1,2,4-triazole (1,2,4-T), 4-amino-1,2,4-triazole (ATC), 3-methyl-1H-pyrazole (3-MP), reaction mass of N-((5-Methyl-1H-pyrazol-1-yl)methyl)acetamide and N-((3-Methyl-1H-pyrazol-1-yl)methyl)acetamide (MPA), 3,4-dimethylpyrazole phosphate (DMPP), reaction mass of 2-(3,4-dimethylpyrazole-1-yl)-succinic acid and 2-(4,5-dimethylpyrazole-1-yl)-succinic acid (DMPSA), Dicyandiamide (DCD), 2-chloro-6-(trichloromethyl)-pyridine (Nitrapyrin) as well as N-(2-nitrophenyl)-phosphoric triamide (2-NPT), N-(n-Butyl)-thiophosphoric triamide (NBPT), N-(n-Propyl)-thiophosphoric triamide (NPPT).

The results showed that the availability of information varies greatly among the inhibitors. DCD, which was already used as a fertilizer at the beginning of the 20^th century as well as nitrapyrin, are among the most widely used nitrification inhibitors and those with the most available information about their environmental behavior. Key parameters influencing degradation include temperature, soil water content and inhibitor concentration, which are closely linked to microbial processes; however, prior exposure to soil organisms and soil composition were also found to be influential.

In many publications about inhibitors, such as in the case of DMPP, the dissipation of the substance is focused. However, the whole transformation path and the dissipation processes such as volatilisation can be relevant, as dissipation does not necessarily imply the absence of environmental risk. Furthermore, DT50 values were found to be calculated inconsistently across studies; therefore, all values were recalculated using the same methodology. Also, new data was created using figures from publications which didn’t provide DT50 values or degradation rates were calculated using the emergence of its transformation products.

Information about other NIs, such as ATC, which is not among the most used inhibitors, or DMPSA, which was just introduced to the market in the last years, is scarce or not available. Regarding UIs, much information about the fast-dissipating NBPT is available, such as transformation paths and influences such as concentration and pH value. Almost no information was available about the structurally similar NPPT and scarce information for 2-NPT.