Nitrification and urease inhibitors - From fields to drinking water resources?

Applications of nitrification and urease inhibitors (NUI) with nitrogen fertilisers on agricultural soils are intended to improve the efficiency of nitrogen fertilisers and also to prevent nitrogen emissions from the fertilisers. However, the deliberate release of chemicals into the environment carries a certain degree of risk due to spreading in the water systems. Both sensitive ecosystems and water supplies may be affected. Processes that need to be considered for a reliable assessment regarding the fate and distribution of these substances in the different compartments are manifold. So far, reliable predictions in literature are scarce and regulatory approaches are based on limited information on substance fate.

Yet, two out of the ten NUIs currently approved in Germany, have been found in surface waters, prompting further investigation. In this context, no analytical method has been established so far for some of the other NUIs, thus there is no information available on the occurrence of these substances. In order to gain transparency on the actual environmental fate and implications for drinking water production of these NUI has been analyzed In addition to legal approval procedure different processes and scenarios relevant to their fate were investigated in the INHIBIT project.

Following a comprehensive literature review, experimental investigations were carried out to further evaluate the environmental behaviour of these substances on an empirical basis. Initially, a multi-method was developed for the simultaneous determination of 1H-1,2,4-triazole (triazole), dicyandiamide (DCD), 3,4-Dimethylpyrazol-phosphate (DMPP), 3-Methylpyrazole (3-MP), N-(n-butyl)thiophosphoric triamide (NBPT) and N-(2-nitrophenyl)phosphoric triamide (2-NPT) in soil pore water. Experiments conducted in this research project provided essential empirical information on the hydrolysis stability, degradation behaviour and sorption tendency in soils for selected NUI. In addition, vessel, column, lysimeter and practical field application tests were carried out to obtain empirical information on the leaching risk of these substances via the transport pathway unsaturated zone-leachate-groundwater. Furthermore, the indirect input pathway via infiltrating surface waters (bank filtration) was investigated. Studies were carried out on different soils and using different parameters in order to depict different site conditions.

Results indicated a high hydrolysis stability for the nitrification inhibitors (NI) DCD, DMPP, 3-MP and triazole. The hydrolysis stability of the urease inhibitors (UIs) NBPT and 2-NPT is strongly pH-dependent. While NBPT is particularly unstable in an acidic environment, 2-NPT shows the lowest stability in a more alkaline environment. Sorption tendency to soils of all compounds was low. Microbial degradation of NI in soils was lower compared to urease inhibitors (UIs). Overall, the NI triazole, DCD, 3-MP and DMPP were found to be potentially relevant substances for drinking water production. The NI active substances DCD and triazole were additionally monitored in several surface waters and were frequently detected, in some cases at very high concentrations of several µg/L. These findings underline the relevance of these substances for water resources.