Fate of urease and nitrification inhibitors in surface water and saturated sediment

Nitrification and urease inhibitors (NI,UI) added to fertilisers can increase the availability of nitrogen to plants. By inhibiting certain processes, they could contribute to a reduction in the emission of N₂O and NH₃ compounds and a reduction in nitrate leaching. Some of these substances have already been detected in surface water and groundwater and are considered to be harmful to human health. The inhibitors are therefore an area of conflict between climate change mitigation and increased fertiliser efficiency on the one hand, and soil and groundwater protection on the other. However, knowledge of their abiotic and biotic degradation in water and saturated sediment systems is currently very limited.

The aim of this study is to determine the fate of the 6 most commonly used inhibitors 1H-1,2,4-Triazol (Triazole), Dicyandiamide (DCD), 3,4-Dimethylpyrazol (3,4-DMP), N-[(3 or 5-methyl-1H-pyrazol-1-)methyl]acetamid (MPA), 3-Methylpyrazol (3-MP) and N-(2-Nitrophenyl) phosphoric triamide (2-NPT) in surface water and saturated sandy sediments.

Triplicate batch samples containing either saturated water-sediment mixtures (solid-solution-ratio 1:3) or surface water only were spiked with six inhibitors (target concentration 1.5 µg/L each). Both sets were maintained under biotic or abiotic (autoclaved water and sediment) conditions at room temperature. The supernatant was sampled periodically for 90 days and analysed for inhibitors, pH, dissolved organic carbon and electrical conductivity.

None of the inhibitors were sorbed to the sediment except Triazole, which showed only minimal sorption of less than 10%. The urease inhibitor 2-NPT was partly decomposed by hydrolysis alone under the studied pH between 7-8.6. Degradation in general was most pronounced in the biotic water-sediment mixture where DCD and MPA were completely degraded and 3,4-DMP, 2-NPT and 3-MP were partially degraded. The Inhibitor MPA was very susceptible to biodegradation even in surface water, however, its forming metabolite 3-MP is not. Triazol was not degraded under any conditions in this study.

With the exception of Triazole, saturated sediments (for instance in a bank filtration scenario) could probably reduce most of the inhibitors’ concentrations if the microbial community is intact. However, four out of six inhibitors were not completely degraded even under biotic conditions within 90 days, making them susceptible to breakthroughs into groundwater. Therefore, their fate in the environment should be studied further.