Stability of selected nitrification and urease inhibitors in surface water

Nitrification (NI) and urease inhibitors (UI) are added as organic trace substances to agricultural land during the application of nitrogen and urea fertilizers. They inhibit the nitrification processes and urease activity in soil and thus ensure a reduced emission of gaseous ammonia and nitrous oxide. The application of UI has been mandatory since 2020 following the amendment to the German Fertilizer Ordinance, which means that increased use is to be expected. The substances can enter surface waters through translocation and leaching processes in soils. So far, the stability of NI and UI in the aqueous phase has only been investigated in ultrapure water and tap water. Therefore, the aim of this study was to investigate the stability of five NI, i. e. 1H-1,2,4-triazoles (triazole), dicyandiamides (DCD), 3,4-dimethylpyrazoles (3,4-DMP), 3-methylpyrazoles (3-MP), N-((3(5)-Methyl-1H-pyrazol-1-yl)methyl)acetamid (MPA) and one UI, i. e. N-(2-nitrophenyl)phosphoric acid triamide (2-NPT), in two natural surface waters at 20 °C and at different pH values (i. e. pH 5, 7 and 9) using batch experiments. We distinguished between the processes of hydrolysis, sorption and microbial degradation. Hence, three differently treated triplicate batch samples were set up after the removal of suspended matter (> 0.63 mm). To investigate hydrolysis, the test water was filtered through a 0.22 μm polyamide membrane. For the investigation of sorption on suspended solids, a sodium azide solution was added to the water to inactivate microorganisms (final concentration in batch samples 100 mg/L). To investigate microbial degradation, the test water was used in its natural composition. pH values were adjusted using dilute HNO₃ and NaOH, respectively. The six inhibitors were added as a mixture to each batch sample with a target concentration of 5 μg/L each. Batch samples were taken, subsequently filtered (0.45 µm, regenerated cellulose) and measured by HPLC-MS/MS over a period of 8 days for the sorption tests and 83 days for the hydrolysis and microbiological degradation experiments.

None of the investigated inhibitors showed any sorption on suspended solids. With regard to hydrolysis and microbial degradation, the results differed depending on inhibitor and pH. For MPA no decomposition by hydrolysis could be detected at all three pH values. However, MPA was microbially degraded at pH 7 and pH 9 and was no longer detectable after about 55 days. 2-NPT was hydrolytically degraded at pH 5 and 9 over the entire test period, but no hydrolysis was observed at pH 7. At pH 7, no microbial degradation could be detected for 70 days. Thus, 2-NPT is persistent at pH 7. At pH 9, our results did not show any microbial degradation for 2-NPT. The stability of inhibitors in surface waters is driven by hydrolysis and microbial degradation and may vary greatly with pH.