Nitrification inhibitors in the soil-groundwater-river continuum of Germany

Nitrification inhibitors and urease inhibitors are organic chemicals that have been used in agriculture for decades to slow down nitrification in order to keep plant-available ammonium in the soil for longer and to reduce the leaching of nitrate. Furthermore, they serve to reduce the rapid conversion of urea to ammonia, which also has a positive effect on climate protection. Being applied to agricultural fields, there is a high risk that these substances are contaminating water bodies. We therefore initiated both an environmental sampling campaign and a lab study of the environmental fate characteristics of selected nitrification inhibitors and urease inhibitors.

In our first study, we took samples in Northern Hesse and the area of Goettingen (Germany) from streams, lakes and groundwater between October and December, mainly in agricultural areas. The samples were then analysed for six different nitrification and urease inhibitors (1) 3,4-dimethylpyrazole phosphate (DMPP), (2) 4-amino-1,2,4-triazole (ATC), (3) Dicyandiamide (DCD), (4) N-(2-nitrophenyl)phosphoric triamide (2-NPT), (5) Mixture of N-((5-methyl-1H-pyrazol-1-yl)methyl)acetamide and N-((3-methyl-1H-pyrazol-1-yl)methyl)acetamide (MPA) and (6) H-1,2,4-Triazol. We found solely two of the inhibitors in the samples, DCD and H-1,2,4-Triazol, which corresponds to the results of a study from 2014 published by the DVGW.

In a second study, we wanted to examine under what circumstances and with what dynamics the inhibitors are transferred to deeper soil zones and what influence they have on the leaching of nitrate at different temperatures and in different soils. For this purpose, we chose the above mentioned five nitrification or urease inhibitors (1-5), which are currently used in agricultural fertilizers in Germany. In order to obtain information about their behavior in the environment, we have planned a study to investigate their leaching and transformation behavior in the unsaturated soil zone. For this purpose, agricultural topsoils were selected and filled into 25 cm high columns with a diameter of 7 cm. Three fertilizers containing the above-mentioned compounds were applied separately to the soil columns at an application rate of 150 kg N/ha. Each of the variants was tested in triplicate in two different temperature ranges (12 °C, 20 °C). Irrigation was carried out over 12 weeks with a groundwater recharge rate of the fall/winter period. The leachate was analyzed 1-2 times a week and at the end of the study the soil was analyzed at two different depths for the inhibitors as well as nitrate and nitrite.