The fate of urease inhibitors in two agricultural soils

Urease inhibitors (UI) are organic trace substances, which are applied along with urea fertilizers to reduce NH₃ emissions from agricultural fields. Due to the recent amendment to the German fertilizer act (DüngG) which now dictates the use of UI, increasing amounts of these substances will be applied to arable soils.  So far, little is known about the fate of UI in soils and there is only few data on the leaching of UI from soil to groundwater, especially with respect to field data. However, first studies have proven traces of UI in surface and ground waters, raising concern among drinking water suppliers. Therefore, the aim of this study was to investigate the fate of two different UI, i.e. N-(2-Nitrophenyl) phosphoric acid triamide (2-NPT) and N-(n-Butyl)thio-phosphoric triamide (NBPT) in two agricultural soils, addressing the following questions:

• How long do UI remain in the topsoil following application?
• Which portion will be translocated to deeper soil layers?

 

On two agriculturally used fields in the state of Brandenburg (Germany) with sandy soils, which differ in their topsoil total carbon concentration (Berge 0.96 %, Ribbeck 1.39 %) and pH (Berge 5.9, Ribbeck 7.6), 2-NPT (as urea prills) and NBPT (as a mixture with urea solution) were applied along with urea. Soil samples were taken from the topsoil at 0-5 cm depth (using a soil sampling ring with a volume of 100 cm³) and from 5-15 cm and 15-30 cm depth (using a Pürckhauer sampler) of 4 different plots each prior to the application of the substances. Afterwards, samples from the topsoil were taken 1, 3, 6, 8, 10, 12, 14 and 21 days following the application. On the last day of the sampling period, samples from 5-15 cm and 15-30 cm depth were taken in addition. Samples were stored at -18°C until analysis.

Based on recovery tests by spiking the study soils with UI, the field-moist samples (sieved to 2 mm) were extracted according to the following procedure: 20 ml extraction solution (50 Vol.-% Acetonitrile/50 Vol.-% H₂0 for 2-NPT and 0.1 M KCl for NBPT) were added to 10 g soil, then shaken on a horizontal shaker (30 min, 120 rpm). After centrifugation (30 min, 3830 g), the supernatant was filtered through cellulose acetate filters (0.45 µm) and transferred to vials. The extracts were adjusted to neutral pH using dilute NaOH solution and stored in the refrigerator until measurement by HPLC-MS.

The concentration on 1 day of 2-NPT in the Berge topsoil amounted to 353 ± 151 µg/kg and in the Ribbeck topsoil 302 ± 148 µg/kg. NBPT was not found in any of the two soils. Whilst 2-NPT was no longer detectable in the Ribbeck topsoil after 10 days, 2-NPT decreased much slower in the Berge topsoil, reaching concentrations of 15.4 ± 15.7 µg/kg after 21 days. None of the inhibitors could be detected in deeper soil layers. Results will be discussed in the context of the site-specific soil parameters and the local precipitation data.