Urease inhibitor still active at low concentration

Urease inhibitors (UI) decrease NH3 volatilization losses but the degradation of UI after urea storage reduces its effectiveness. Urea coated with UI (550 and 733 mg NBPT+NPPT kg-1 urea) stored at 20°C and 30° for up to 12 months was applied to an Oxisol. Ammonia losses from urea varied from 40 to 53% of the applied N and decreased by 33% to 58% with UI. Differences in the effectiveness of UI to reduce NH3 loss were relatively small from urea freshly treated with UI or stored for 12 months, even if the residual concentrations in samples stored at 30°C were as low as 138 and 213 mg UI kg-1. UI remains relatively effective at much lower concentrations than presently recommended.

In 2018 8.2 Mt of N fertilizer were coated with UI worldwide. UI applied to urea tends to slowly degrade upon storage and degradation increases with increasing temperature (Cantarella et al. 2016). Therefore, it is important to understand how UI at low active ingredient concentration affects NH3 volatilization. Urea coated with a mixture of NBPT and NPPT at two rates (550 and 733 mg UI kg-1) was stored at 20°C and 30°C. After time intervals up to 12 months urea was applied to soils in volatilization chambers and NH3 losses were measured for 30 days.