Transformations of surface-applied urea in an alkaline calcareous soil as affected by urease inhibitor N-(n-butyl) thiophosphoric triamide at different temperatures

Abstract

Effects of the urease inhibitor N-(n-butyl) thiophosphoric triamide (NBPT) on transformations of urea N in an alkaline calcareous soil were investigated at different temperatures. The urease inhibitor was incorporated in urea granules at 7 different concentrations ranging from 0 to 6% of N. The amended urea granules were applied at 250 mg N kg−1 on the soil surface and incubated for 20 days at 18, 25 and 35°C. Effects of NBPT on NH3 volatilization from the surface applied urea were also investigated at different temperatures. In the absence of urease inhibitor, most (96–98%) of the applied urea was hydrolyzed within 5 days. During this period, NBPT significantly inhibited urea hydrolysis at all temperatures (6–46% reduction); the extent of inhibition increased with increasing NBPT application rate and with decreasing temperature. After 10 days, almost all urea was hydrolyzed at all temperatures with NBPT applied up to 2% of N, whereas up to 23% inhibition was recorded with higher NBPT application rates. In the absence of NBPT, 23–41% of the applied urea-N was lost as NH3 during 20 days, with the highest loss recorded at 35°C. However, the application of NBPT significantly reduced NH3 volatilization loss at all temperatures; the beneficial effect of NBPT increased with increasing application rate. Results suggested that application of the urease inhibitor NBPT can cause up to 50% reduction in NH3 volatilization loss when applied at economically feasible application rates ranging from 0.5% of urea-N (during winter season; 18°C) to 1% of urea-N (during fall and summer seasons; 25–35°C).