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

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 degrees 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 degrees 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 degrees C) to 1% of urea-N (during fall and summer seasons; 25-35 degrees C).