Ammonia Volatilization, Rice Yield, and Nitrogen Uptake Responses to Simulated Rainfall and Urease Inhibitor

The effect of rainfall between urea application and flood establishment on rice (Oryza sativa L.) grain yield has not been studied. We compared the effects of simulated rainfall amount and N-(n-butyl) thiophosphoric triamide (NBPT) urease inhibitor rate on NH3 volatilization and rice growth. Three field experiments were conducted and NH3 volatilization was measured in two trials for 11 d after urea application (DAA). Untreated urea (Urea) or NBPT-treated urea (NBPT-Urea) was subjected to six simulated rainfall amounts (0-25.4 mm) applied 5 to 15 h after fertilizer application and flooded 7 to 12 DAA. Cumulative NH3 loss from Urea accounted for 8.6% of the applied N (112-118 kg N ha(-1)) with no simulated rainfall and decreased quadratically to 0.6% with 25.4 mm of simulated rainfall. Cumulative NH3 loss from NBPT-Urea decreased quadratically as simulated rainfall amount increased but loss was 0.2 to 2.0% of the applied N. Depending on the site, yields of rice fertilized with Urea decreased linearly or nonlinearly as simulated rainfall increased with the greatest yield produced by rice receiving no simulated rainfall. The yields of rice fertilized with NBPT-Urea were not affected by simulated rainfall amount in two trials. In the third trial, the yields of rice receiving NBPT-Urea decreased nonlinearly as simulated rainfall amount increased but were 8.9 to 18.1% (746-1376 kg ha(-1)) greater than the yields of Urea-fertilized rice. Rainfall following urea application reduces NH3 loss but appears to increase N loss via denitrification. Based on yield results, total N loss was reduced when urea was treated with NBPT.