Ammonia Volatilization and Rice Grain Yield As Affected By Simulated Rainfall Amount and Nitrogen Fertilizer Amendment.

Urea is the most common N fertilizer used for rice (Oryza sativa L.) production in the mid-South USA and has a high potential for NH₃ volatilization if not incorporated by timely rainfall or flooding. We compared the effects of simulated rainfall amounts and N-(n-butyl) thiophosphoric triamide (NBPT) urease inhibitor rate on NH₃ volatilization and rice N uptake and grain yield. Three experiments were conducted during 2013 and 2014 on an alkaline Calhoun silt loam in Arkansas. Ammonia volatilization was measured in two trials. Urea or NBPT-treated urea (NBPT-Urea, 0.88 g NBPT kg^-1 urea) was subjected to six simulated rainfall amounts ranging from 0-to 25-mm within 5 to 12 h after urea application. The flood was established no earlier than 8 d after urea-N application. Ammonia volatilization, N uptake, and grain yield were regressed on simulated rainfall amount, allowing for linear, quadratic, and cubic terms with coefficients depending on N source and trial. After 11 d, cumulative NH₃ loss from NBPT-Urea ranged from 0.2 to 2.0% of the applied-N (112 kg N ha^-1) and was similar across simulated rainfall amounts. Cumulative NH₃ loss from Urea ranged from 0.7 to 8.6% of the applied-N and was greatest with no simulated rainfall and decreased quadratically as rainfall amount increased. Cumulative NH₃ loss from NBPT-Urea was significantly lower than from Urea when simulated rainfall was <21.8 mm. Grain yield of rice receiving Urea decreased as simulated rainfall increased (180-206 kg ha^-1 Trial A, 173-197 kg ha^-1 Trial B) with the greatest yield when no simulated rainfall was applied. However, rice grain yield fertilized with NBPT-Urea (203-211 kg ha^-1 Trial A, 178-198 kg ha^-1 Trial B) was constant across rainfall amounts and greater than rice yields fertilized with Urea. Sufficient rainfall after urea-N application reduced NH₃ loss but apparently increased N loss via nitrification-denitrification.