Ammonia Volatilization and Rice Growth As Affected By Rainfall Amount and Urease Inhibitor.

Ammonia Volatilization and Rice Growth as Affected by Rainfall Amount and Urease Inhibitor

 

R.J. Dempsey*, N.A. Slaton, C.G. Massey, and R.E. DeLong

Dept. of Crop, Soil, and Environmental Sciences, University of Arkansas, Fayetteville, AR.

Urea is the most common N source used in the direct-seeded, delayed-flood method of rice (Oryza sativa L.) production in Arkansas and has a high potential for ammonia (NH₃) volatilization if not quickly incorporated into the soil by timely rainfall or flooding.  Our objective was to compare the effects of simulated rainfall amounts and N-(n-butyl) thiophosphoric triamide rate on NH₃ volatilization and rice N uptake and grain yield.  Two experiments were conducted in 2013 on an alkaline Calhoun silt loam in Arkansas.  Ammonia volatilization loss was measured in only one trial.  Urea or NBPT-treated urea (urea-NBPT, 0.88 g NBPT kg^-1 urea) was applied at 112 kg N ha^-1 and compared to a no N control.  Each N source was subjected to six simulated rainfall amounts of 0, 3.18, 6.35, 12.70, 19.05, or 25.40 mm.  The permanent flood was established 12 d after N application and rainfall simulation in the trial where NH₃ volatilization was measured and 6 d after in the second trial.  Ammonia volatilization, N uptake, and grain yield were regressed on simulated rainfall amount, allowing for both linear and quadratic terms with coefficients depending on N source.  Cumulative NH₃ volatilization was influenced by a significant N source by rainfall interaction (P<0.0001).  Cumulative NH₃ loss from urea-NBPT ranged from 0.3 to 2.4% of the applied N and was similar across simulated rainfall amounts.  In contrast, cumulative NH₃ loss from urea ranged from 0.3 to 12.5% of the applied N and was greatest with no simulated rainfall and decreased quadratically as rainfall amount increased.  Cumulative NH₃ loss from urea-NBPT was significantly lower than urea when simulated rainfall was <19.05 mm, but similar when simulated rainfall amounts were ≥19.05 mm.  Nitrogen uptake and grain yield data will also be presented.