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NUTRIENT MANAGEMENT & SOIL & PLANT ANALYSIS

Nitrogen Fertilizer Sources and Timing before Flooding Dry-Seeded, Delayed-Flood Rice

^a Dep. of Crop, Soil, and Environmental Sciences, 115 Plant Science Building, Univ. of Arkansas, Fayetteville, AR 72701
^b Rice Research and Extension Center, 2900 Hwy. 130 East, Stuttgart, AR 72160
^c Dep. of Crop, Soil, and Environmental Sciences, 1366 W. Altheimer Dr., Fayetteville, AR 72704
^d USDA-NRCS, 4900 Oklahoma Ave., Ste. 300, Woodward, OK 73801
^e Agricultural Statistics Lab., Univ. of Arkansas, Fayetteville, AR 72701

^* Corresponding author (rnorman{at}uark.edu).

Urea is the primary N source used for the large preflood N application in delayed-flood rice (Oryza sativa L.) production in the southern United States. Urea is prone to substantial NH₃ volatilization losses, however, if fields are not flooded quickly. Most delayed-flood rice fields require 5 to 10 d to flood. Consequently, a study was conducted to evaluate the use of less NH₃–volatile N sources for the preflood N application. The objectives were to evaluate the NH₃ volatilization loss and impact on N uptake and rice yield when urea, urea plus the urease inhibitor N-(n-butyl)thiophosphoric triamide (NBPT), (NH₄)₂SO₄, or a urea-(NH₄)₂SO₄ (UAS) blend were applied preflood and a flood established 1, 5, or 10 d after N application. When flooding was delayed for 5 or 10 d after N application, NH₃ volatilization was the least for urea + NBPT (2–10%) and (NH₄)₂SO₄ (4–5%) and they produced the highest rice N uptake and yield. The UAS blend that had NH₃ volatilization losses (11–15%) at 5 and 10 d after application that were intermediate between urea (17–24%) and (NH₄)₂SO₄ or urea + NBPT also had N uptake and grain yield intermediate between these N sources. Urea should only be used if 2 d are required to flood a field. If 3 to 5 d are required to flood a field, then UAS has some merits but it is not as consistent as (NH₄)₂SO₄ or urea + NBPT. When >5 d are required to flood, (NH₄)₂SO₄ or urea + NBPT should be used.

Abbreviations: NBPT, N-(n-butyl)thiophosphoric triamide • UAS, urea–ammonium sulfate