Ammonia Volatilization from Urea and Mitigation by NBPT following Surface Application to Cold Soils

Urea is frequently surface applied to winter wheat (Triticum aestivum L.) during cold weather months (October-April) in the semiarid northern Great Plains. This study was conducted to quantify NH(3) volatilization loss from surface-applied urea (100 kg N ha(-1)) and urea amended with N-(n-butyl) thiophosphoric triamide (NBPT) during this period. Ammonia emissions were quantified by a micrometeorological integrated horizontal flux method with samplers placed on a mast in the center of circular plots (20-m radius). Cumulative NH(3) losses from urea varied but averaged 20.5% of applied N across 12 trials. The largest losses (30-44% of applied N) occurred aft er urea was applied to high-water-content soil surfaces, followed by a period of slow drying with little or no precipitation. Emissions occurred for a prolonged period oft en lasting > 42 d. Periods (1-2 wk) of high NH(3) flux (> 30 g N ha(-1) h(-1)) frequently occurred when mean daily soil temperatures (1-cm depth) were -2 to 5 degrees C. In one trial, 24.3% of applied N was lost aft er urea was applied to a 140-mm snowpack. Ammonia losses were moderated by applying urea to dry soil surfaces. If precipitation events that followed were light (< 8 mm), losses were reduced to 10 to 20% of applied N. If the events were heavy (> 18 mm), then losses were < 10%. Coating urea with NBPT (1 g kg(-1)) reduced cumulative NH(3) losses by 66%. Volatilization protection lasted 2 to 3 wk on acidic soils, and > 7 wk on an alkaline soil. This study demonstrated that significant NH(3) losses from surface-applied urea could occur during cold weather months.