Late-Fall, Winter and Spring Broadcast Applications of Urea to No-Till Winter Wheat I. Ammonia Loss and Mitigation by NBPT

Surface urea applications in the late-fall, winter, or early-spring are common in Montana, but N applied at these timings is susceptible to volatilization. This study was conducted to quantify and contrast ammonia (NH3) loss from urea and urea plus N-(n-butyl) thiophosphoric triamide (NBPT) following late-fall, winter, and spring applications (100 kg N ha(-1)). Experiments were run over 3 yr in farmer-cooperator fields (>60 ha) under no-till winter wheat (Triticum aestivum L.)-chemical fallow management. Ammonia emissions were quantified by the integrated horizontal flux method with samplers placed on a mast in the center of circular plots (20-m radius). Prolonged periods of modest NH3 flux (<= 33 g N ha(-1) h(-1)) were found following late-fall (87-106 d) and winter (48-62 d) urea applications before flux fell to nominal levels (<= 3 g N ha(-1) h(-1)). In contrast, NH3 flux following spring urea was generally lower in intensity (<= 7 g N ha(-1) h(-1)) and shorter in duration (14-30 d). Cumulative NH3 loss (% applied N) from urea was greater (P < 0.05) for late-fall (16.4%) and winter (11.4%) than for spring (2.0%) applications. The weather pattern of light (<= 6 mm), scattered precipitation events from December throught March vs. larger precipitation events (>= 12 mm) in April and May was an important factor which helped explained this response. Addition of NBPT (1 g kg(-1)) to urea reduced cumulative NH3 loss by 65.6%. Urea applications during the over-winter period (December-March) should be avoided, in favor of spring applications to minimize NH3 loss.

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