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

Core Ideas Urea volatility was greater following late‐fall and winter applications to cold soils compared with spring applications to warmer soils. NBPT reduced cumulative NH3 loss from urea by 66%. Urea applications in the late‐fall and winter resulted in prolonged periods of NH3 flux. 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 (NH 3 ) 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 NH 3 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, NH 3 flux following spring urea was generally lower in intensity (≤7 g N ha –1 h –1 ) and shorter in duration (14–30 d). Cumulative NH 3 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 NH 3 loss by 65.6%. Urea applications during the over‐winter period (December–March) should be avoided, in favor of spring applications to minimize NH 3 loss.