Late‐Fall, Winter and Spring Broadcast Applications of Urea to No‐Till Winter Wheat I. Ammonia Loss and Mitigation by NBPT. Issue 2 (20th April 2017) - Engel, Richard
Abstract : 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 (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.