A Micrometeorological Study to Quantify Ammonia Volatilization Losses From Urea Applied to Cold Soils in the Semiarid Northern Great Plains.

In Montana, winter wheat growers frequently apply urea to the soil surface during the late-fall, winter, and early spring. Although, surface applied urea is known to be susceptible to volatilization losses, growers believe this problem is minimized if applications are deferred to cold weather months. The objectives of this study are to quantify ammonia losses from urea appliedto the soil surface during the fall to early spring period; and to evaluate the use of NBPT (N-(nbutyl) thiophosphoric triamide) to mitigate losses.  Losses are quantified via a micrometeorological mass-balance or integrated horizontal flux approach.  Studies are being conducted in two no-till winter wheat fields with a loam (pH 5.5) and silt loam (pH 6.2) texture.    Leuning shuttles are placed on masts 0.25, 0.50, 1.00, 1.50, and 2.75 m above ground level, and masts are located in the center of urea and urea+NBPT treated (100 kg N/ha) circular-plots (40 m dia.).  Background ammonia is accounted for with a third mast, 200 m distant from the treated plots. Shuttles are exchanged weekly over 8-wk gas sampling campaigns.  Ammonia-N losses from urea have ranged from 3 to 40% of the application rate over the 5 gas sampling campaigns conducted to date.  Ammonia-N fluxes as great as 22 kg N/ha/wk occurred during one campaign.  In this semiarid region, ammonia losses are sometimes delayed (>2 wk) until sufficient precipitation falls to dissolve urea prills.  Significant ammonia losses may then occur over a 3 to 6-wk period. Applying urea to frozen soils does not guarantee losses will be minimized. Surprisingly, some of the greatest ammonia losses occurred when urea was applied to surface soils near 0 °C combined with high moisture. Coating urea with NBPT (4.2 ml/kg urea) provided two wks of protection against volatilization losses following fertilizer dissolution, and reduced ammonia losses by ~60%.