Ammonia Volatilization From Surface-Applied Urea and NBPT Coated Urea Under Cold Temperatures.

Urea is frequently surface-applied to winter wheat in Montana during the late-fall to early spring (October-April).  Although, urea is known to be susceptible to volatilization the losses are believed to minimized  if applications are deferred to cold weather months. The objectives of this study were to quantify ammonia losses from urea surface-applied during the fall to early spring period; and to evaluate the use of NBPT (N-(nbutyl) thiophosphoric triamide) to mitigate losses.  Losses were quantified via a micrometeorological mass-balance or integrated horizontal flux approach.  Studies were conductedon privateon-farms with varying soil textures.  Leuning shuttles were placed on masts 0.25, 0.50, 1.00, 1.50, and 2.75 m above ground level in the center of urea and urea+NBPT treated (100 kg N/ha) circular-plots (40 m dia.).  Background ammonia was accounted for with a third mast, 200 m distant from the treated plots.   Ammonia-N losses from urea ranged from 3 to 44% of the application rate over 12 gas sampling campaigns.   The highest ammonia fluxes (up to 22 kg N/ha/wk) occurred when urea was applied to soils with a high surface water content followed by a period of rapid drying. Applying urea to frozen soils or onto snow did not provide protection against volatilization losses. Surprisingly, some of the greatest ammonia losses occurred when urea was applied to surface soils near 0 °C combined with high moisture.   At field sites with acidic soils (pH 5.5 to 6.5) coating urea with NBPT (4.2 ml/kg urea) mitigated volatilization losses over a two-wk period following fertilizer dissolution.   At a field site with a calcareous soil (pH 8.3) the protection from NBPT lasted seven weeks.  The results from this study demonstrate that significant losses of ammonia can occur when urea is surface-applied to cold soils.