Volatilization losses from surface-applied
urea during cold weather months Surface application of urea is common management practice on winter wheat farms in the semiarid climate of Montana. Often this occurs during the late fall, or winter if the snowpack is shallow, or early spring when soil temperatures are cold (e.g. < 5 °C). The objectives of this study were to quantify NH3 losses from urea applied during this period; and to evaluate the use of the urease inhibitor, NBPT (N-(nbutyl) thiophosphoric triamide), to mitigate losses. Ammonia losses were quantified on-private farms using a micrometeorological mass-balance approach. Leuning samplers were placed on a mast in the center of circular plots (40 m dia.) receiving urea (100 kg N/ha) and NBPT-coated (1.0 g/kg) urea. Ammonia losses were variable but averaged 20.5% of the application rate over the 13 trials conducted. The largest cumulative losses (30-44%) occurred after urea was applied to high water content soil surfaces, followed by a period of slow drying with little or no precipitation. Losses were moderated by applying urea to dry soil surfaces. Emissions were found to occur over a prolonged time period often lasting 6 wks. Applying urea to cold soils, or onto snow packs of modest depth (<15 cm), did not provide protection against volatilization losses. Surprisingly, some of the greatest NH3 losses occurred periods when mean daily soil temperatures were -2 to 5°C. Coating urea with NBPT reduced cumulative NH3 losses by approximately 2/3, though the efficacy of this compound was tied to the size and distribution of precipitation events following fertilization. Protection from NBPT lasted 2-3 wk on acidic soils, but >7 wk at one field site with a calcareous soil (pH 8.4) suggesting NBPT breakdown occurs more slowly at high pH. This study demonstrated that significant NH3 losses from surface-applied urea could occur during cold weather months in a semiarid climate.
|