Effects of Soil Drying on Ammonia Volatilization from Surface‐applied Urea

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Abstract
Two laboratory experiments were conducted to evaluate the effects of rapid soil drying on the potential for ammonia (NH3) volatilization from urea. Urea was applied to the surface of a sandy loam soil in acrylic chambers at a rate equivalent to 200 kg N ha−1. Static traps for NH3 and carbon dioxide (CO2) were added to the chambers and left in place for 48 h. Humid or relatively dry air was passed over the soil in the chambers between 48 and 72 h at rates sufficient to not inhibit NH3 volatilization. Concentrations of NH3 in the airstream leaving the chambers was monitored during this period. At 48 and 72 h, soil in selected chambers was sampled in 2‐mm depth increments and analyzed for pH, urea‐N, and ammonium‐nitrate (NH4+‐N). Rapid drying of the soil was found to rapidly decrease the rate of NH3 loss from the soil. Drying also reduced the amount of urea that hydrolyzed during the period from 48 to 72 h to practically zero. In the surface 2 mm, the urea concentration of the chambers receiving the dry air treatment increased substantially during the period of airflow. This increase was due to the mass flow of urea towards the soil surface as water evaporated from the soil. These data indicate that the potential for NH3 volatilization from urea applied to a soil that is initially moist but dries rapidly is low, if the soil water content/potential falls to a level at which urea hydrolysis is inhibited.
Funding Information
  • U.S. Department of Agriculture