ASPECTS OF AMMONIA VOLATILIZATION FROM SURFACE-APPLIED UREA FERTILIZERS (INTEGRATED HORIZONTAL FLUX, NITROGEN, RESIDUE)

Urea is now the most popular form of N fertilizer in the world. As such it deserves special attention. Unincorporated surface application of urea risks the loss of nitrogen as ammonia to the atmosphere. Extensive laboratory and field measurements have been made to characterize the ammonia volatilization process; however, almost all previous experiments have modified the physical environment, which significantly influences loss.; The aims of this research were to measure ammonia volatilization using a micrometeorological mass balance method that produces only minimal disturbances of the environment, and to model changes in the physical environment at the soil surface that might influence loss. A method of simplifying the mass balance approach based on an analytical solution to the diffusion profile was found to be less sensitive to errors in atmospheric ammonia density measurements than a recently published simplified method. Volatilization rates from urea-ammonium nitrate broadcast on wheat straw residue were found to be as high as 1.5 kg-N(ha h)('-1) after 0.25 cm irrigations simulating light rains. Rates of loss without irrigation or after heavy rains were found to be considerably less. Wheat straw was found to have a high pH, a high H('+) buffering capacity, and a high urease activity. Initial rates of ammonia loss from straw residue fertilized with N-solution fertilizers were in the order urea-ammonium nitrate > ammonium nitrate > urea. Maximum rates of loss from urea solutions applied to bare soils were found to be less than 0.26 kg-N(ha h)('-1) under all conditions. Diurnal patterns of loss coinciding with fluctuations in surface temperature, surface water content, and wind speed were observed in both the residue and bare soil studies. Over the duration of the volatilization experiments, water was the major factor determining ammonia loss through its influence on urea hydrolysis. The osmotic coefficient of urea in saturated solutions was found to be related to molal concentration by (phi) = (1 + c)('-0.08). A model based on a finite difference solution of the coupled soil water and heat flow equations and on an energy balance at the soil surface favorably simulated diurnal fluctuations in both surface soil water content and soil temperature.;...