UREA TRANSFORMATIONS IN FLOODED SOIL COLUMNS .2. DERIVATION OF MODEL AND IMPLICATIONS TO AMMONIA VOLATILIZATION

Ammonia volatilization from rice (Oryza sativa L.) paddies following urea application reduces the effectiveness of the urea-N. The objectives of this study were to: (i) derive, for flooded soils, a model describing hydrolysis and diffusion of urea and diffusion of ammoniacal N; (ii) derive the necessary parameters from experimental data, and (iii) use the model to study implications of hydrolysis/transport parameters and management variables to NH3 volatilization. The parameters are diffusion coefficients of urea and NH4 in soil, hydrolysis coefficients of urea in the soil and overlying flood-water, and the sorption coefficient for NH4 on the soil. The effects on potential for NH3 volatilization of variation in these parameters and the management factors of depth of flood water and depth of fertilizer incorporation were calculated. The results illustrate that urea-hydrolysis rates in the floodwater and soil are the most important factors influencing the potential for NH3 volatilization. If urea-hydrolysis rates are very high, incorporation and shallow floodwater are the indicated management while, with low hydrolysis rates, deep floodwater and no incorporation are the indicated management.