Ammonia Volatilization from Flooded Soil Systems: A Computer Model. I. Theoretical Aspects

Ammonia volatilization from flooded rice ( Oryza sativa L.) is a major mechanism for N loss and poor fertilizer use efficiency. Ammonia volatilization is influenced by five primary factors: NH 4 ‐N concentration, pH, temperature, depth of floodwater, and wind speed. This NH 3 ‐volatilization model is based on chemical and volatilization aspects. The chemical aspects of the model deal with the NH 4 /NH 3 (aq) equilibrium in floodwater. Ammonium ions undergo dissociation with a first‐order rate constant, while NH 3 (aq) and H undergo a diffusion‐controlled association reaction with a second‐order rate constant. The transfer of NH 3 across the water‐air interface of flooded soil systems is characterized by a first‐order volatilization rate constant. By utilizing the chemical dynamics of the NH 4 /NH 3 (aq) system in association with transfer of gaseous NH 3 across the interface, an equation was derived to determine the rate of NH 3 volatilization from flooded systems as a function of the five primary factors. The chemical aspects of the model include the derivation of association and dissociation rate constants. The volatilization aspects of the model, which is based on the two‐film theory, allows it to compute the volatilization rate constant for NH 3 . Expressions are derived to compute the Henry's law constant, gas‐phase and liquidphase exchange constant, and the overall mass‐transfer coefficient for NH 3 .