Diffusion of NH+4 and NO−3 mineralized from organic N in soil

Abstract

SummaryThe mineralization of native soil organic matter and the simultaneous diffusion of zero NH+4 and NO−3 to a solution sink of zero N concentration was analysed experimentally and theoretically for a fine sandy loam soil. Experimentally, the NH4 and NO3 ions produced in an incubated unsaturated soil column were allowed to diffuse through a sintered glass plate into a stirred solution sink. The distribution of NH+4 and NO−3 in the soil column was measured after various incubation times. The rate of ammonification was measured directly during incubation and the rate of nitrification modelled from nitrifier growth kinetics. A Freundlich equation was used to describe the equilibrium between soluble and exchangeable NH+4 in the soil. Terms for the microbial transformation of N and the adsorption‐desorption of NH+4 were combined with diffusion equations which were solved numerically using finite difference methods. The model constructed was used to predict the NH+4 and NO−3 con‐centration distributions in the soil column, and good agreement was obtained between the experimental and predicted concentration profiles. The use of the model for predicting the diffusive flux of mineral N to the outer surfaces of soil peds, where it is vulnerable to leaching, was demonstrated.