FINITE-ELEMENT MODEL OF NITROGEN SPECIES TRANSFORMATION AND TRANSPORT IN THE UNSATURATED ZONE

A two-dimensional finite element model based on Galerkin's weighted residual approach and incorporating an upstream weighting technique was developed to predict the simultaneous transformation and transport of nitrogen species. The nitrogen cycle used in the analyses assumes first-order rate coefficients for nitrification, denitrification, immobilization and mineralization between the nitrogen species NH4+, NO3-, organic-N and denitrified fractions. The NO3--N component in the soil solution was assumed to have negligible adsorption on exchange sites while adsorption of NH4+ was assumed to be represented by a Freundlich type nonlinear isotherm (S=kdCp). The accuracy and validity of the proposed model were examined by comparison with analytical model results and available field data.