A detailed model of the behaviour of charged species in the afterglow of a nitrogen plasma in a tubular geometry has been developed. The axisymmetric model for the species concentrations (Nk+, where k=1, 2, 3 and 4) accounted for radial and axial ambipolar diffusion, homogeneous recombination, homogeneous ionization, ion-neutral species rearrangements and recombination reactions at the sheath boundary at the wall. Comparison of the model results with experimental measurements showed agreement to within the experimental uncertainties. By systematically varying the terms in the model, it was possible to identify the important reactions in the afterglow. In terms of total ion concentrations, the homogeneous ionization reactions were a significant source term, particularly near the entrance of the afterglow, and recombination at the sheath boundary layer was the dominant sink term (homogeneous recombination was negligible). Indeed, the rate of recombination at the sheath boundary was so fast that the overall process is limited by the rate of radial diffusion of ions to the sheath. The ion-neutral species reactions, being charge-conserving, had little effect on the total concentration of ions; however, their importance in determining the relative concentrations was identified. It was also demonstrated that a fully two-dimensional model of the afterglow was necessary to predict the experimental measurements accurately. Truncation of the model to one dimension produced results that disagreed with experiment by several orders of magnitude.