A model for nitrate leaching in undisturbed structured clay soil during unsteady flow

Abstract

A two-domain mixing model is described for predicting nitrate leaching during unsteady unsaturated flow through soil. The model assumes that water infiltrating the soil surface mixes incompletely with resident water to form a miscible transport volume, the size of which can vary with time. Changes in solute storage within the transport volume can occur by diffusion and biological transformations. Output from the transport volume is by drainage from a lower surface in the soil. The model's output was tested against the measured amounts of NO 3-N leached from large undisturbed cores of structured clay soil during constant flux irrigation with 10 m M CaCl 2. The average concentration of NO − 3 and Cl − in the soil solution was determined by extraction of small cores taken around the large cores prior to leaching. The rate of nitrification in the soil was also measured. The change in the transport volume with time was calculated from equations for the mass balance of water and Cl − in the transport volume. Knowing the transport volume, analogous equations in terms of NO − 3 were solved to predict the NO − 3 concentration in the effluent. Good agreement between predicted and observed NO 3-N leached was obtained for cores showing divergent trends in NO − 3 leaching. A large difference between cores in the fraction of soil NO 3-N leached (0.34 compared to 0.13) was attributed to the difference in the volume of high-nitrate soil water that mixed with infiltrating water to form the transport volume, and to differences in the distribution of NO − 3 between ped faces and ped interiors. The merit of incorporating the concept of a variable transport volume into a transfer function model for predicting NO − 3 leaching is briefly discussed.