A SIMPLE, INVERSE MODEL FOR ESTIMATING NITROGEN REACTION RATES FROM SOIL COLUMN LEACHING EXPERIMENTS AT STEADY WATER FLOW

Abstract

Many soil nitrogen studies have included estimations of nitrification or denitrification rates in soil column systems based on simple mass balance considerations. This approach does not take into account the effect of the solute dispersion. The present study presents a simple, inverse finite difference model (IFDM) for estimating temporal and spatial variations in nitrification or denitrification rates from measured nitrogen concentration profiles at steady water flow. The model takes into account both convective and dispersive transport of nitrogen by using a Crank-Nicolson finite-difference discretization of a paraphrased convection-dispersion equation for solute transport including a reaction term. A test procedure is suggested and used to find criteria for avoiding numerical errors in the calculations. Significant numerical errors are inherent in the IFDM but can be avoided if the depth increment and the ratio of time increment to depth increment are chosen carefully. The possible applications of the simple IFDM are illustrated by using the model on soil column data for rapid, continuous leaching of nitrate through water-saturated porous media at steady water flow. The IFDM-based analysis showed a large influence of soil composition and temperature on the temporal and spatial variations in nitrate removal rates. The IFDM assumes constant pore water velocity and solute dispersion coefficient. In order to use the IFDM not only for qualitative but also for quantitative predictions, these two parameters need to be carefully measured.