ADAPTED FICK LAW APPLIED TO SOIL RESPIRATION

In soil respiration studies the diffusive gas fluxes are often calcula ted using Fick's law. However, results obtained with Fick's law deviat e from those obtained with the theoretically sound Stefan-Maxwell equa tions. In the present study a numerical model based on an adapted form of Fick's law is applied to soil respiration. A pressure adjustment f lux to maintain isobaric equilibrium in the system is employed to corr ect errors related to the usage of Fick's law. The results of the abov e-mentioned model are compared with those of analytical solutions of F ick's law and the Stefan-Maxwell equations to check the model's accura cy. The analytical solutions are derived for steady state transport at constant respiration rates in a hypothetical ternary system with N-2, O-2, and CO2. Calculations are performed at various constant rates of CO2 production and O-2 consumption throughout the soil. Differences b etween the mole fraction gradients calculated with Fick's law and the Stefan-Maxwell equations are substantial. If Fick's law is combined wi th the isobaric equilibrium correction procedure, the similarity with the Stefan-Maxwell equations is much better. The numerical model emplo ying the adapted Fick's law is subsequently tested against field measu rements. Field measurements were carried out in large outdoor lysimete rs filled with oil-contaminated soil containing nonvolatile hydrocarbo ns. Nonsteady gas transport due to dynamic soil respiration during bio degradation of the hydrocarbons in the lysimeters is modeled at known boundary conditions. The result of the model agree with measurements o f CO2 fluxes and O-2 and CO2 concentration profiles in the lysimeters.