DYNAMIC OXYGEN AND CARBON-DIOXIDE EXCHANGE BETWEEN SOIL AND ATMOSPHERE .1. MODEL DEVELOPMENT

The rate of exchange of O2 and CO2 gases between soil and atmosphere is controlled by atmospheric and soil physical and biological conditions. Analysis and evaluation of problems in soil biology and soil ecology would be greatly facilitated by the availability of a mathematical model that predicts concentrations of these gases as a function of time and soil depth as well as the rate of gas exchange at the soil surface. Our objective was to develop such a model, driven by time-dependent functions for solar radiation, rainfall, air temperature, and relative humidity of the atmosphere. Outputs of the model include concentrations of CO2 and O2 soil water content, soil temperature, and rate of consumption of O2 and production of CO2 by roots and soil microorganisms, all as functions of time and soil depth. Dissolution of O2 and CO2 in soil water, adsorption of O2 and CO2 onto colloidal surfaces of soil particles, and replacement of air by water due to soil wetting and vice versa are accounted for in the model. Rates of respiration by microorganisms and roots and rate of root growth are given as functions of O2 and CO2 concentrations. Not all of the processes or parameters could be included in the model development because of their large number and complexity. The structure of the model consists of time-dependent simultaneous equations for vertical transport of water, heat, O2, and CO2 through unsaturated soils. Finite difference methods are used to solve the mathematical model.