Studies on the Gaseous Environment in Soil (3)

Simulation of the One Dimensional Distribution of CO₂ Concentrations in Soils

Abstract

An investigation of simulations of one dimensional distributions of CO₂concentrations in soils was made. For the simulation, the coefficients of CO₂ diffusion in a soil D (x) were required. The relation between the diffusion coefficient (D) and air permeability (K) in this experiment, was found the following equations.
D=0.093 cm²/sec at K≥1.2 cm/sec
D=0.0845K^0.432cm²/sec at K≤1.2cm/sec
As it was very difficult to determine the D (x) of a soil, the D (x) was estimated from the equations above after determining the air permeability K (x) . Then, we made the assumption that the amount of CO₂ generated in a soil is expressed by the following equation
Pco₂ (t) =Ae^{-B (logt/Tm) 2}+C_const
C (l) _pv was the CO₂ concentration at the bottom of a soil, and was calculated by the Pco₂ change with time. C (l) _pc also was the CO₂ concentration at the same place, and was calculated by assuming that the amount of PCO₂ when the CO₂ concentration was determined was constant throughout the experiment.
Comparing C (l) _pv with C (l) _pc, we found that C (l) _pv was about 0.9-1.6 times as great.
Therefore, distribution of the CO₂ concentration could not be expressed as C (x) so far as the parameters in the equation of PCO₂ are concerned. When the CO₂ concentration at the bottom of a soil was taken as 1 (=RC (l) ), and all the CO₂ concentrations in a soil were taken as RC(x) =C (x) /C (l), distributions of the CO₂ concentrations in soils were found to be regular regardless of the parameters in the equations of Pco₂. Therefore, to express the distributions of the CO₂ concentrations, we used RC (x) which denotes the relative concentrations of CO₂.
The distributions determined and those simulated by the expression above agreed very wll. Thus, the one dimensional distributions in a soil could be simulated.