DISTRIBUTION OF CARBON DIOXIDE IN THE AQUEOUS PHASE OF AEROBIC SOILS

Theory predicts that in the aqueous phase of aerobic soils carbon dioxide concentration/distance gradients are 1·35 times as large as those for oxygen. Because of differences in solubility, the partial pressure/distance gradients of carbon dioxide should be less than one-twentieth of the gradients of oxygen partial pressure. Experiments were carried out to test this prediction. In some experiments oxygen partial pressures were measured directly by means of a micro-electrode and carbon dioxide indirectly by methods involving microelectrode determinations of specific conductance in agar-CaCO₃ gel. Measurements were made in models of water-saturated soil consisting of yeast cells embedded in agar-CaCO₃ gel, around plant roots in agar-CaCO₃ gel and within water-saturated soil. Oxygen partial pressures fell by 5–15 per cent of an atmosphere over distances of 0·2–0·4 cm whereas carbon dioxide partial pressures increased by less than 0·3 per cent of an atmosphere over the same distance. In other experiments different thicknesses of water-saturated soil were incubated with their upper surfaces exposed to air. The mean oxygen partial pressure fell by 7–13 per cent of an atmosphere with increasing the thickness from 0·2 to 0·4 cm whereas the carbon dioxide partial pressure never increased by more than 0·2 per cent of an atmosphere. Both sets of evidence confirm the theoretical prediction. It is concluded that the partial pressure of carbon dioxide in the aqueous phase of soils containing no oxygen-free zones, would never, even at the surfaces of roots, be more than 1 per cent of atmosphere greater than in the gas phase.