The effect of oxygen concentration on the decomposition of organic materials in soil

Summary

1. Techniques are described for relating the oxygen concentrations in the soil water on the surfaces of micro-organisms to their metabolizing activities.

2. Studies were made on the decomposition of organic materials in water-saturated crumbs (mean radius 1.55 × 10⁻¹ cm) of a loam soil.

3. Respiration of water-saturated crumbs was not inhibited unless the oxygen concentration was less than about 10⁻⁶ M. Evidence was obtained that above a similar low oxygen concentration there was no inhibition of respiration in soils of widely different type.

4. Anaerobic decomposition of the soil organic matter was very slow. Anaerobic decomposition of casein digest was more rapid than that of any other material tested; the products were water soluble and included 83 µ-equivalents of volatile fatty acid per mg of α-amino-N decomposed.

5. Casein digest percolation of soil crumbs under air resulted in the formation of micro-organisms that respired at 70 per cent of their maximum rate when the oxygen concentration was about 2.7 × 10⁻⁶ M.

6. No products of anaerobic casein digest decomposition could be detected on percolating casein digest through soil crumbs when 80 per cent of the soil contained no oxygen and the maximum concentration in any part of the soil was about 3 × 10⁻⁵ M.

7. The kinetics of oxygen uptake consequent on the decomposition of casein digest and of other simple organic compounds in soil crumbs were similar and were only slightly affected by reduction of oxygen partial pressure in the atmosphere from 15 to 1.7 cm of mercury.

8. It is concluded that ‘change-over’ from aerobic to anaerobic metabolism of organic materials takes place in widely different soils at an oxygen concentration less than about 3 × 10⁻⁶ M.