Organic matter decomposition in soil as influenced by pH and redox conditions

Organic matter decomposition in soils and sediments is known to be less under anaerobic as compared to aerobic conditions. It is also known that soil pH can also influence rate of breakdown. The authors report the effect of sediment pH and oxidation-reduction potential (redox conditions) on oxidation of super(14)C-labelled plant material in a Mississippi River alluvial soil (Mhoon silt loam) which was maintained at controlled pH (5.0, 6.5, 8.0) and redox potentials (- 150, 50, 250 and 500 mV). The results suggest that not only is decomposition slower under anaerobic conditions than under aerobic conditions, as is generally recognized, but that the intensity of aerobic and anaerobic conditions have an effect on decomposition. For example, O sub(2) is generally absent and a soil or sediment is thus considered anaerobic at redox potentials of less than + 350mV at pH 7. However, the rate of decomposition fell with decreasing sediment redox potential below this critical potential for O sub(2). Also, soil pH is interrelated with the redox potential and can also influence the rate of decomposition at the various redox levels which are found in wetland soil and sediments. Redox potential and pH probably govern orgainc matter decomposition by determining the kind, numbers and activity of microorganisms involved in the breakdown processes.