Aerobic and anaerobic decomposition of organic matter in marine sediment: Which is fastest?

The enigma of aerobic vs, anaerobic decomposition in marine sediments was addressed by means of a thin-layer incubation technique. Two different C-14-labeled plant materials, aged diatoms (Skeletonema costatum) and fresh barley hay, were each mixed into intertidal sediment and spread in a 1.5-mm layer on the bottom of oxic and anoxic chambers. After a 27-d incubation, conditions in all chambers were switched from aerobic to anaerobic and vice versa for 11 d. Rates of (CO2)-C-14 evolution in diatom chambers showed that aerobic carbon mineralization was similar to 10 times faster than anaerobic both before and after the switch. Low rates of [C-14]DOC release suggested that the limiting step of anaerobic decay was the initial hydrolytic and fermentative enzymatic attack on the predecomposed diatoms. Initial carbon mineralization of barley hay was not affected by the presence or absence of oxygen. Leaching of DOC from the fresh barley hay supplied anaerobic respirers with labile substrates. When leaching ceased and after the aerobic-anaerobic switch, the rate of anaerobic mineralization was reduced. Mineralization of leachable and easily hydrolyzable compounds from fresh plant detritus is equally fast under aerobic and anaerobic conditions. When structural components dominate the particulate remains, anaerobic processes are hampered by inefficient and slow bacterial hydrolysis of structurally complex macromolecules.