Bioturbation and remineralization of sedimentary organic matter: effects of redox oscillation

A variety of field and laboratory observations demonstrate that particle reworking and irrigation activities of benthic fauna promote the remineralization of organic matter. Of the many simultaneous factors involved, repetitive oscillation of redox conditions may be one of the most important. In bioturbated C_org-rich sediments with restricted O₂ penetration, particles constantly cycle between oxic and anoxic zones but typically spend ∼ 10-100 × longer under anoxic than oxic conditions. Cyclic redox patterns are also common within individual burrow structures and are accompanied by rapid switching in dominant metabolic processes. Geometrically and temporally complex redox mosaics are the rule. Experimental evidence and theoretical considerations indicate that even brief, periodic re-exposure to O₂ results in more complete and sometimes rapid decomposition than is possible under constant conditions or unidirectional redox change. Redox oscillation apparently results initially in net remineralization of existing microbial biomass followed by stimulated renewed synthesis (self-priming) in a manner similar to many disturbances or grazing effects. Some properties, such as sedimentary P storage, are comparable under fully oxic and oscillating redox conditions but differ under anoxic. The relative frequency and duration of redox change are presumably critical properties governing response. Redox oscillation common in bioturbated sediments or the terrestrial rhizosphere likely represents a distinct functional environmental state with unique biogeochemical properties. Studies of decomposition and C_org preservation processes should take this possibility into account.