Decomposition of plant materials in marine sediment exposed to different electron accepters (O2, NO3-, and SO42-), with emphasis on substrate origin, degradation kinetics, and the role of bioturbation

Carbon mineralization of fresh and aged diatoms (Skeletonema costatum) and barley hay (Hordeum vulgare) was followed for 23 to 35 d in sandy and silty sediment. By the use of a thin-layer flow-through technique. it was possible to expose the sediment selectively for oxygen, nitrate or sulfate as electron accepters in the terminal oxidation of organic carbon. Decomposition took place in two basic stages. Mineralization of the rapidly leachable fraction of the fresh materials occurred rapidly and with the same constant rate regardless of the electron acceptor available, indicating that the dissolved organic carbon released initially was labile and readily available for all heterotrophic respirers. In the case of diatoms, decay of the remaining, more refractory, particulate fraction of fresh and aged diatoms were strikingly similar, although both were degraded 5 to 10 times faster under oxic than anoxic conditions. Most of the particulate remains of diatoms after leaching apparently belong to one fraction, which maintains the same degradability even after prolonged aging. With respect to hay, the late divergence in rates of aerobic and anaerobic decay (a factor of 4 to 5 for aged hay only after 20 d) indicated that the larger hay particles (<500 m) became exhausted in labile organic matter much slower through time than fine-particulate diatoms (similar to 20 mum). Anaerobic carbon mineralization rates of diatoms and hay particulates with sulfate and nitrate as electron accepters were similar or up to two times faster with sulfate. The generally low levels of dissolved organic carbon in all incubations after the initial leaching phase suggest that the limiting step of decomposition under both aerobic and anaerobic decay is the initial hydrolytic attack on the complex particulate remains. Based on a volumetric model, we show that the exposure of anoxic subsurface sediment containing partly degraded organic material to oxygen via irrigated worm burrows or by reworking may significantly enhance total sediment carbon oxidation. The enhancement in the irrigation case increases linearly with density (up to 80%) and is higher than the density-independent enhancement (100%) in the reworking case when abundance is above a lower limit of similar to 400 individuals/m(2). Copyright (C) 2001 Elsevier Science Ltd.