The fate of organic carbon and nitrogen in experimental marine sediment systems: Influence of bioturbation and anoxia

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Abstract
The decay rate of particulate organic carbon (POC) and nitrogen (PON) was followed during 94 days in three homogenized sediment microcosms: 1. With a natural density of the polychaete Nereis virens (NOx-cores); 2. Defaunated, with an aerobic water phase (Ox-cores); and 3. Defaunated, with an anaerobic water phase (An-cores). In all cores there was a marked preferential mineralization of PON compared to POC. The presence of Nereis increased the net decomposition of POC and PON 2.6 and 1.6 times relative to Ox-cores. Ventilation of burrow structures by the worms increased the flux of O2, TCO2 and DIN across the sediment-water interface 2.5-3.5 times. This significantly decreased the pore water concentrations of TCO2 and DIN. Similarly, nitrification and denitrification were stimulated 2.3-2.4 times due to nereid activity. Oxygen did not increase organic degradation: in fact, the decay of POC and PON was faster in An- than in Ox-cores, 1.5-1.6 and 1.2 times, respectively. Sulfate reduction, measured at the end of experiment, was surprisingly low in the aerobic NOx- and Ox-cores relative to An-cores. Net ammonium production measured at the end of the experiment agreed with the means loss of PON for Ox- and An-cores, but was low for NOx-cores, suggesting that a high C:N substrate was being degraded in these cores at the end. An empirical model describing the temporal decay pattern of POC and PON is presented: the detritus in all cores were initially composed of two fractions (similar C:N); a readily degradable (.apprx. 43%) and a low degradable (.apprx.57%) fraction. A substantial part of the degradable fraction in NOx-cores was used during the experiment, with nitrogen being mineralized preferentially. The mean C:N molar ratio of detritus used was 5.9, compared to a value of 15.5 determined at the end. The OX- and An-cores, however, showed similar C:N ratios for the detritus used during the experiment (3.7 and 4.8) and that measured at the end (4.2 and 4.6). Presumably not all the low C:N detritus had yet been mineralized in these cores at the end of experiment.