Aerobic and anaerobic mineralization of organic material in marine sediment microcosms

Fluxes of total CO2, 02, NO3-+NO2-, NH,', DON (dissolved organic nitrogen) and HSwere measured across the interface of a coastal bay sediment for 43 d. The seawater overlying the defaunated sediment cores was changed continuously. Two treatments were employed: oxygenated overlying water (OX-cores) and anoxic water (AN-cores). Fluxes were measured before and after addition of an organic substrate, and loss of POM (particulate organic matter) was measured at the end of the experiment. Loss of POC (particulate organic carbon) from the sediment was the same under aerobic and anaerobic conditions. However, the highest efflux of CO2 was measured in the OX-cores, and the highest efflux of DOC occurred in the AN-cores before and after addition of fresh substrate. Similarly, loss of PON (particulate organic nitrogen) from the sediment was the same in the 2 treatments, but the highest fluxes of NH,' and DON were measured In the AN-cores. Our conclusion is that degradation of POM is the same under both aerobic and anaerobic condltlons, but that mineralization of organic molecules is less effective under anaerobic condlhons. Before substrate addi t~on there was an influx of NO3in both OXand AN-cores, and the estimated nitrification rate in the OX-cores was 1 to 2 mm01 m-' d-l The estimated denitrification rate contributed 3 to 6 '10 to total carbon respiration. After addition of the substrate, fluxes of all constituents increased. The estimated denitrification rate constituted < 3 '10 of total respiration. There was a large efflux of HSin AN-cores During the experimental period, the losses of original POC and PON were calculated to be 2.6 and 4.0 %, respectively. Relative to the initial values, the pore water constituents increased with incubation, and the integrated AN/OX ratios for NHdf , DON and total CO2 were 1.2, 1.5 and 1.6, respectively, after 43 d.