DECAY OF PLANT DETRITUS IN ORGANIC-POOR MARINE SEDIMENT - PRODUCTION-RATES AND STOICHIOMETRY OF DISSOLVED C-COMPOUND AND N-COMPOUND

Initial rates (30-60 days) and C:N stoichiometry of decomposition were examined in an organic-poor sediment (0.5% LOI) amended with fresh an d dried yeast (Y) and Ruppia maritima (R) detritus by the use of ''ope n system'' core incubations and ''closed system'' jar incubations. Hig h organic additions (0.5% dw) inhibited anaerobic carbon mineralizatio n (i.e. sulfate reduction) and stimulated DOC production and nitrogen mineralization 3(R) to 15(Y) times (i.e. hydrolysis and fermentation). This indicated that carbon and nitrogen mineralization in the highly amended anaerobic sediments were uncoupled. Low organic additions (0.0 8% dw), on the other hand, stimulated both carbon and nitrogen mineral ization by 1-2(R) and 3(Y) times. The comparison of reaction rates inv olving CO2, SO42- and NH4+ estimated from (1) modeling of porewater pr ofiles (''open system''), (2) temporal changes in jars (''closed syste m'') and (3) sediment-water fluxes, documented equal applicability of these techniques in non-bioturbated sediment (except for NH4+ in (3) w here nitrification interfered). The modeling approach (1) also suggest ed that the TCO2 deficiency observed in the uppermost oxidized zone of the sediment can be explained by rapid CO2 fixation by e.g. sulfide o xidizing chemoautotrophs. Although the C:N stoichiometry of inorganic decomposition products based on estimate (1) and (2) generally agreed well, it was found crucial to include dissolved organic pools (i.e. DO G) in estimates from highly amended anaerobic sediments due to the unc oupling of carbon and nitrogen mineralization. The stoichiometry of in organic mineralization products can only be used to describe particula te organic matter decay in sediments where the concentration of DOC is negligible. C:N ratios obtained in the present study indicated that t he major compounds being degraded in unamended (with an indigenous dia tom pool) and yeast amended sediment were proteins (C:N = 4-5), wherea s in Ruppia amended sediment carbohydrates were more important (C:N = 6-9).