Initial rates (30–60 days) and C:N stoichiometry of decomposition were examined in an organic-poor sediment (0.5% LOI) amended with fresh and dried yeast (Y) and Ruppia maritima (R) detritus by the use of “open system” core incubations and “closed system” jar incubations. High organic additions (0.5% dw) inhibited anaerobic carbon mineralization (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 (0.08% dw), on the other hand, stimulated both carbon and nitrogen mineralization by 1–2(R) and 3(Y) times. The comparison of reaction rates involving CO2, SO42− and NH4+ estimated from (1) modeling of porewater profiles (“open system”), (2) temporal changes in jars (“closed system”) and (3) sediment-water fluxes, documented equal applicability of these techniques in non-bioturbated sediment (except for NH4+ in (3) where nitrification interfered). The modeling approach (1) also suggested that the TCO2 deficiency observed in the uppermost oxidized zone of the sediment can be explained by rapid CO2 fixation by e.g. sulfide oxidizing 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. DOC) in estimates from highly amended anaerobic sediments due to the uncoupling of carbon and nitrogen mineralization. The stoichiometry of inorganic mineralization products can only be used to describe particulate organic matter decay in sediments where the concentration of DOC is negligible. C:N ratios obtained in the present study indicated that the major compounds being degraded in unamended (with an indigenous diatom pool) and yeast amended sediment were proteins (C:N = 4–5), whereas in Ruppia amended sediment carbohydrates were more important (C:N = 6–9).