Budgets of sediment nitrogen and carbon cycling in the shallow water of Knebel Vig, Denmark

Sediment was sampled from a shallow coastal area (Knebel Vig, Denmark). The vertical distribution of pigments, Pb-210 and Cs-137 indicated that organic matter was mixed into the sediment. On an area basis, sediment acid hydrolyzable amino acids accounted for 24 % of the particulate organic carbon pool and 53 % of the particulate organic nitrogen pool. Similarly, porewater add hydrolyzable amino acids were an important component of dissolved organic carbon and dissolved organic nitrogen (9 and 27%, respectively). It was inferred that ribonucleic acids potentially were an important component of dissolved organic nitrogen. The estimated efflux of dissolved organic nitrogen from the sediment was higher (3.9 mmol N m(-2) d(-1)) than the estimated efflux of dissolved inorganic nitrogen (<2 mmol N m(-2) d(-1)). The high efflux of dissolved organic nitrogen was explained by organic matter hydrolysis close to the sediment surface. The low rates of inorganic nitrogen efflux together with a high carbon oxidation rate suggested degradation of organic matter with a low average nitrogen content and possible bacterial nitrogen assimilation. This was further supported by mass balance calculations on nitrogen incorporation into microbial biomass. The calculated average C/N ratio in the organic matter degraded suggested that only part of the sediment acid hydrolyzable amino acids were available for bacterial degradation. The efficiency of bacterial carbon incorporation was 0.33 and within the range previously encountered in sediments of Danish waters.