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ABSTRACT. 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 muted into the sediment.On an area basis, sediment acid hydrolyzable amino acids accounted for 24 '4, of the particulate organic carbon pool and 53":, of the part~culate organlc nitrogen pool. Similarly, porewater acid 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 mnlol N m-' d-l) than the estimated efflux of dissolved inorganic nitrogen ( < 2 mm01 N m-' d-l). 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 wlth a high carbon oxidation rate suggested degradation of organic matter with a low average nitrogen content and possible bacterial nitrogen assimdation. 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 degradat~on. The efficiency of bacterial carbon incorporation was 0.33 and within the range prev~ously encountered in sediments of Danish waters

Nitrous oxide (N 2 O) is a potent greenhouse gas with a high contribution from agricultural soils and emissions that depend on soil type, climate, crops and management practices. The N 2 O emissions therefore need to be included as an integral part of environmental assessments of agricultural production systems. An algorithm for N 2 O production and emission from agricultural soils was developed and included in the FASSET whole-farm model. The model simulated carbon and nitrogen (N) turnover on a daily basis. Both nitrification and denitrification was included in the model as sources for N 2 O production, and the N 2 O emissions depended on soil microbial and physical conditions. The model was tested on experimental data of N 2 O emissions from grasslands in UK, Finland and Denmark, differing in climatic conditions, soil properties and management. The model simulated the general time course of N 2 O emissions and captured the observed effects of fertiliser and manure management on emissions. Scenario analyses for grazed and cut grasslands were conducted to evaluate the effects of soil texture, climatic conditions, grassland management and N fertilisation on N 2 O emissions. The soils varied from coarse sand to sandy loam and the climatic variation was taken to represent the climatic variation within Denmark. N fertiliser rates were varied from 0 to 500 kg N ha À1 . The simulated N 2 O emissions showed a non-linear response to increasing N rates with increasing emission factors at higher N rates. The simulated emissions increased with increasing soil clay contents. N 2 O emissions were slightly increased at higher temperatures, whereas increasing annual rainfall generally lead to decreasing emissions. Emissions were slightly higher from grazed grasslands compared with cut grasslands at similar rates of total N input (fertiliser and animal excreta). The results indicate higher emission factors and thus higher potentials for reducing N 2 O emissions for intensively grazed grasslands on fine textured soils than for extensive cut-based grasslands on sandy soils.

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The initial benthic degradation of senescent Zostera marina leaves was studied in controlled flowthrough microcosm chambers for 33 d. Sediment chambers without added eelgrass leaves served as control chambers. The inflowing artificial seawater and ouflowing seawater were analyzed for dissolved organic carbon and nitrogen (DOC and DON), total acid-hydrolyzable amino acids (THAA), dissolved free amino acids (DFAA), urea, NH , NO , incubation, 24.3% of the DON efflux was identified as acid-hydrolyzable amino acids, dissolved free amino acids, and urea in the chambers with eelgrass addition, whereas these compounds accounted for 33.8% of the DON efflux in the control chambers. There were indications of a stimulated bacterial growth on the eelgrass leaves during the first 7 d after leaf addition that was measured as an increase in acid-hydrolyzable amino acids. Further, there was a gradual increase in acid-hydrolyzable amino acids in the sediment throughout the incubation that could only be explained as bacterial growth (and/or protein synthesis). Most of the nitrogen for microbial growth was mobilized from the indigenous particulate organic nitrogen pool, whereas it could be inferred that the energy source for bacterial growth was mainly supplied from the added eelgrass leaves. Most of the nitrogen mineralized within the sediment was incorporated into the microbial biomass with a resultant low efflux of inorganic nitrogen from the sediment to the water column.