Nitrogen fluxes and processes in sandy and muddy sediments from a shallow eutrophic lake

Nitrogen fluxes and processes were estimated in sandy and muddy sediments f rom the shallow eutrophic lake Nuldernauw, The Netherlands. N-2, NH4+, NOx- and CH4 fluxes were measured from sediment samples collected throughout th e year and incubated under both oxic and anoxic conditions at 2, 12 and 23 degrees C. Fluxes increased with temperature with a mean temperature factor of 1.9 +/- 0.3 for a 10 degrees C increase for both sediment types. At the same temperature the total N fluxes (N-2 + NOx- + NH4+) from the muddy sed iments were generally larger than those from the sandy sediments. These dif ferences are related to the relatively high availability of decomposable or ganic matter in the muddy sediments compared to the sandy sediments. Especi ally the denitrification was influenced by the organic matteI content: 75-9 0% of the total N flux was denitrified by the muddy sediment whereas only 4 5-65% was denitrified by the sandy sediments. NH4+ fluxes were much higher and NOx- fluxes were much lower in cores collected just after spring bloom of phytoplankton, compared to cores collected during other periods. This ef fect was most pronounced at the high incubation temperature. The freshly se ttled and easily degradable organic matter at the top of the sediment appea red to be of great influence. Based on the results a general concept, combi ning the effects of temperature and (easily degradable) organic matter on N loss due to the coupled denitrification, was postulated. The concept impli es that the coupled denitrification initially increases with increasing con tents of (fresh) organic matter and/or temperature. By a further increase o f organic matter or temperature, however, more oxygen is consumed by the ae robic mineralization and the CH4 oxidation and little or no oxygen remains available for the oxidation of nitrogen. Consequently no or less coupled de nitrification can occur. Although high temperatures are not often found in the Dutch surface waters, these conditions can occur in spring and summer. Then nitrogen removal from the sediment-water system by the coupled nitrifi cation-denitrification will be reduced and ammonium will be released to the overlying water where it can be consumed by algae. (C) 1998 Elsevier Scien ce Ltd. All rights reserved.