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

Abstract

Nitrogen fluxes and processes were estimated in sandy and muddy sediments from the shallow eutrophic lake Nuldernauw, The Netherlands. N 2, NH 4 +, NO x − and CH 4 fluxes were measured from sediment samples collected throughout the year and incubated under both oxic and anoxic conditions at 2, 12 and 23°C. Fluxes increased with temperature with a mean temperature factor of 1.9±0.3 for a 10°C increase for both sediment types. At the same temperature the total N fluxes (N 2+NO x −+NH 4 +) from the muddy sediments were generally larger than those from the sandy sediments. These differences are related to the relatively high availability of decomposable organic matter in the muddy sediments compared to the sandy sediments. Especially the denitrification was influenced by the organic matter content: 75–90% of the total N flux was denitrified by the muddy sediment whereas only 45–65% was denitrified by the sandy sediments. NH 4 + fluxes were much higher and NO x − fluxes were much lower in cores collected just after spring bloom of phytoplankton, compared to cores collected during other periods. This effect was most pronounced at the high incubation temperature. The freshly settled and easily degradable organic matter at the top of the sediment appeared to be of great influence. Based on the results a general concept, combining the effects of temperature and (easily degradable) organic matter on N loss due to the coupled denitrification, was postulated. The concept implies that the coupled denitrification initially increases with increasing contents of (fresh) organic matter and/or temperature. By a further increase of organic matter or temperature, however, more oxygen is consumed by the aerobic mineralization and the CH 4 oxidation and little or no oxygen remains available for the oxidation of nitrogen. Consequently no or less coupled denitrification 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 nitrification–denitrification will be reduced and ammonium will be released to the overlying water where it can be consumed by algae.