Temporal Variation of Denitrification Activity in Plant-Covered, Littoral Sediment from Lake Hampen, Denmark

Diel and seasonal variations in denitrification were determined in a littoral lake sediment colonized by the perennial macrophyte Littorella uniflora (L.) Aschers. In the winter, the activity was low (5 μmol of N m⁻² h⁻¹) and was restricted to the uppermost debris layer at a depth of 0 to 1 cm. By midsummer, the activity increased to 50 μmol of N m⁻² h⁻¹ and was found throughout the root zone to a depth of 10 cm. The root zone accounted for as much as 50 to 70% of the annual denitrification. A significant release of organic substrates from the roots seemed to determine the high activities of root zone denitrification in the summer. The denitrification in the surface layer and in the root zone formed two distinct activity zones in the summer, when the root zone also contained nitrification activity, as indicated from the accumulations of NO₃⁻. Light conditions inhibited denitrification in both the surface layer and the upper part of the root zone, suggesting that a release of O₂ by benthic algae and from the roots of L. uniflora controlled a diel variation of denitrification. In midsummer, the rate of denitrification in both the surface layer and the upper part of the root zone was limited by NO₃⁻. In the growth season, there was evidence for a significant population of denitrifiers closely associated with the root surface.