ABSTRACT: Coupled nitrification-denitrification rates in the rhizosphere of 4 different aquatic macrophytes, Zostera marina, Lobelia dortmanna, Littorella uniflora and Potamogeton pectinatus, were measured by means of both an indirect and a direct ¹⁵N technique. Using the indirect technique we estimated the fraction of O₂, excreted by plant roots in the rhizosphere, that was used by coupled nitrification-denitrification. Incubations were carried out in ¹⁵NH₄⁺ amended oxic slurries. This fraction was multiplied with values for root O₂ excretion reported in the literature. The indirect technique allowed us to quantify both the oxidation of NH₄⁺ to NO₃^- , and the reduction of NO₃^- to NH₄⁺ compared to denitrification. Using the direct technique we estimated coupled nitrification-denitrification from the amount of ¹⁵N₂ labeled gas that accumulated in intact sediment-plant systems. The 2 techniques recorded similar trends. In these cores ¹⁵NH₄⁺ was added homogeneously to the rhizosphere by perfusion. Denitrification activity in the rhizosphere of Z. marina and P. pectinatus was low (1.5 to 5 µmol N m^-2 h^-1) compared to the activity measured in L. dortmanna and L. uniflora vegetated sediments (24 and 30 µmol N m^-2 h^-1). The variability in denitrification activity was due to differences in the ability of the plants to oxidize the rhizosphere, and difference in the character of sediment O₂ and NO₃^- consumption. This study attempts to clarify how rooted aquatic vegetation affects the sediment nitrogen cycle via oxygen excretion from the roots. Rhizosphere associated nitrogen transformations (coupled nitrification-denitrification) are central to the understanding of nutrient cycling in shallow aquatic ecosystems.

KEY WORDS: Rooted macrophytes · Rhizosphere · O₂ excretion · Coupled nitrification-denitrification

Published in AME Vol. 19, No. 1 (1999) on September 6
ISSN: 0948-3055. Copyright © Inter-Research, Oldendorf/Luhe, 1999