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Coupled nitrification-denitrification rates in the rhizosphere of 4 different aquatic macrophytes, Zostera manna, Lobelia dortmanna, Littorella uniflora and Potamogeton pectinatus, were measured by means of both an indirect and a direct l s ~ technique. Using the indirect technique we estimated the fraction of 02, excreted by plant roots in the rhizosphere, that was used by coupled nitrification-denitrification. Incubations were carried out in ''NH,' amended oxic slurries. This fraction was multiplied with values for root 0, excretion reported in the literature. The indirect technique allowed us to quantify both the oxidation of NH4+ to NO3 . and the reduction of NO3to NH,+ compared to denitrification. Using the direct technique we estimated coupled nitrification-denitrification from the amount of I5Nz labeled gas that accumulated in intact sediment-plant systems. The 2 techniques recorded similar trends. In these cores '"F&+ was added homogeneously to the rhizosphcrr by peifusion. Denitrification activity in the rhizosphere of 2, and P. pectinatus was low (1.5 to 5 1.1rnol N m-' h-') compared to the activity measured in L. dortmanna and L. uniflora vegetated sediments (24 and 30 pm01 N m-2 h-'). 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 NO, 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

Direct and indirect measurements of nitrification and denitrification in the rhizosphere of aquatic macrophytes