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

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 NO3-to 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 N O , 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