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

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 pmol 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.