Nitrification and denitrification in the rhizosphere of the aquatic macrophyte Lobelia dortmanna L.

Nitrogen and O-2 transformations were studied in sediments covered by Lobelia dortmanna L.; a combination of N-15 isotope pairing and microsensor (O-2, NO3-, and NH4+) techniques were used. Transformation rates and microprofiles were compared with data obtained in bare sediments. The two types of sediment were incubated in double-compartment chambers connected to a continuous flow-through system. The presence of L. dortmanna profoundly influenced both the nitrification-denitrification activity and porewater profiles of O-2, NO3-, and NH4+ within the sediment. The rate of coupled nitrification-denitrification was greater than sixfold higher in L. dortmanna-vegetated sediment than in bare sediment throughout the light-dark cycle. Illumination of the Lobelia sediment reduced denitrification activity by similar to 30%. In contrast, this process was unaffected by light-dark shifts in the bare sediment. Oxygen microprofiles showed that O-2 was released from the L. dortmanna roots to the surrounding sediment both during illumination and in darkness. This release of O-2 expanded the oxic sediment volume and stimulated nitrification, shown by the high concentrations of NO3- (similar to 30 mu M) that accumulated within the rhizosphere. Both N-15(2) isotope and microsensor data showed that the root-associated nitrification site was surrounded by two sites of denitrification abo-ye and below, and this led to a more efficient coupling between nitrification and denitrification in the Lobelia sediment than in the bare sediment.