Effects of macrophytes on potential nitrification and denitrification in oligotrophic lake sediments

Macrophytes may either stimulate or depress nitrogen-related microbial processes via radial oxygen loss (ROL), production of exudates or uptake of inorganic N. ROL can favor aerobic processes as nitrification, exudates may stimulate denitrification, whereas N assimilation and competition with microbes may depress both processes. We measured rates of potential nitrification (PN) and denitrification (PD) in oligotrophic lacustrine sediments colonized by submersed and emergent macrophytes. Potential rates were also analyzed in adjacent control sediments devoid of vegetation. Aim of the work was to verify if the presence of macrophytes alters the potential activity of nitrifying or denitrifying bacteria. Vertical profiles (0−10 cm depth) of PN and PD rates were measured via oxic (nitrification) and anoxic (denitrification) slurries, where we measured the accumulation of NOx− from added NH4+ and the production of 30N2 from added 15NO3−, respectively. Results suggest that under oligotrophic settings macrophytes produced small effects on potential nitrification and denitrification activities. Despite elevated oxygen release demonstrated for most of the tested macrophytes, nitrification was likely constrained by significant plant-bacteria competition. Potential denitrification was comparatively more stimulated by macrophytes, but we address this result to a general increase of heterotrophic microbial activity in organic-richer vegetated sediments, due to dead root biomass or exudates. The highest PN and PD rates were measured in sediments colonized by Littorella uniflora, likely due to its large underground biomass (root:shoot ratio ∼3.5), root porosity and oxygen leakage.