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The effects of nitrate availability and the presence of Glyceria maxima on the composition and activity of the dissimilatory nitrate-reducing bacterial community were studied in the laboratory. Four different concentrations of NO 3 ؊ , 0, 533, 1434, and 2,905 g of NO 3 ؊-N g of dry sediment ؊1 , were added to pots containing freshwater sediment, and the pots were then incubated for a period of 69 days. Upon harvest, NH 4 ؉ was not detectable in sediment that received 0 or 533 g of NO 3 ؊-N g of dry sediment ؊1. Nitrate concentrations in these pots ranged from 0 to 8 g of NO 3 ؊-N g of dry sediment ؊1 at harvest. In pots that received 1,434 or 2,905 g of NO 3 ؊-N g of dry sediment ؊1 , final concentrations varied between 10 and 48 g of NH 4 ؉-N g of dry sediment ؊1 and between 200 and 1,600 g of NO 3 ؊-N g of dry sediment ؊1 , respectively. Higher input levels of NO 3 ؊ resulted in increased numbers of potential nitrate-reducing bacteria and higher potential nitratereducing activity in the rhizosphere. In sediment samples from the rhizosphere, the contribution of denitrification to the potential nitrate-reducing capacity varied from 8% under NO 3 ؊-limiting conditions to 58% when NO 3 ؊ was in ample supply. In bulk sediment with excess NO 3 ؊ , this percentage was 44%. The nitrate-reducing community consisted almost entirely of NO 2 ؊-accumulating or NH 4 ؉-producing gram-positive species when NO 3 ؊ was not added to the sediment. The addition of NO 3 ؊ resulted in an increase of denitrifying Pseudomonas and Moraxella strains. The factor controlling the composition of the nitrate-reducing community when NO 3 ؊ is limited is the presence of G. maxima. In sediment with excess NO 3 ؊ , nitrate availability determines the composition of the nitrate-reducing community.

The influence of nitrate addition and the presence of Glyceria maxima (reed sweetgrass) on the composition and nitrate metabolism of the dissimilatory nitrate‐reducing bacterial community was investigated. Anoxic freshwater sediment was incubated in pots with or without G. maxima and with or without addition of nitrate. After incubation the sediments were sampled. Dilution series of these sediment samples were incubated in deep agar tubes and almost all colonies from the most diluted tubes were isolated and purified. When the nitrate concentration was low, 80% of the nitrate‐reducing community in the rhizosphere of G. maxima consisted of NO2−‐accumulating or NH4+‐producing Bacillus strains. In bulk sediment with low nitrate concentrations, denitrifying Pseudomonas and Acinetobacter strains were dominant. The difference in the composition of the nitrate‐reducing community between sediment with or without plants disappeared when nitrate was added. Denitrifying pseudomonads then made up 78–86% of the nitrate‐reducing community. This shows that when the nitrate concentration was low, the presence of G. maxima greatly influenced the nitrate‐reducing community. However, when nitrate was present and available, the composition and nitrate metabolism of the dominant nitrate‐reducing community was probably not affected by G. maxima.

A B S T R A C TThe dissimilatory nitrate-reducing processes, denitrification, and dissimilatory nitrate-reduction to ammonium were studied in freshwater lake sediments within healthy and degrading Phragmites australis The nitrate reduction rates were calculated based on an incubation period of one hour. The denitrification rate in sediment from the degrading vegetation site was higher than from the healthy vegetation site. The rate of dissimilatory nitrate reduction to ammonium was almost tenfold higher in sediment from the degrading vegetation site compared to sediment from the healthy vegetation site. The significantly lower percentages of dissimilatory nitrate reduction to ammonium and denitrification in the healthy stand compared to the degrading stand was probably due to the presence of roots and rhizomes.In the sediments of healthy and degrading P. australis stands, denitrification was the main nitratereducing process.Publication nr. 2202 NIOO-CTO.

The dissimilatory nitrate-reducing bacterial community in the rhizosphere of aerenchymatous plant species such as Glyceria maxima, consists of oxidative, denitrifying and fermentative nitrate-ammonifying bacteria. To study the respective ecological niches of both types of nitrate-reducing bacteria, competition for nitrate or glucose between the representative denitrifier Pseudomonas fluorescens and the representative fermentative nitrate-ammonifying Bacillus licheniformis under continuous or fluctuating anoxic conditions were performed in continuous culture. Competition started by mixing the separate, steady-state mono-cultures of the two species at different ratios. All the experiments were performed at a dilution rate of 0.05 h 3I . The competition was followed by measuring concentrations of nitrogen, glucose and fatty acids and by determining the cell numbers of P. fluorescens and B. licheniformis. Under continuous anoxic nitrate-limited conditions and under certain fluctuating anoxic conditions (8 h 10% and 16 h 0% air saturation), B. licheniformis was able to maintain itself in the chemostat at a low percentage of 4^7%. Under continuous anoxic glucose-limited conditions and under specific fluctuating anoxic (16 h 10% and 8 h 0% air saturation) conditions, B. licheniformis washed out. The outcome of the competition was explained by a higher affinity of P. fluorescens for nitrate and glucose compared to B. licheniformis. B. licheniformis was able to maintain itself in the chemostat under continuous anoxic nitrate-limited conditions and under certain fluctuating anoxic conditions (8 h 10% and 16 h 0% air saturation) due to the fermentation of the remaining glucose. z