The fate of N-15-nitrate in healthy and declining Phragmites australis stands

The dissimilatory nitrate-reducing processes, denitrification, and dissimilatory nitrate-reduction to ammonium were studied in freshwater lake sediments within healthy and degrading Phragmites australis (reed) stands. The samples from the healthy vegetation site contained roots and rhizomes. Cores were supplied with 1.9-5.2 mu g N-15-NO3- g(-1) dry sediment in the laboratory and subsequently incubated for 8 h at 20 degrees C, in the dark. The N-15 compounds were determined before (natural percentage of N-15) and after 1 and 8 h of incubation. The uptake of N-15 by the roots and rhizomes in the healthy vegetation was 61%. Nitrogen losses, interpreted as denitrification, accounted for 25 and 84% of the added N-15- NO3- in sediment from the healthy and degrading vegetation sites, respectively. The percentages of nitrate reduced to ammonium were 4 and 9% in sediment from the healthy vegetation and degrading vegetation sites, respectively. The percentage of N-15-total N in the sediment of the healthy vegetation site was 10%, whereas for the degrading vegetation site this percentage was 7%. The percentage of nitrate reduced to ammonium could be potentially underestimated by the percentage of N-15 measured in the sediment. In this case, in healthy and degenerating P. australis stands, the percentage of produced ammonium accounted for 14-16%. 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 nitrate-reducing process. [KEYWORDS: Nitrate reduction; waterlogged soils; n-15 dilution; denitrification; ammonium; nitrification; sediment; nitrogen; oxygen; competition]