Elevated nitrogen (N) deposition changes the retention, transformation, and fluxes of N in ombrotrophic peatlands. To evaluate such effects we applied a N-15 tracer ((NH4NO3)-N-15) at a rate of 2.3 g Nm(-2) yr(-1) to mesocosms of five European peatlands with differing long-term N deposition rates for a period of 76 days of dry and 90 days of wet conditions. We determined background N content and moss length growth, and recovered the N-15 tracer from the mosses, graminoids, shrubs, the peat, and dissolved N. Background N contents in Sphagnum mosses increased from 5.5 (Degero Stormyr, deposition <0.2 g N m(-2) yr(-1)) up to 12.2 mg g(-1) (Frolichshaier Sattelmoor, 4.7-6.0 gNm(-2) yr(-1)). In peat from Degero, nitrate and ammonium concentrations were below 3 mg L-1, whereas up to 30 (nitrate) and 11 mg L-1 (ammonium) was found in peat from Frolichshaier Sattelmoor. Sphagnum mosses (down to 5 cm below surface) generally intercepted large amounts of N-15 (0.2-0.35 mg g(-1)) and retained the tracer most effectively relative to their biomass. Similar quantities of the N-15 were recovered from the peat, followed by shrubs, graminoids, and the dissolved pool. At the most polluted sites we recovered more N-15 from shrubs (up to 12.4 %) and from nitrate and ammonium (up to 0.7 %). However, no impact of N deposition on N-15 retention by Sphagnum could be identified and their length growth was highest under high N background deposition. Our experiment suggests that the decline in N retention at levels above ca. 1.5 g m(-2) yr(-1), as expressed by elevated near-surface peat N content and increased dissolved N concentrations, is likely more modest than previously thought. This conclusion is related to the finding that Sphagnum species can apparently thrive at elevated long-term N deposition rates in European peatlands.
