The fate of ¹⁵N-nitrate in mesocosms from five European peatlands differing in long-term nitrogen deposition rate

Abstract. Elevated nitrogen (N) deposition changes the retention, transformation, and fluxes of N in ombrotrophic peatlands. To evaluate such effects we applied a ¹⁵N tracer (NH₄ ¹⁵NO₃) at a rate of 2.3 g N m⁻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 ¹⁵N tracer from the mosses, graminoids, shrubs, the peat, and dissolved N. Background N contents in Sphagnum mosses increased from 5.5 (Degerö Stormyr, deposition < 0.2 g N m⁻² yr⁻¹) up to 12.2 mg g⁻¹ (Frölichshaier Sattelmoor, 4.7–6.0 g N m⁻² yr⁻¹). In peat from Degerö, 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 Frölichshaier Sattelmoor. Sphagnum mosses (down to 5 cm below surface) generally intercepted large amounts of ¹⁵N (0.2–0.35 mg g⁻¹) and retained the tracer most effectively relative to their biomass. Similar quantities of the ¹⁵N were recovered from the peat, followed by shrubs, graminoids, and the dissolved pool. At the most polluted sites we recovered more ¹⁵N from shrubs (up to 12.4 %) and from nitrate and ammonium (up to 0.7 %). However, no impact of N deposition on ¹⁵N 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.