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

[摘要] Elevated nitrogen (N) deposition changes the retention, transformation, andfluxes of N in ombrotrophic peatlands. To evaluate such effects we applied a¹⁵N tracer (NH₄ ¹⁵NO₃) at a rate of2.3 g N m⁻² yr⁻¹ to mesocosms of five European peatlands withdiffering long-term N deposition rates for a period of 76 days of dry and 90days of wet conditions. We determined background N content and moss lengthgrowth, and recovered the ¹⁵N tracer from the mosses, graminoids,shrubs, the peat, and dissolved N. Background N contents in Sphagnummosses increased from 5.5 (Degerö Stormyr, deposition < 0.2 g N m⁻² yr⁻¹) up to 12.2 mg g⁻¹ (FrölichshaierSattelmoor, 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 to30 (nitrate) and 11 mg L⁻¹ (ammonium) was found in peat fromFrölichshaier Sattelmoor. Sphagnum mosses (down to 5 cm belowsurface) generally intercepted large amounts of ¹⁵N (0.2–0.35mg g⁻¹) and retained the tracer most effectively relative to theirbiomass. Similar quantities of the ¹⁵N were recovered from the peat,followed by shrubs, graminoids, and the dissolved pool. At the most pollutedsites we recovered more ¹⁵N from shrubs (up to 12.4 %) and fromnitrate and ammonium (up to 0.7 %). However, no impact of N deposition on¹⁵N retention by Sphagnum could be identified and their lengthgrowth was highest under high N background deposition. Our experimentsuggests that the decline in N retention at levels above ca.1.5 g m⁻² yr⁻¹, as expressed by elevated near-surface peat Ncontent and increased dissolved N concentrations, is likely more modest thanpreviously thought. This conclusion is related to the finding thatSphagnum species can apparently thrive at elevated long-term Ndeposition rates in European peatlands.

[学科分类] 地球化学与岩石