Articles | Volume 13, issue 3
Research article
04 Feb 2016
Research article |  | 04 Feb 2016

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

K. Zając and C. Blodau

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 (NH415NO3) 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.

Short summary
Peatlands have been exposed to nitrogen (N) deposition in Europe for decades. In this greenhouse study we investigated how N concentration and mobility in plants and peat have responded by sampling five sites across Europe and experimentally depositing labeled nitrogen on samples in a greenhouse. The peat moss retained its ability to absorb labeled nitrogen, but in the polluted sites more of it reached the deeper peat and was taken up by shrubs and grasses, indicating increased mobility of N.
Final-revised paper