The Fate of ¹⁵N Tracer in Waterlogged Peat Cores from Two Central European Bogs with Different N Pollution History

Under low nitrogen (N) input into rain-fed peat bogs, Sphagnum moss efficiently filters incoming N, preventing invasion of vascular plants and peat oxygenation. Elevated atmospheric N deposition, in combination with climatic warming, may cause retreat of bryophytes and degradation of peat deposits. There are concerns that higher emissions of greenhouse gases, accompanying peat thinning, will accelerate global warming. Breakthrough of deposited N below living moss has been quantified for two Central European peat bogs dominated by Sphagnum magellanicum. In the 1990s, the northern site, ZL, received three times more atmospheric N (> 40 kg ha⁻¹ year⁻¹) than the southern site, BS. Today, atmospheric N inputs at both sites are comparable (15 and 11 kg ha⁻¹ year⁻¹, respectively). Replicated peat cores were collected from the wet central segments of both study sites, ¹⁵N-NO₃⁻ tracer was applied on the moss surface, and the peat cores were incubated under water-logged conditions. After 40 weeks, the rate of downcore leaching of the ¹⁵N tracer was assessed. The recent history of high N pollution at ZL did not accelerate ¹⁵N penetration into deeper peat layers, relative to BS. At both sites, less than 3% of the ¹⁵N tracer reached the shallow depth of 9 cm. Analysis of control peat cores, along with a ²¹⁰Pb chronology, revealed removal of the "excess" N from the ZL peat profiles prior to sampling. Following a decrease of atmospheric N pollution in the past two decades, efficient filtering of atmospheric N by Sphagnum at ZL has been renewed.