Effects of Reduced and Oxidised Nitrogen on Rich-Fen Mosses: a 4-Year Field Experiment

Abstract

Dutch fens, subjected to high nitrogen (N) deposition levels with reduced N (NHy) highly dominating over oxidised N (NOx), have since the second half of the past century seen a significant decline of Scorpidium and other characteristic brown moss species, while several Sphagnum species have increased rapidly. This promotes acidification and the transition from rich to poor fens. In line with the outcomes of previous short-term water culture experiments, we hypothesised that Scorpidium growth is negatively affected by NHy due to ammonium toxicity, but not by NOx deposition, and that Sphagnum grows equally well on both N forms. To test this hypothesis under field conditions, we carried out a 4-year N addition experiment (5.0 g N m−2 year−1, applied either as NO3−-N or as NH4+-N) on natural mixed Scorpidium revolvens–Sphagnum contortum stands in a rich fen with relatively low background N deposition. After 4 years, ammonium addition had significantly reduced Scorpidium growth, while Sphagnum had not significantly been affected by N additions. Increased ammonium levels were directly toxic to Scorpidium, while Sphagnum was not affected. Furthermore, N addition (in particular nitrate) also indirectly influenced moss growth through promoting vascular plants. Our study confirms that it is ecologically relevant to consider the specific form in which N enrichment occurs, i.e. the ratio of NHy vs. NOx. We conclude that in rich fens, the risk of rapid transition of the moss layer to dominance of poor-fen species is strongly promoted by increased deposition of reduced N.