Nitrogen or phosphorus limitation in rich fens? - Edaphic differences explain contrasting results in vegetation development after fertilization

BACKGROUND AND AIMS: Many rich fens are threatened by high nutrient inputs, but the literature is inconsistent with respect to the type of nutrient limitation and the influence of edaphic characteristics. METHODS: We performed experiments with N- and P-fertilization in three endangered rich fen types: floating fen with Scorpidium scorpioides, non-floating fen with Scorpidium cossonii, floodplain fen with Hamatocaulis vernicosus. In addition, K-fertilization was carried out in the floodplain fen. RESULTS: The floodplain fen showed no response to P-addition, but N- and K-addition led to grass encroachment and decline of moss cover and species richness. In contrast, in the P-limited floating fen with S. scorpioides, P-addition led to increased vascular plant production at the expense of moss cover. Scorpidium scorpioides, however, also declined after N-addition, presumably due to ammonium toxicity. The fen with S. cossonii took an intermediate position, with NP co-limitation. These striking contrasts corresponded with edaphic differences. The N-limited fen showed low Ca:Fe ratios and labile N-concentrations, and high concentrations of plant-available P and Fe-bound P. The P-limited fen showed an opposite pattern with high Ca:Fe ratios and labile N-concentrations, and low P-concentrations. CONCLUSIONS: This implies that edaphic characteristics dictate the nature of nutrient limitation, and explain contrasting effects of N- and P-eutrophication in different fens.