1.Low phosphorus (P) availability limits plant biomass production in fens, which is a prerequisite for the persistence of many endangered plant species. We hypothesized that P limitation is linked to soil iron (Fe) content and soil Fe:P ratios as iron compounds provide binding sites for dissolved P, presumably reducing P availability to plants. 2.We sampled 30 fens in a trans-European field survey to determine how soil Fe pools relate to pools of P and Fe-bound P, and we measured vegetation P uptake and N:P ratio to assess where P limitation occurs. Next, we determined P uptake by Carex rostrata in experimental fen mesocosms to investigate interactive effects of soil Fe- and P pools (and -NDASH-fractions) and water levels (drained or rewetted). 3.The field survey revealed that soil P pools correlate positively with soil Fe pools, regardless of fen degradation level, location, or sampling depth. Moreover, soil Fe- and P pools correlated positively with P uptake by the vegetation and negatively with vegetation N:P ratios. Generally, N:P ratios dropped below 10 g g−1 whenever thresholds of 15 mmol Fe L−1 soil and 3.3 mmol P L−1 soil were exceeded. Endangered fen species mainly thrived in Fe- (and thus P-) poor fens. 4.The mesocosm experiment further showed that interactions between water levels and P pools determined plant P uptake: although fen rewetting led to an overall increase in P uptake, plants that had grown on drained Fe-rich soils with large acid-extractable P pools (>1.6 mmol Pacid L−1) could still sequester large quantities of P. Soil Fe:P ratio had no effect on P uptake. 5.Synthesis and applications. Our findings have important implications for the management and restoration of endangered fen communities. We demonstrated the existence of an iron-phosphorus (Fe-P) binding ambiguity in fens: large Fe pools “trap” mobile P, thereby enhancing overall P availability to plants rather than diminishing it. For P limitation we suggest an empirical threshold of < 3.3 mmol P L−1 soil, which is mainly found in Fe-poor fens. Restoring fens by rewetting increases the relative availability of P and may not always result in favourable conditions for endangered fen communities. Rewetting of drained fens is most likely to be successful if soil P and Fe pools are well below 3.3 mmol L−1 and 15 mmol L−1 respectively.

Emsens et al. DatafileThis file contains data of the field survey, vegetation relevees and the mesocosm experiment (3 tabs).Emsens et al. Data.xlsx