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. 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 fractions)
and water levels (drained or rewetted). 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. 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.
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 and 15 mmol L−1 respectively.
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