Restoration of endangered fen communities: the ambiguity of iron-phosphorus binding and phosphorus limitation

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 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 ironphosphorus (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 L⁻¹ soil, which is mainly found in Fe-poor fens. Restoring fens by rewetting increases the relative availability of 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.