Data from: 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 -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.1. 低磷（P）有效性限制了碱沼（fen）的植物生物量生产，而这是诸多濒危植物物种得以存续的前提条件。本研究提出假说：磷限制与土壤铁(Fe)含量及土壤铁磷比(Fe:P)密切相关，因为铁化合物可为溶解态磷提供结合位点，推测会降低植物可获取的磷有效性。 2. 本研究通过跨欧洲野外调查采集了30处碱沼的样本，以明确土壤铁库与磷库及铁结合态磷库之间的关联；同时测定了植被的磷吸收量与氮磷比(N:P)，以此评估磷限制发生的区域。随后，我们在中型实验生态系统(mesocosm)中测定了喙状苔草(Carex rostrata)的磷吸收量，以探究土壤铁库、磷库（及其组分）与水位条件（排水或复湿）之间的交互效应。 3. 野外调查结果显示，无论碱沼的退化程度、采样位置或采样深度如何，土壤磷库均与土壤铁库呈显著正相关。此外，土壤铁库与磷库均与植被的磷吸收量呈正相关，而与植被氮磷比呈负相关。通常而言，当土壤铁含量超过15 mmol Fe·L⁻¹、土壤磷含量超过3.3 mmol P·L⁻¹时，植被氮磷比会降至10 g·g⁻¹以下。濒危碱沼植物主要在铁（及相应的磷）匮乏的碱沼中长势良好。 4. 中型实验生态系统实验进一步表明，水位条件与磷库之间的交互作用决定了植物的磷吸收量：尽管碱沼复湿整体上提升了植物的磷吸收量，但在经排水形成的富铁土壤且酸可提取态磷库（>1.6 mmol Pacid·L⁻¹）较高的生境中生长的植物，仍能固存大量磷。土壤铁磷比对植物磷吸收量无显著影响。 5. 结论与应用展望。本研究结果对濒危碱沼植物群落的管理与修复具有重要指导意义。我们证实了碱沼中铁磷(Fe-P)结合存在模糊性：大型铁库会“困住”移动态磷，反而提升了植物整体可获取的磷有效性，而非降低其有效性。针对磷限制，我们提出经验阈值：土壤磷含量<3.3 mmol P·L⁻¹，该阈值主要存在于铁匮乏的碱沼中。通过复湿修复碱沼会提升磷的相对有效性，因此未必总能为濒危碱沼植物群落提供适宜的生境。当土壤磷库与铁库分别远低于3.3 mmol·L⁻¹与15 mmol·L⁻¹时，对排水后碱沼的复湿修复最易取得成功。