How helophytes influence the phosphorus cycle in degradedinundated peat soils – Implications for fen restoration
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SubjectLeaching; Phosphorus retention; Phragmites australis; Top soil removal; Redox interface; Rewetting
When severely degraded fens are rewetted, they often become shallow lakes with an average water depthof less than 1 m. The additional high nutrient availability in highly decomposed peat soils of these newlyformed ecosystems favours the fast establishment of a small number of helophytes while the return of losttarget species like low sedges and brown mosses could be delayed for decades. We hypothesise that thephosphorus (P) uptake of the newly developed vegetation substantially influences the P cycle in rewettedfens. Therefore, we investigated how much of the P released in upper degraded peat soils is pumpedacross the redox-interface between the soil and surface water (=‘P barrier’) during the growing season(∼150 days) by six helophytes (Phragmites australis, Typha latifolia, Glyceria maxima, Carex acutiformis,Carex riparia, and Phalaris arundinacea) in five rewetted fens. We then assessed how this would affect thedifferent plant-available P fractions in the rooted degraded peat layers. The highest P uptake during thegrowing season (duration 150 days from May to September) was recorded for T. latifolia and G. maxima(3.0 and 2.8 g m−2, respectively). Overall, the P uptake was in the range of the P mobilisation rates wemeasured in highly decomposed peat soils (range: 0.8–15.6 g P m−2, n = 30), but four to 10-fold higherthan diffusive net P fluxes at the interface between soil and surface water. Accordingly, helophytes areable to compensate for the high P mobilisation in degraded peat soils during the growing season, byincorporating this P into biomass. On the other hand a large part of the plant-P stock is released after dieback through leaching and mineralisation, which increases the P load of these newly formed shallow lakesand possibly also of adjacent water courses. We estimated that it would still take 20–50 years to exhaustthe large pool of plant-available P in highly decomposed peat soils if aboveground biomass was removed.Without any further management apart from fen rewetting it is unlikely that the fens will return to lownutrient levels within a human life time.