Effects of Ca- and Fe-rich seepage on P availability and plant performance in calcareous dune soils

Ca- and Fe-rich seepage in wet dune slacks often sustains oligotrophic, species rich vegetation as a result of reduced P availability. While this effect has been attributed to pH buffering, we tested whether Ca- and Fe-rich seepage also immobilised P in calcareous soils with a strong pH buffer. Two oligotrophic species (Carex flacca and Schoenus nigricans) and two eutrophic species (Calamagrostis epigejos and Molinia caerulea) were planted in experimental sods. After 4 months supply with water of seepage or infiltration quality, with or without the addition of P, soil P fractions and a number of plant physiological responses were measured. A field validation was performed in a flow-through lake in calcareous dunes where the seepage flow had been restored recently. The readily available water soluble P fraction (Pw) was reduced by more than 80% by seepage, both in the greenhouse experiment and in the field, but the P Olsen-inorganic fraction was unaffected. All four test species had elevated N: P ratios in aboveground tissues when treated with seepage, indicating that seepage water had indeed reduced P availability to plants. Formation of dauciform roots by Carex flacca was diminished by P addition to less than 25% of treatments without P addition, indicating sensitivity to P availability, while seepage on average halved production of these root structures. Dauciform root formation by Schoenus nigricans was unaffected by the P addition and the hydrological treatment. Biomass of the test species in the experiment as well as vegetation biomass and relative abundance of oligotrophic species in the field were unrelated to seepage patterns, suggesting that compensatory mechanisms enabled the plants to sustain biomass production within the time frame of this experiment. In conclusion, Ca- and Fe-rich seepage can lower P availability in soils with a strong pH buffer. In the long term, this may create favourable conditions for species that have low P requirements or efficient P uptake. In the short term, however, existing vegetation seems to be resilient to changes in P supply.