Inorganic phosphates (ferric phosphate, calcium phosphate and aluminium phosphate) and organic forms of phosphorus (inosito) hexaphosphate and β-glycerophosphate) were tested as phosphorus (P) sources for the growth of three Carex species from fens differing in productivity- and nutrient availability. Aluminium phosphate and β-glycerophosphate were equally effective sources of P for all three Carex species as KH2PO4, which was used as a control treatment. Phosphorus from ferric phosphate and calcium phosphate was also taken up bv Carex acutiformis Ehrh., a species of high productivity from eutrophic fen areas, but not by the other two species. Only Carex lasiocarpa Ehrh., a species from P-limited fens, showed a higher P-uptake when grown with inositol phosphate than in the second control treatment without P. The P-source also affected, with differences between species, the biomass allocation to the below-ground plant parts and the area of absorption surface of the roots.
The mobilization of organic sources of P by fen Carex species found in the present study might be an important factor in the P dynamics of fens because of the high contents of organic P in the fen peat soils where these species grow. In the same way, the utilization of inorganic sources of P can play a relevant role in the P-cycle in these systems where the discharge of ground water rich in ions can lead to the fixation of P into inorganic compounds. However, the differing abilities of the various Carex species to mobilize P from P-pools does not explain clearly their preferential presence in different fens in relation to the P-availability.