For seven years we studied the recovery of vegetation in a Belgian P limited rich fen (Caricion davallianae), which had been fertilized with nitrogen (200 g.m−2) and phosphorus (50 g.m−2) in 1992. The vegetation in this fen has low above-ground biomass production (< 100 g m−2) due to the strong P limitation. Above-ground biomass was harvested from 1992 to 1998 and P and N concentrations measured. In 1998, below-ground biomass was also harvested. The response to fertilization differed markedly between below- and above-ground compartments. Above-ground, P was the single most important factor that continued to stimulate growth 7 yr after fertilization. Below-ground, N tended to decrease live root biomass and increase dead root biomass and seemed to have a toxic effect on the roots. In the combined NP treatment the stimulating effect of P (an increase of live root biomass) was countered by N. The 1998 soil analysis showed no difference in soil P in the plots. Thus, Fe hydroxides are not capable of retaining P in competition with fen vegetation uptake. However, higher capture of P in root Fe coatings from N plots may partially explain this negative N effect. The results suggest that N root toxicity will be amplified in strongly P limited habitats but that its persistence will be less influenced by P availability. This mechanism may be a competitive advantage for N2 fixing species that grow in strongly P limited wetlands.
Abbreviations: DCB = Dithionite citrate bicarbonate.