Conclusions
Black locust and black alder contained significantly more P in the foliage than silver birch and Scots pine indicating higher P uptake by the N-fixing tree species. However, the P concentrations in the O horizon followed a different trend with the highest concentration measured under silver birch and not under the N-fixers. We presume that this was due to more efficient P resorption from leaves by the N-fixing species (in particular by black alder). The effect of tree species on Porg concentrations in the uppermost mineral soil was limited. Only Scots pine exhibited significantly lower Porg concentrations in the mineral soil probably due to lower concentration of P in pine needles and the O horizon and slower decomposition rate of pine litter. The other tree species studied did not differ in the Porg concentrations in the mineral soil. Tree species did not affect the concentrations of organic P fractions with different lability as well. The concentrations of fulvic, humic and residual P were lower in the mineral soil under Scots pine only because there was generally less organic P under this tree. The percentages of labile, moderately labile, fulvic, humic and residual fractions in soil Porg were nearly the same under all tree species. The largest effect on the Porgconcentration in the mineral soil as well as on the contents of its particular fractions was from the substrate type. Sands contained much less Porg than Clays and Ashes. However, the percentage labile P in Porg was much higher there resulting in better P supply for plants. We conclude that N-fixing trees do not affect the concentration of labile organic P as the major factor controlling this Porg fraction is the soil substrate quality.