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.