NITROGEN ISOTOPE RATIOS IN DIFFERENT COMPARTMENTS OF A MIXED STAND OF SPRUCE, LARCH AND BEECH TREES AND OF UNDERSTOREY VEGETATION INCLUDING FUNGI

Natural nitrogen isotope ratios were measured in different compartments (needles or leaves and twigs of different age classes and crown positions, roots and soil of different horizons) of spruce (Picea abies), larch (Larix decidua) and beech (Fagus sylvatica) trees in an 11-year-old mixed stand in the Fichtelgebirge, NE Bavaria, Germany. In addition, samples of understorey vegetation (mainly ericaceous shrubs and grass) and of ectomycorrhizal and saprophytic fungi were analyzed. The deltaN-15 values found for all samples ranged between -7.5 and +4.5 parts per thousand. No significant differences were found for the nitrogen isotope ratios of the three tree species despite of their evergreen versus deciduous foliage and despite of their different rooting depth. Ericaceous shrubs had the most negative and fungi and soil from the mineral horizon the most positive deltaN-15 values. Positive deltaN-15 values of the fungi indicate their ability to utilize organic soil nitrogen, but the data do not unequivocally show that plants forming mycorrhizas profit from this organic nitrogen source.