NITROGEN-FIXATION BY ALNUS-INCANA AND NITROGEN TRANSFER FROM A-INCANATO PINUS-SYLVESTRIS INFLUENCED BY MACRONUTRIENTS AND ECTOMYCORRHIZA

The aims of this study were to evaluate the effect of macronutrients o n nitrogen fixation in mycorrhizal and nonmycorrhizal grey alder (Alnu s incana (L.) Moench), and to evaluate the effect of ectomycorrhizal m ycelium on the transfer of symbiotically fixed nitrogen from grey alde r to Scots pine (Pinus sylvestris L). One alder and one pine were grow n together in pots with root systems separated by a 20 mu m mesh nylon filter which allowed hyphae but not roots to penetrate. Half the plan ts of both species were inoculated with Paxillus involutus (Fr.) Fr. a nd all alders were inoculated with Frankia. Nutrient solutions were ad ded with macronutrient (N, K, P, Ca, Mg and S) concentrations varied a ccording to a two-level fractional factorial design. The plants were h arvested after two growing periods in a growth chamber. Nitrogen fixat ion by alder and transfer of symbiotically fixed N from alder to pine was measured by N-15-dilution. Fixed hi (mg) correlated with nodule bi omass in both mycorrhizal and nonmycorrhizal alders. On average, speci fic nodule activity over the two periods was 510 mg N fixed g(-1) nodu le d. wt. This was not affected by mycorrhizal infection or by the dif ferent nutrient treatments. By contrast there was a strong nutrient ef fect on the proportion of N derived from fixation. These results indic ate that the regulation of nitrogen fixation was via nodule growth rat her than via nodule specific activity. Nitrogen had a strong negative effect and P a positive effect on the percentage of N derived from fix ation (%Ndfa). However, the effect of N depended on the level of P. Th is N x P interaction resulted in a %Ndfa when N was high, of 5-10 % at low P and 45-48 % at high P. The highest value of 90 % Ndfa was found at the combination of low N and high P. Potassium had a small but sta tistically significant effect on the %Ndfa but Ca, Mg and S had no sig nificant effects. No mycorrhizal effect was found on the %Ndfa in alde r. By contrast, the %Ndfa and biomass were lower in mycorrhizal than i n non-mycorrhizal alders. The proportion of fixed N in pine, transferr ed from alder, was greatest (9 %) when the pine was nitrogen starved a nd mycorrhizal and the alder was fixing maximally (low N and high P). However, the amount of fixed N transferred to pine was not statistical ly different from zero.