Biomass production and nitrogen utilization by Alnus incana when grown on N2 or NH 4 + made available at the same rate

A single clone of Alnus incana (L.) Moench was grown in a controlled-environment chamber. The plants were either inoculated with Frankia and fixed atmospheric nitrogen or were left uninoculated but received ammonium at the same rate as the first group fixed their nitrogen. Nitrogen fixation was calculated from frequenct measurements of acetylene reduction and hydrogen evolution. The diurnal variation of acetylene reduction was also taken into account. The relative efficiency of nitrogenase could be used in the calculations of fixed nitrogen since the Frankia used did not show any detectable hydrogenase activity. Alders fixing nitrogen developed more biomass, longer shoots, larger leaf areas and contained more nitrogen than alders receiving ammonium. In one experiment, almost all ammonium given to the non-nodulated alders was taken up and 15% of the nitrogen taken up was excreted. In the other experiment, 34% of the ammonium was left in the nutrient solution and 8% of the nitrogen taken up was excreted. Alders inoculated with Frankia did not excrete any detectable amount of nitrogen. It seems that the energy demand for nitrogen fixation is not so high that biomass production in alders is retarded. The symbiotic system of A. incana and Frankia seems to be more efficient in utilizing its nitrogen than non-symbiotic A. incana receiving ammonium.