A clone of Alnus incana (L.) Moench was grown in symbiosis with a local source of Frankia or with Frankia Ar14. Seven to 9‐week‐old plants were given 20 mM NH4Cl (20 mM KCl = control) for 3 days. Nitrogenase activity of intact plants decreased gradually within the 3 days of treatment to about 10% of the initial rates. Hydrogen evolution in air and total nitrogenase activity responded similarly to the treatment. Relative efficiency of nitrogenase thus remained the same throughout the study period. Control plants were not affected. Measurements of nitrogenase activity in root nodule homogenates (in vitro measurements) indicated loss of active nitrogenase rather than shortage of energy for nitrogenase activity in Frankia from ammonium‐treated plants. Shoots were exposed to 14CO2 and translocation of 14C to Frankia vesicle clusters prepared from root nodules was studied. Frankia vesicle clusters from ammonium‐treated plants contained about half as much 14C as those of control plants during all 3 days studied. One explanation for the observed effects is that a reduced supply of carbon to Frankia vesicles in the root nodules caused a reduced metabolic rate, including reduced protein synthesis and synthesis of nitrogenase.

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The possibility that respiration limits oxygen access to nitrogenase was tested by artificially upsetting the balance between oxygen consumption (respiration) and oxygen influx (diffusion). Argon treatment of the nodulated root system on intact plants stopped in vivo nitrogenase activity almost completely. Upon return to air, nitrogenase activity was very low and recovered gradually to full activity after about 5 h. In vitro measurements on nodule homogenates indicated that active nitrogenase was lost upon the shift from low (argon) to normal (air) oxygen. Maintenance of nodulated root systems at low temperature (2°C) inhibited both respiration and in vivo nitrogenase activity. Upon return to normal temperature (22°C), oxygen uptake recovered very rapidly, but nitrogenase activity recovered only gradually to full activity after about 5 to 6 h. Again, loss of active nitrogenase could, at least partly, explain the reduced in vivo nitrogenase activity. The effects from a temporarily impaired balance between oxygen consumption and oxygen influx thus point to the importance of respiration for limiting oxygen access to nitrogenase.

A survey was made of the occurrence of Frankia, infective on Alnus, in some 40 soils from the whole circumpolar area. Some of these soils were also tested for the occurrence of Rhizobium infective on Trifolium pratense. Infectivity tests were performed by growing test seedlings in soil or soil suspensions. Frankia was detected only in very few soils, in spite of extended experimental periods. When nodulation took place, nodulation was observed in few test plants. Several of nodulated test seedlings never turned green, suggesting that Frankia was ineffective in N 2 fixation. An exception was soil from a site in the Faeroe Islands where nodulated Alnus had been introduced. This soil showed high nodulation ability and N 2 fixation was likely. It is suggested that lack of infective Frankia in the circumpolar soils studied may be because Frankia had not been spread to these sites, but does not necessarily mean that soil conditions are negative for Frankia. Infective Rhizobium was rare in the soils studied. Lack of infective root nodule bacteria in potential sites for soil reclamation calls for the need to inoculate the plants and also provides the opportunity for introduction of selected bacterial strains without competition from an endogeneous soil microflora.