Effect of elevated carbon dioxide in the root atmosphere on nitrogenase activity in three actinorhizal plant species

In wet soils, nitrogen-fixing root nodules are subjected to elevated CO2. Only a few studies have examined the effect of elevated CO2 on nitrogenase activity, and the results have been mixed. We examined intact black alder (Alnus glutinosa (L.) Gaertn.) and sweet gale (Myrica gale L.) seedlings and field-collected speckled alder (Alnus incana ssp. rugosa (Du Roi) Claus.) nodules to clarify the effects of elevated CO2. Nitrogenase activity was measured via acetylene reduction in an open, flow-through system. We found that repeated measurements of the peak rate of nitrogenase activity, the only reliable measure of nitrogenase activity, could be made on the same plant via 150-s exposures to acetylene separated by 20 min without acetylene. Our results for elevated CO2 consistently showed that it had little effect on nitrogenase activity at low concentrations and increasingly inhibited nitrogenase activity as the CO2 concentration increased. In black alder, 0.5 kPa CO2 had little effect, whereas 3.0 kPa CO2 reduced nitrogenase activity 31–35%. Sweet gale, was less sensitive to elevated CO2 and was unaffected by 1.5 kPa CO2. Black alder grown with the roots, but not the shoots, in 1.3 kPa CO2 showed only minimal acclimation to elevated CO2.Key words: acetylene reduction technique, actinorhizal plants, Alnus, carbon dioxide, Myrica gale, nitrogen fixation.