Nitrogen fixation in root nodules of Alnus is brought about by:

Alnus (alder) is a non-leguminous plant that bears nitrogen-fixing nodules on its roots.
These nodules are formed by Frankia, a filamentous bacterium (actinomycete) that lives as a symbiont and fixes atmospheric nitrogen.
Frankia is also found in the root nodules of other non-legumes such as Casuarina.
Bradyrhizobium forms root nodules in legumes such as soybean, and Azorhizobium forms stem nodules on Sesbania.
Clostridium is a free-living anaerobic nitrogen-fixing bacterium that does not form nodules.
So nitrogen fixation in the root nodules of Alnus is brought about by Frankia.