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

Nitrogen is the most critical element. Atmosphere is the greatest reservior of nitrogen. Molecular nitrogen cannot be utilized directly by plants. It has to be fixed or converted into compounds prior to utilization. Symbiotic nitrogen fixation is accomplished by Rhizobium species, which occurs on the roots of leguminous plants. Certain non-leguminous plants also form nodules to fix nitrogen. The best known example in temperate region is alder (Alnus sp.) The bacteria involved in nodule formation is an Actinomycetes, the Frankia. Clostridium is anaerobic, saprotrophic, free-living nitrogen fixing bacteria. Bradyrhizobium is symbiont in plants of Parasponia and soyabean. The Azorhizobium forms both stem and root nodules in Sesbania (aquatic plant).