Biological Nitrogen Fixation

Root nodule‐based nitrogen fixation associations are exemplified by Rhizobium–legume and actinorhizal symbioses. These interactions between diazotrophic bacteria and plants are major biological contributors of fixed nitrogen in soil‐based ecosystems. Due to the extensive cultivation of legumes, the greatest documented contribution of fixed nitrogen into land‐based systems results from the infection of legume roots by species of the bacterial genera Rhizobium and Bradyrhizobium. Serological typing, DNA analysis, and legume infection have also been used to detect, identify, and quantify rhizobia. The foregoing evaluation of the Rhizobium–legume symbiosis suggests the existence of a relationship that is of interest not only for basic studies but also for a variety of practical applications. Two sources of rhizobia for root infection can occur in many managed agricultural soils: native populations and crop inoculants.