ABSTRACT
Nitrogen (N) is an essential nutrient for all forms of life because it is required for their synthesis of amino acids, proteins, and DNA nucleotides. Biological nitrogen fixation (BNF) converts N<sub>2</sub>, an abundant and inert gas in the atmosphere, into ammonia, a bioreactive form that is the primary natural source of N for plant and animal life in both terrestrial and aquatic ecosystems. In agroecosystems, much of the BNF is accomplished through a symbiotic association between rhizobial bacteria and leguminous plants. This chapter focuses on the BNF that results from the legume-rhizobia symbiosis because it has the most relevance for regenerative and resilient agroecosystems. The taxonomy of rhizobia bacteria is in a state of flux due to the advent of whole-genome–based taxonomy. Plant hosts have adapted several mechanisms to minimize oxidative damage to the nitrogenase complex in their nodules.
