page_title: Nitrogen fixation
section_title: Non-leguminous
hierarchical_section_title: Nitrogen fixation / Biological / Root nodule symbioses / Non-leguminous
caption_attribution_description: A whole Alder root nodule gall.
context_page_description: Nitrogen fixation is the process by which nitrogen in the air is changed into ammonia or other nitrogenous compounds in the soil.
  Nitrogen fixation is essential to life because nitrogen compounds are needed for making all nitrogen-containing organic compounds, such as amino acids, proteins, and nucleic acids.
  Atmospheric nitrogen is a relatively unreactive molecule. It is useless to all except a few bacteria and archaea. Biological nitrogen fixation converts N
  ₂ into ammonia, which is used by most organisms.
  Nitrogen is fixed in biological and non-biological ways:
context_section_description: Although most plants able to form nitrogen-fixing root nodules are in the legume family Fabaceae, there are a few exceptions:
  Parasponia, a tropical genus in the Cannabaceae also able to interact with rhizobia and form nitrogen-fixing nodules
  Actinorhizal plants such as alder and bayberry, can also form nitrogen-fixing nodules, thanks to a symbiotic association with Frankia bacteria. These plants belong to 25 genera in eight plant families. In these families, not all can fix nitrogen. For example, of 122 genera in the Rosaceae, only 4 genera are capable of fixing nitrogen.
  All these families belong to the orders Cucurbitales, Fagales, and Rosales, which together with the Fabales form a clade. In this clade, Fabales were the first lineage to branch off; thus, the ability to fix nitrogen was lost in most descendants of the original nitrogen-fixing plant. 
  Biological nitrogen fixation was discovered by Hermann Hellnegel (1831–1895) and Martinus Beijerinck (1851–1931).
caption: A whole alder tree root nodule.
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