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Symbiotic nitrogen (N) fixation is a key ecological process in which bacteria in roots convert inert atmospheric N gas into forms of N that plants can use for growth. These N-fixing symbioses occur in only 4% of plants worldwide but are critical for sustaining soil fertility and forest growth, especially with increasing atmospheric carbon dioxide.

root nodules
Root nodules, such as these observed on an alder, are a symbiotic relationship between a plant and nitrogen-fixing bacteria. A symbiotic relationship is one where both organisms benefit. 


Strategies for symbiotic N-fixation vary; some fix N continuously whereas others reduce fixation when soil N is plentiful, and this difference controls how N-fixers influence ecosystems. Researchers used a multi-site field experiment to study symbiotic N-fixing strategies, and examined differences between temperate and tropical trees, and between the two major types of N-fixing bacteria. Contrary to current theories, all tree species at all locations continued to fix N even when soil N was abundant. These findings help explain why forests with N-fixing trees often have high rates of N leaching and N greenhouse gas emissions.  

Menge, D.N., Wolf, A.A., Funk, J.L., Perakis, S.S., Akana, P.R., Arkebauer, R., Bytnerowicz, T.A., Carreras, P.K., Huddell, A., Kou-Giesbrecht, S., and Ortiz, S., 2022, Tree symbioses sustain nitrogen fixation despite excess nitrogen supply: Ecological Monographs, Online.