New figures on biological nitrogen fixation in nature: Climate models may need to be adjusted
The total amount of nitrogen acquired in forests, prairies and other natural areas via biological nitrogen fixation might be lower than previously estimated. This is indicated in a new study, which has climate implications, as plants need the element to remove carbon dioxide from the atmosphere.
Nitrogen is essential for all living organisms, but only a few types of bacteria are able to convert the inert form of nitrogen in the atmosphere into a form that plants and animals are able to use, through a process referred to as biological nitrogen fixation. “Fixed” nitrogen is needed by plants to make proteins and chlorophyll, the green pigment in leaves that enables photosynthesis, and which controls the uptake of carbon dioxide from the atmosphere.
Results imply lower carbon dioxide uptake in nature
“In natural ecosystems, nitrogen fixation improves soil fertility and supports plant growth, thereby increasing carbon storage,” said Carla Reis Ely, a postdoctoral scholar at the Oregon State University College of Forestry, who led an international team of 24 scientists on the study, recently published in Nature. “However, our new estimate of natural nitrogen fixation, based on a much-improved data synthesis, suggests a lower estimate of nitrogen fixation in natural ecosystems than previous syntheses”.
The international group of researchers arrived at a more conservative global estimate by considering the abundance of nitrogen fixing organisms in all major biomes more carefully. They compiled 1177 field measurements of biological nitrogen fixation from around the world and found that past analyses were over-estimates due to a skewed sampling bias: Field measurements of nitrogen fixation in natural areas had been taken in places where nitrogen-fixing organisms were 17 times more abundant than their worldwide distribution.
“The findings in this study will likely influence predictions of how much carbon dioxide ecosystems will take up in the future, says Michael Gundale, Professor at the Department of Forest Ecology and Management at SLU, and co-author of the study.
“Many of the models that are used to predict future CO2 uptake and storage in terrestrial ecosystems rely on assumptions about the rate of biological nitrogen fixation, based on its relationship with ecosystem productivity or plant water loss during photosynthesis (evapotranspiration). Our study removes some uncertainty about these relationships, and will likely lead to somewhat lower estimates of the future capacity for terrestrial ecosystems to sequester carbon”, says Gundale.
In agriculture, nitrogen fixation is more extensive than previously thought
Meanwhile, a dramatic rise in agricultural nitrogen fixation, from pre-industrial to present, has occurred mainly due to large-scale planting of legume crops, such as soybeans and alfalfa, that host nitrogen-fixing bacteria. The scale of this legume-driven nitrogen fixation on crop land is notable, as the amount is now almost equal to all natural nitrogen-fixation in other land ecosystems globally.
“This increase is both a positive and a negative”, says study lead author Reis Ely. “On the one hand, nitrogen fixation is essential for producing food for a growing global population, and is generally more environmentally sustainable than the use of synthetic nitrogen fertilizers. But release of this agricultural nitrogen from the production and consumption chain can be a big contributor to nitrogen pollution in certain regions, and result in nitrous oxide emissions, which is a potent greenhouse gas.”
“This large and growing source of biological nitrogen fixation from agriculture may contribute to nitrogen enrichment in some delicate ecosystems that are adapted to very low nitrogen availability. In these types of ecosystems, nitrogen pollution from agriculture and fossil fuel combustion can have negative consequences for biodiversity,” says Gundale.
Contact at SLU
Michael Gundale, Professor
Department of Forest Ecology and Management
Swedish University of Agricultural Sciences, Umeå
+46 90 7868427, michael.gundale@slu.se
Michael Gundale was part of a working group that planned the study. His specific roll was to compile data on the abundance of mosses in different biomes, especially boreal forests. Mosses are an important N fixer in boreal forests. To do this, he acquired data from the Swedish National Forest Inventory, as well as Forest Inventories of Canada and Alaska, USA.
The article
Carla R. Reis Ely et.al. 2025. Global terrestrial nitrogen fixation and its modification by agriculture. Nature. DOI: 10.1038/s41586-025-09201-w. https://www.nature.com/articles/s41586-025-09201-w
Press images
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Field work in Lapland. Mosses, in symbiosis with nitrogen-fixing bacteria, are important nitrogen fixers in boreal forests. Photo: Vincent Buness
Michael Gundale. Photo: Malin Grönborg
