Plants use nitrogen to produce proteins, enzymes, and chlorophyll: all necessary components to perform photosynthesis, in which plants remove carbon from the atmosphere and store it in their leaves, roots, and soil.
However, though the atmosphere is made up of more than 78% nitrogen, the element is unusable for plants in its natural form. Tiny microorganisms called diazotrophs are responsible for “fixing” nitrogen into a form that plants can absorb and use. Diazotrophs live in the soil and in living and decaying plants, creating important partnerships with both naturally growing vegetation and agricultural crops.
Because plants need the nitrogen to grow and remove carbon from the atmosphere, understanding the global distribution of biological nitrogen fixation (BNF) is crucial for building accurate climate models.
But a new study makes a surprising update to global BNF estimates: Forests, grasslands, and other natural areas may have access to between a quarter and two thirds less biologically fixed nitrogen than previously thought. In previous studies, most field measurements of BNF in natural settings were taken from locations such as tropical forests, where nitrogen-fixing organisms are 17 times more abundant than the global average, creating an overestimation of nitrogen availability. This new work, coauthored by a team of 24 international scientists, examines a broader range of ecosystem types and provides a more detailed picture of the global distribution of nitrogen fixation.
Modernized Mapping
A group of researchers, many of whom are involved in the new study, first published a paper on how to model BNF in 1999, explained lead author Carla Reis Ely, an ecosystem ecologist at the Oak Ridge Institute for Science and Education. “But they knew that there were some issues, particularly with data on the abundance of nitrogen fixers, that needed to be addressed.”
The scientists involved with the updated project started by reviewing a compilation of field measurements and distribution data on BNF across natural ecosystems. They found that the sampling bias in past research had produced an overestimation of global nitrogen availability.
Reis Ely said that “it makes sense” that scientists hoping to measure BNF would do their research in places where they know BNF is occurring. “It’s very hard to propose a project where scientists were going to go to a place to measure nitrogen fixation where they know nitrogen fixation is not happening.”
They compiled more than 1,100 existing measurements of BNF rates from natural field sites, ranging from tropical forests to the Arctic. In doing so, they aimed to build a much larger and more representative dataset on how common nitrogen-fixing organisms and their hosts (such as shrubs and mosses) are across various regions and ecosystems. Once they had gathered and organized the measurements of BNF rates from specific sites, they upscaled those rates to estimate and map global nitrogen fixation rates for each of Earth’s biomes.
From Forests to Farms—and Beyond
According to the study’s findings, the amount of nitrogen fixation by microbes in natural environments is approximately 25 million tons lower than previously estimated.
According to the study’s findings, the amount of nitrogen fixation by microbes in natural environments is approximately 25 million tons lower than previously estimated—the equivalent of 113 fully loaded cargo ships. Most of it occurs in tropical forests and drylands, but Reis Ely noted that soils, biocrusts, mosses, and lichens also conduct high amounts of nitrogen fixation.
Though naturally occurring nitrogen fixation is lower than previous estimates, agriculturally based nitrogen fixation has actually been underestimated, the researchers discovered after sorting through thousands of measurements of agricultural BNF. When natural and agricultural datasets were combined, “we found both lower natural nitrogen fixation and higher agricultural nitrogen fixation than prior estimates, [indicating] an increasing human signal on this essential process worldwide,” said Steven Perakis, an ecologist with the U.S. Geological Survey at the Forest and Rangeland Ecosystem Science Center and one of the study’s authors.
Crops like soybeans and alfalfa host bacteria that are fixing much more nitrogen than the natural systems that they replaced were fixing. Even though agricultural nitrogen-fixing crops cover only 6% of Earth’s land, they have boosted global nitrogen fixation by 64% since preindustrial levels.
This increase comes with pros and cons: Nitrogen-fixing crops can help feed Earth’s growing population, and they tend to be more eco-friendly than crops requiring chemical fertilizers. But too much nitrogen can upset the nutrient balance in soils and threaten biodiversity by feeding the growth of invasive plants. Further, excess nitrogen can be converted into the greenhouse gas nitrous oxide, and runoff from these soils can leach into groundwater and cause algal blooms.
“It’s a Goldilocks sort of thing. You want just enough, but not too much, for healthy functioning of ecosystems.”
“Less nitrogen fixation in natural areas could mean reduced capacity [for plants] to uptake carbon from the atmosphere and help mitigate climate change,” Reis Ely said. “On the other hand, if we underestimate how much agricultural nitrogen fixation is happening, we are also underestimating how much excess nitrogen we are adding to natural environments.”
Understanding this balance has implications for estimating nitrogen needs in agriculture as well as how forests grow and store carbon as carbon dioxide levels rise. “It’s a Goldilocks sort of thing. You want just enough, but not too much, for healthy functioning of ecosystems,” said Eric Davidson, a biogeochemist at the University of Maryland Center for Environmental Science who was not involved in the study.
With this new dataset, researchers can now update their models, which may have been under- or overestimating the nitrogen fixation occurring in natural and agricultural settings. Correct estimates can factor into plans for mitigating climate change. “Could these numbers, these global estimates, change in the future?” Davidson said. “Yes, they could with better understanding. But for the time being, it would appear that this is a significant improvement.”
—Rebecca Owen, Science Writer (@beccapox.bsky.social)