‘Nitrogen fixing’ trees could help tropical forests bounce back, research suggests
An extra dose of nitrogen can double the growth of tropical trees in a regenerating forest, significantly increasing the amount of carbon dioxide (CO2) they can absorb for a decade, according to a new study.
The researchers found that adding a nitrogen fertilizer to the soil in the youngest forests – those that were pastures less than a year ago – increased their tree biomass by 95% compared to an unfertilized control group. Ten-year-old forests also rebounded from the nitrogen treatment, showing a 48% increase in growth compared to the control group.
Researchers at the Smithsonian Tropical Research Institute (STRI) tracked the growth of trees and woody vines over a four-year period, monitoring the impact of nitrogen, phosphorus, or a combination of fertilizer on growth. Working on plots around the Panama Canal watershed, they also tested the answers across a gradient of forest types, including areas that were livestock pastures less than a year ago, 10-year-old recovering forests, 30-year-old recovering forests, and 600-year-old forests.
For three months each year, field crews fertilized the trees at regular intervals. “You go up and down these steep hills to get to the field,” Batterman said. “And it’s super beautiful. You can see the Panama Canal in the distance, with the big ships crossing it. And then you cross this landscape of pastures with cows and some forests in different stages of recovery.”
After hikes ranging from five minutes to an hour and a half, field crews fertilized the trees and measured their trunks. “It’s really hot and there’s a lot of sweat, there’s a lot of mosquitoes and bugs,” Batterman said.
From the diameter of tree trunks, researchers can extrapolate the above-ground biomass of trees and, above all, their carbon storage.
The team’s findings, published January 13 in the journal Natural communicationsshowed that nitrogen nearly doubled growth in areas that were farmland up to a year ago and boosted growth by almost 50% in forests that had been regenerating for 10 years.
Older forests showed no response to supplemental nitrogen, and no forests showed a response to phosphorus fertilizer.
When trees are harvested from tropical rainforests, the soil underneath is also degraded, with nutrients like nitrogen and phosphorus depleted. This degradation is always detectable decades after deforestation.
But rather than suggesting that we are physically fertilizing vast tropical forests with nitrogen, the new findings can be used to plan forest restoration projects that prioritize tree species that can convert atmospheric nitrogen into a nutrient. These are called “nitrogen-fixing trees”, co-author of the study Jefferson Roomdirector of the Agua Salud project at STRI, which provided some of the forest plots where the experiment took place.
“It’s not practical for people to go out and, you know, fertilize all the forests in the world to capture CO2“, Hall told Live Science. “The natural way to improve the nitrogen system would be to plant more nitrogen-fixing trees.”
Richard Birdseya senior scientist at the Woodwell Climate Research Center who was not involved in the study, said the results confirmed a long-standing observation about nutrients. “When I was in school 50 years ago, the problem of nutrient depletion in tropical forests was known at that time. But no such experiments had been done. These were just a few observations,” he told Live Science.
Ancient rainforests that have been destroyed, most often for agricultural purposes, lack nutrients in the soil, and it often takes a long time for these nutrients to be replenished, even when the land is reforested. “The study, in some ways, confirms these long-held beliefs about how rainforests work and what happens to them when they are logged,” Birdsey said.
Birdsey, who worked in the U.S. Forest Service for more than four decades, said regenerating tropical forests are a vital global carbon sink, meaning they absorb more carbon than they release.
“They absorb about 2.5 pentagrams of carbon per year,” he said. “Globally, forests take up about 3.5 pentagrams. So tropical forests, as a whole, make up the largest component of the carbon sink. And tropical regrowth forests, or regenerating forests, make up the largest part of the rainforest sink.”
Tang, W., Hall, JS, Phillips, OL, Brienen, RJW, Wright, SJ, Wong, MY, Hedin, LO, Van Breugel, M., Yavitt, JB, Hannam, PM, & Batterman, SA (2026). Carbon sequestration of tropical forests accelerated by nitrogen. Natural communications17(1), 55. https://doi.org/10.1038/s41467-025-66825-2
