'Nitrogen fixing' trees could help tropical forests bounce back, research suggests

An extra helping of nitrogen can double the growth of tropical trees in a recovering forest, vastly boosting the amount of carbon dioxide (CO₂) they can absorb for a decade, according to a new study.

Researchers found that adding a nitrogen fertilizer to the soil in the youngest forests — those that had been pastures less than a year ago — increased their tree biomass by 95% compared with a non-fertilized control group. Ten-year-old forests also bounced back with the nitrogen treatment, showing a 48% increase in growth compared with the control group.

"We all rely on tropical forests to stabilize our climate," study co-author and principal investigator Sarah Batterman, an associate professor at the University of Leeds and ecosystem ecologist at the Cary Institute of Ecosystem Studies, told Live Science. "They store about half of forest carbon and sequester about 20% of our carbon emissions. But there's huge uncertainty in whether tropical forests will continue to take up CO2 or will become a source of carbon into the atmosphere in the future. One of the key uncertainties is the role of nutrients in supporting more carbon sequestration and recovery from disturbance."

Researchers with the Smithsonian Tropical Research Institute (STRI) tracked the growth of trees and woody vine lianas across a four-year period, monitoring how fertilizers of nitrogen, phosphorus or a combination of both would impact growth. Working in plots around the Panama Canal watershed, they also tested responses across a gradient of forest types, including areas that had been cattle pastures less than a year prior, 10-year-old recovering forests, 30-year-old recovering forests and 600-year-old forests.

For three months each year, field teams fertilized the trees at regular intervals. "You're driving up and down these steep hills to get to the field site," Batterman said. "And it's super beautiful. You can see the Panama Canal in the distance, with the big ships driving through. And then you're driving through 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, the field teams would fertilize the trees and measure their trunks. "It's superhot and sweaty, lots of mosquitoes and insects," Batterman said.

From the diameter of the tree trunks, researchers can extrapolate the aboveground biomass of the trees and, importantly, their carbon storage.

The team's findings, published Jan. 13 in the journal Nature Communications, showed that nitrogen almost doubled growth in the areas that had been agricultural land until a year prior and boosted growth by almost 50% in forests that had been recovering for 10 years.

Older forests showed no response to the extra nitrogen, and no forests showed a response to the phosphorus fertilizer.

When trees are harvested from tropical rainforests, the soil below is also degraded, with nutrients like nitrogen and phosphorus being depleted. This degradation is still detectable decades after deforestation.

But rather than suggesting we physically fertilize vast tropical forests with nitrogen, the new findings can be used to plan forest recovery projects that prioritize tree species that can convert atmospheric nitrogen into a nutrient. This is known as "nitrogen fixing trees," study co-author Jefferson Hall, director of the Agua Salud project at STRI, which provided some of the forest plots where the experiment took place.

"It's not practical that people are going to go out and, you know, fertilize all the forests of the world to capture CO₂," Hall told Live Science. "The natural way of enhancing the nitrogen system would be to plant more nitrogen-fixing trees."

Richard Birdsey, a senior scientist at the Woodwell Climate Research Center who was not involved in the study, said the findings confirmed a long-standing observation about nutrients. "When I went to school, 50 years ago, the issue of nutrient depletion in tropical forests was known at that time. But no experiments like this had been done. It was simply some observations," he told Live Science.

Former tropical forests that had been removed, most often for agriculture, lack nutrients in the soil, and it often takes a long time for these nutrients to rebuild — even when the land is reforested. "The study, in a way, confirms these long-held beliefs about how tropical forests operate and what happens to them when they're harvested," Birdsey said.

Birdsey, who was with the U.S. Forest Service for over four decades, said recovering tropical forests are a vital global carbon sink, meaning they absorb more carbon than they release.

"They take up about 2.5 pentagrams of carbon per year," he said. "Globally, forests take up something like 3.5 pentagrams. So tropical forests, overall, are the biggest component of the carbon sink. And tropical regrowth forests, or regenerating forests, are the biggest part of the tropical forest sink."