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

Photograph of a grassy hill with cows in the background and a tall tree.

Many tropical forests are regenerating on land where forests have previously been chopped down for cattle grazing. Practices like these cause nitrogen to be lost from the soil, which can slow the regrowth of forests. (Image credit: Sarah Batterman / Cary Institute of Ecosystem Studies)

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, 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."

A team member spreads fertilizer on a recently abandoned pasture plot. White tubes mark locations in the plot so researchers can locate trees as they grow back. After four years, the trees in this plot were taller than the researchers, and the forest stored twice as much carbon when they had sufficient nitrogen compared to when they did not. (Image credit: Sarah Batterman / Cary Institute of Ecosystem Studies)

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.

A root nodule on a legume tree where symbiotic bacteria fix nitrogen from the atmosphere into a form of nitrogen that the trees can use to grow. Legume trees are abundant in tropical forests and can be used in reforestation efforts to naturally enrich the soil with nitrogen that speeds up carbon sequestration and storage. (Image credit: Sarah Batterman / Cary Institute of Ecosystem Studies)

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.

A tropical forest. — A tropical forest that is about thirty years old following deforestation and agricultural use. By thirty years, the forests show no evidence of nutrient limitation on carbon accumulation. Orange paint on tree trunks allows researchers to find and measure the same trees every year to track their growth and carbon storage. (Image credit: Sarah Batterman / Cary Institute of Ecosystem Studies)

"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.