New research finds that tropical forests can grow significantly faster and sequester more climate-warming carbon dioxide when additional nitrogen is available in the soil.
“With this information we can prioritise management and conservation practices to maximise forest regrowth,” Kelly Anderson, a research scientist at Missouri Botanical Garden in the U.S., told Mongabay by email. Anderson wasn’t involved with the study but does work with NEXTropics, a network of scientists who collaborate on forest nutrient studies.
During the recent study, researchers “wanted to test how either nitrogen or phosphorus limit forest recovery and specifically if there was a shift in that limitation from really young forests to older forests,” Sarah Batterman, corresponding author of the study with the Cary Institute and the University of Leeds, told Mongabay in a video call.
To test both nutrients the research team conducted a long-term field experiment in Panama. Research plots were established in 2015 and 2016 in recovering forests of three different ages: those on recently abandoned pasture; young secondary forest (10 years); and older secondary forests (30 years). They also looked at mature forest plots established in 1997, for a total of 76 experimental plots.
For each age of forest, plots received one of four treatments: added nitrogen, added phosphorus, both nutrients, and control plots where nothing was added. They also established several replicate plots where they repeated the experiments.
Batterman said the strongest response was in young trees that received additional nitrogen.
“So, in the first 10 years of forest recovery, the forests grow back about twice as fast when they have sufficient nitrogen in the soil compared to when they don’t,” Batterman told Mongabay. “That’s really fast recovery.”
The nitrogen-boosted growth declined sharply in the middle-aged trees and was undetectable in mature forests.
“We were really surprised because the strength of nitrogen limitation was so high. We were kind of blown away by that,” Batterman said.
While additional nitrogen can significantly help young tropical trees grow and sequester carbon, the researchers caution that the source of nitrogen is important. Excess synthetic fertilizer can leach into waterways, creating pollution or forming nitrous oxide, a powerful greenhouse gas.
Instead, the researchers suggest incorporating native nitrogen-fixing trees into reforestation projects. Another option is targeting reforestation efforts in areas with excess nitrogen from industry, cattle ranching and vehicles.
“Then it’s kind of a win-win because the forest will suck up that extra nitrogen, as opposed to [polluting] waterways, or it’s lost as gaseous emissions, including of nitrous oxide,” Batterman said.
“The next steps will be to replicate this type of experiment across more tropical forests,” Anderson said.
If the same results hold up globally, nitrogen-supported forest regrowth could have a significant climate impact; approximately 0.69 billion metric tons of carbon dioxide could be sequestered each year, the researchers found. That’s roughly the annual emissions of Indonesia.
“It’s not going to solve climate change but it’s a piece in the puzzle,” Batterman said.
