Left alone, tropical forests may recover—but experiments show that targeted nitrogen can dramatically accelerate their first ten years of regrowth and carbon storage, buying time in a warming world.
Lots of people are rooting for logged tropical forests to grow back. Here’s one way to jumpstart it: Add a splash of nitrogen.
Perhaps it should come as no surprise that a key component in agricultural fertilizer would help trees grow faster. But new research clarifies how much it helps and for which forests, as well as suggesting ways to give forests a boost without the ecological damage that can come with fertilizers.
It turns out that in the first 10 years of growing, fledgling forests are particularly ravenous for nitrogen. Supplementing the nutrient supply can make the vegetation explode, nearly doubling growth rates in newly abandoned farm fields and increasing growth by 48% in 10-year-old forests, researchers reported this month in Nature Communications.
“It was pretty amazing to see,” said Sarah Batterman, an ecologist at the Cary Institute of Ecosystem Studies and a co-author the new paper. “The plots with added nitrogen looked so much bigger than the ones where we didn’t add nitrogen—the trees were just huge. We were surprised at how quickly the forest grew back and how strong the effect of nitrogen was.”
The discovery emerged from 76 plots of land in Panama, ranging from ancient, intact forest to cattle pasture. Scientists left some of the areas to grow on their own. Other parcels were dosed with a combination of nitrogen and phosphorous, two nutrients needed by plants. Researchers returned over the course of four years to track vegetation growth in newer forests and for more than two decades for the old-growth jungle.
While the immediate findings are confined to a single forest, the researchers suspect young forests in many parts of the tropics confront a nitrogen shortage.
The nitrogen boost they observed could make a noticeable, though temporary, dent our climate problem. Batterman and colleagues calculated that the added growth spurt, if applied to tropical forests around the world, could pull nearly a gigaton of carbon dioxide from the air every year, equal to roughly 2% of annual human-caused greenhouse gas pollution.
But the research also shows there’s a time-limit to those gains. As forests aged, the bonus nitrogen didn’t have a noticeable effect. Thirty-year-old forests and ancient forests showed no response. Older forests appeared able to get enough nitrogen without help, as nitrogen depletion common to farmland was reversed by forests building up natural nitrogen stores in the soil.
Given the urgency of climate change, even focusing on young forests could be worth it, says Batterman. “In the long-term, the forests are not going to sequester extra carbon, but in that first 10 years, they can do the job faster, and 10 years is what we really need right now.”
This doesn’t mean people should be spraying fertilizer across the tropics. Synthetic nitrogen comes with its own headaches. Industrial fertilizer production creates its own climate pollution. Runoff from fertilizer-enriched fields can sparking algae blooms in water bodies, creating low-oxygen “dead zones” such as the one covering a part of the Gulf of Mexico larger than the state of Connecticut.
Rather, the scientists suggested nature-based strategies. People replanting tropical forests could make sure they include nitrogen-fixing plants, which use bacteria in their roots to convert atmospheric nitrogen into a form the plant can use. People could also concentrate reforestation work in places on more nutrient rich soil.
If people are going to try to help create a recipe for climate-smart reforstation, the new research adds another piece to the ingredient list. Don’t forget the nitrogen.