As greenhouse gases like carbon dioxide continue to pile up in the atmosphere, the world is experiencing more destructive extreme weather events like hurricanes, heatwaves, floods and droughts. A new study, published earlier this month in the scientific journal Nature Plants, finds that as Earth continues to warm, a certain group of trees, called nitrogen-fixing trees, may be able to help forests remove more heat-trapping CO2 from the atmosphere than previously thought.
If confirmed for more species of nitrogen-fixing trees and over larger scales, the trees could play more of a role in slowing climate change than accounted for in current climate models.
"The response of nitrogen fixation to warming is an important thing to get right," said Thomas Bytnerowicz, a Stengl-Wyer Postdoctoral Scholar at The University of Texas at Austin and first author of the study. "Accurate forecasts are critical in allowing governments and economies to plan for the future and prevent worst-case scenarios."
With the help of certain bacteria, nitrogen-fixing trees are able to convert atmospheric nitrogen that is in a form that plants cannot use into a bio-available form, akin to fertilizer, that helps them grow. Nitrogen fixation is the largest source of new nitrogen into the terrestrial biosphere.
Earlier research suggested that bacteria are unable to fix nitrogen at elevated temperatures. Using a novel method for studying individual trees, a team from Columbia University and The University of Texas at Austin discovered that the optimal temperature range for nitrogen fixation is actually 4 to 12 degrees Celsius higher than previously believed.
"Our work suggests that higher temperatures caused by global warming are likely to increase nitrogen-fixation, even in the tropics, which is in direct contrast to past [climate model] projections," Bytnerowicz explained.
By pulling carbon out of the atmosphere and storing it in their tissues, trees and other plants can help combat climate change.
"How much carbon terrestrial ecosystems can store is driven by plant growth, which is driven by the amount of nutrients, particularly nitrogen, these plants have to grow," Bytnerowicz said.
About 78% of our atmosphere is nitrogen. But, it's inaccessible to almost all biological life forms, Bytnerowicz said.
"Because the two nitrogen atoms are bonded with a triple bond, it really takes a lot of energy to break," he explained. "And that nitrogen has to be changed to biologically reactive forms before it can be used. So that triple bond has to be broken."
Nitrogen-fixing trees have a symbiotic relationship with bacteria that can "fix" the nitrogen and make it usable. Bytnerowicz raised various species of these trees in growth chambers, allowing him to control light, day length, relative humidity, CO2 levels and temperature. Seedlings were treated with a special mixture of nitrogen-fixing bacteria to encourage the symbiosis.
"We set up our experiment to test how temperature affected nitrogen fixation in plants from temperate to tropical regions and how the temperature response of nitrogen fixation was affected by acclimation to recent growing temperatures," Bytnerowicz said.
Many climate models track the movement of carbon through the atmosphere, oceans and land, and some have started incorporating nitrogen and phosphorus, too. The models currently predict that nitrogen will limit how much carbon dioxide forests will be able to absorb in the future. But greater nitrogen fixation at warm temperatures would suggest more nitrogen in ecosystems to help stimulate growth in plants, which, in turn, absorb carbon dioxide. The researchers have developed mathematical functions based on this latest study that can be used to more accurately represent nitrogen-fixing trees in climate models.
Other researchers that participated in this study are Palani Akana, Kevin Griffin and Duncan Menge, all of whom are affiliated with the Ecology, Evolution and Environmental Biology department at Columbia University.
Funded by the Stengl-Wyer Endowment, the Stengl Wyer Postdoctoral Scholars Program provides up to three years of independent support for talented postdoctoral researchers in the broad area of the diversity of life and/or organisms in their natural environments.
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