Forests worldwide are using up the nitrogen in their soil, a new study found. That’s a bit of good news for the Chesapeake Bay, which is ailing in large part from too much nitrogen getting into the water. But the study’s authors warn that a widespread decline of nitrogen in forest soils bodes ill for efforts to prevent global climate change.Forests absorb nitrogen in the soil that might otherwise pollute local waterways, and they help offset climate change by drawing carbon dioxide out of the atmosphere. (Dave Harp)

The study, published recently in the journal Nature Ecology & Evolution, analyzed the chemistry of more than 43,000 leaf samples collected from hundreds of tree species worldwide from 1980 to 2017 and found that the levels of nitrogen in them, which fuel the trees’ growth, has declined. Rising carbon dioxide levels in the atmosphere and longer growing seasons from warmer temperatures are promoting increased tree growth, the authors suggest. As a result, they wrote, forests’ demand for nitrogen is outstripping what’s available in the soil. 

The Earth’s forests have been expected to mitigate climate change by sequestering carbon as they draw carbon dioxide from the atmosphere to help them grow. But nitrogen helps to provide the energy that trees’ leaves use in photosynthesis to produce food — sugars — from water and carbon dioxide. Trees won’t necessarily get larger if one ingredient in that balanced recipe grows without a corresponding increase in the other.

 “This result suggests that [carbon] sequestration potential is going to hit a limit,’’ said Andrew Elmore, a study co-author and professor of landscape ecology at the University of Maryland Center for Environmental Science Appalachian laboratory. Rising carbon dioxide levels will still boost plant growth, he said, but not by as much if the nutrient is in short supply.

Much of the scientific and public discussion about nitrogen has been about how there’s too much of it in coastal waters, causing algae blooms and oxygen-starved “dead zones” where fish and shellfish struggle to survive. But Elmore, who’s conducting research at the National Socio-Environmental Synthesis Center in Annapolis, said the focus “has hidden this long-term trend in unamended systems that is caused by rising carbon dioxide and longer growing seasons.”

The Chesapeake Bay, though, has benefitted from the increased use of nitrogen by forests. Woodlands in the watershed have been getting a big dose of the plant nutrient because of airborne nitrogen from coal-burning power plants and vehicle emissions.

Research by Elmore’s Appalachian lab colleague Keith Eshleman has shown that federal clean air regulations have curtailed those nitrogen emissions over the last two decades, reducing the amount of nutrients runoff into streams that ultimately flow into the Bay.

Forests already yield some of the lowest nitrogen runoff of any land use in the 64,000-square-mile Bay watershed. This study indicates that rising carbon dioxide levels and longer growing seasons have reduced nitrogen runoff even more, as trees consume even more of it.

“Decreasing atmospheric deposition, longer growing seasons, and elevated atmospheric CO2 are all contributing to declining [nitrogen] export from western Maryland forests,” Elmore said.

While that’s helping to clean up the Bay, Elmore said, the declining nitrogen availability in forests means climate change could be more extreme than previously thought, and it could have far-reaching effects beyond simply rising temperatures.

“There’s a whole slew of insects and birds,” Elmore said, that depend on plants and trees for food. “If nitrogen is not as available to the trees or other plants, then it’s not as available to the insects and birds as well.” He cited press reports of a recently published study that reported a 76 percent decline from 1989 to 2016 in flying insects in German nature preserves.

“Beyond declines in leaf chemistry, we are seeing grazing cattle become more protein limited, pollen protein concentrations decline, and reductions of nitrogen in many streams,” said Joseph Craine, the study’s lead co-author and an ecologist with Jonah Ventures, a private research firm specializing in environmental DNA sequencing. “These dots are starting to connect into a comprehensive picture of too much carbon flowing through ecosystems.”

Elmore said that he thinks the threat of nitrogen depletion in forests is serious enough to consider whether action ought to be taken to somehow “fertilize” pristine wooded landscapes to counter the effects of rising carbon dioxide levels in the atmosphere. Similar “geo-engineering” proposals, such as the suggestion to “seed” the oceans with iron so they’ll lock up more carbon, have been highly controversial, as critics question their effectiveness and the potential for unintended consequences.

“I’ve never been an advocate for geo-engineering,’’ Elmore said, “but on the other hand, if insect populations or bird populations start crashing, that’s biodiversity [at stake].”

A quick calculation by an Appalachian lab graduate researcher suggested that applying chemical fertilizer to forests could yield a net reduction in climate change, as the increased forest growth would more than make up for the carbon emissions from fertilizer production.

“This new study adds to a growing body of knowledge that forests will not be able to sequester as much carbon from the [atmosphere] as many models predict because forest growth is limited by nitrogen,” said Eric Davidson, director of the UMCES Appalachian laboratory in Frostburg. The findings provide new evidence, he said, that “decreases in carbon emissions are urgently needed.”