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As temperatures, CO2 levels rise, plants get less nitrogen

"If nitrogen is less available it has the potential to decrease the productivity of the forest. We call that oligotrophication," said researcher Andrew Elmore.

By Brooks Hays
As temperatures, CO2 levels rise, plants get less nitrogen
Rising CO2 levels are starving many terrestrial ecosystems of their nitrogen supply. Photo by University of Maryland Center for Environmental Science/Andrew Elmore

Oct. 22 (UPI) -- The amount of nitrogen available to terrestrial plants declines as temperatures and CO2 levels in the atmosphere increase.

Many studies have focused on an excess of nitrogen in freshwater and marine ecosystems. Runoff from commercial farms carries nitrogen and other nutrients into watersheds, fueling harmful algal blooms.

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While too much nitrogen is a problem for many human-influenced, coastal ecosystems, new research suggests habitat farther inland is plagued by the opposite problem.

"This idea that the world is awash in nitrogen and that nitrogen pollution is causing all these environmental effects has been the focus of conversations in the scientific literature and popular press for decades," Andrew Elmore, a researcher at the University of Maryland Center for Environmental Science, said in a news release. "What we're finding is that it has hidden this long-term trend in unamended systems that is caused by rising carbon dioxide and longer growing seasons."

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To study changes in local nitrogen supplies, scientists surveyed a database of leaf chemistry, featuring leaf samples collected from hundreds of species around the world between 1980 and 2017.

The leaf chemistry data showed most terrestrial ecosystems, including forests and fields unaffected by fertilization, are oligotrophic, which means they lack sufficient nutrient levels.

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"If nitrogen is less available it has the potential to decrease the productivity of the forest. We call that oligotrophication," said Elmore. "In the forested watershed, it's not a word used a lot for terrestrial systems, but it indicates the direction things are going."

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Researchers think carbon uptake is limiting the ability of microbes to return nitrogen to the soil as they break down decaying plant matter on the forest floor.

"This new study adds to a growing body of knowledge that forests will not be able to sequester as much carbon from the atmospheric as many models predict because forest growth is limited by nitrogen," said Eric Davidson, director of the Center for Environmental Science's Appalachian Laboratory. "These new insights using novel isotopic analyses provide a new line of evidence that decreases in carbon emissions are urgently needed."

According to the new study, published Monday in the journal Nature Ecology and Evolution, the only way to increase the amount of nitrogen available to terrestrial ecosystem is to reduce carbon emissions.

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"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 ecologist Joseph Craine. "These dots are starting to connect into a comprehensive picture of too much carbon flowing through ecosystems."

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