Nitrogen study casts doubt on ability of plants to continue absorbing same amounts of carbon dioxide

A new study casts doubt as to whether plants will continue to absorb as much carbon dioxide in the future as they have in the past due to declining availability of nitrogen in certain parts of the world.

When it comes to the role plants play in keeping the heat-trapping greenhouse gas out of the atmosphere, "it may not be business as usual," said Lixin Wang, an associate professor in the Department of Earth Sciences at IUPUI.

Wang is a co-author of the paper "Isotopic evidence for oligotrophication of terrestrial ecosystems," which reports that finding. It was published Oct. 22 in the journal Nature Ecology and Evolution.

Grasslands View print quality image
In grasslands and forests, which are not directly fertilized, the availability of nitrogen to plants is declining.

The study examines global availability of nitrogen, using a data set that is more than 30,000 data points larger than those previously used to determine nitrogen availability.

An essential nutrient for plants as well as for humans and animals, nitrogen is used widely in more urban, developed countries to fertilize crops. In fact, it has been used so widely that its use has raised serious environmental concerns.

That gave people the impression "that we are kind of nitrogen-saturated everywhere, that we have too much nitrogen," Wang said.

But the researchers found that perception is not true.

In natural systems such as grasslands and forests that are not directly fertilized, the researchers said, the availability of nitrogen to plants is declining. As availability declines, compared to the relative demand for the nutrient due to plants leafing out earlier and the longer growing seasons associated with climate warming, plants are suffering from nitrogen deficiency, Wang said.

"In such systems, which cover a large part of the world, demand for nitrogen is rising at a faster rate than the supply of nitrogen," Wang said.

With nitrogen deficiency, plants are unable to absorb the same quantity of carbon dioxide as they did previously.

"We know that plants reliably suck up carbon dioxide that we emit into the environment," Wang said. "But the problem right now is if plants are suffering more and more nitrogen limitations, it means they will be able to take up less and less of the extra carbon dioxide."

"Not only will plants be more stressed for nitrogen," said Joseph Craine, the paper's lead author, "but so will animals that eat plants. Less nitrogen in plants means less protein for herbivores, which could threaten the entire food chain."

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