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Climate Change

Rocks are a missing piece of the nitrogen cycle

Biogeochemists argue that rocks supply a significant source of the element, which could have implications for climate models

by Katherine Bourzac
April 5, 2018 | APPEARED IN VOLUME 96, ISSUE 15

09615-scicon3-coolrocks.jpg
Credit: Paul White
The Pinnacles at Gunung Mulu National Park in Borneo are made from limestone that can weather and release nitrogen into the soil.

Nitrogen gets around on Earth. The element moves between the atmosphere, oceans, land, crust, and mantle of the planet. But models of this nitrogen cycle are in need of a major update, a group of biogeochemists says. The researchers claim the models omit about 15 tera­grams of nitrogen produced by rock weathering (Science 2018, DOI: 10.1126/science.aan4399). The implications of this omission are unclear, but some researchers say correcting it may fine-tune climate models.

Scientists have always assumed nitrogen enters land-based ecosystems from the air, not from geological sources. But geologists have long known that rocks, especially sedimentary rocks, contain varying concentrations of nitrogen, much of it in forms that organisms can use. Through tectonic upheaval, physical and chemical weathering, and other processes, these nitrogenous compounds can move into the soil, just as other nutrients do.

In a new paper, Benjamin Houlton, a biogeochemist at the University of California, Davis. and colleagues used models of physical and chemical weathering to predict that rocks release 11 to 18 Tg of the nutrient into the soil per year. As a comparison, before the industrial era, the land and atmosphere exchanged about 100 Tg of nitrogen per year.

Houlton expects that adding rocks into the nitrogen cycle may affect climate change forecasts to some extent. Plants currently absorb about 30% of human carbon dioxide emissions, Houlton says. Their ability to do this is limited by how much nitrogen they have access to because they need the nutrient to make biomolecules.

“If there is more nitrogen there than expected, then the constraints on plant growth in a high-CO2 world may not be as great as we think,” says Ronald Amundson, who studies soil biogeochemistry at the University of California, Berkeley.

William Schlesinger, a biogeochemist and president emeritus of the Cary Institute of Ecosystem Studies, is more skeptical. Rock nitrogen is probably insignificant on a planetary scale, though it may be important locally in some places, he says. Schlesinger expects the effects of rock nitrogen on the carbon cycle would be small enough to fall into the margin of error of climate models.

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