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Trees Pull Nitrogen from Rocks and Microbes

The new nitrogen source, previously unsuspected by scientists, may help fuel tree growth and pull excess CO2 from the air



Todd Huffman, courtesy Flickr

Nitrogen-rich forest bedrock -- the geologic rock formation located under forest soil -- may aid trees in better sequestering carbon, according to a recent study that offers a new understanding on why some forests store greenhouse gases more efficiently than others.

While geologic rock isn't a carbon sink itself, it plays an important role in helping the soil and trees above absorb CO2, say the study's authors, who published their findings last week in Nature. But a lack of research on nitrogen has left it largely ignored by climate scientists and policymakers scrambling to identify carbon sinks that mitigate carbon dioxide pollution from large emitters.

A group of University of California, Davis, biogeologists observed two very similar, adjacent forests in Northern California -- South Fork Mountain and the Bear Wallow Diorite Complex.

Both forests had similar soils, vegetation, rainfall and temperature. But the rock below the South Fork Mountain forest was made of nitrogen-rich mica schist, derived from marine sediments from the early Cretaceous Period.

The results were significant: South Fork Mountain held 42 percent more carbon in its trees, as observed in analysis of carbon in leaves and soil, than Bear Wallow.

"This idea that bedrock could also provide an option [for carbon sequestration] is new and an addition to how we think about climate change," said co-author Benjamin Houlton, an assistant professor of Earth systems ecology and biogeochemistry at the University of California, Davis.

The forgotten element in climate change
Nitrogen plays a key role in carbon sequestration, explained Houlton. The enzyme ribulose-1,5-bisphosphate carboxylase oxygenase -- shortened to RuBisCO -- is essential in photosynthesis, the plant process that converts carbon dioxide into oxygen. Nitrogen is an essential building block of RuBisCO, and makes up about 15 percent of the enzyme.

As they break down over time, rocks release nitrogen into soil, a concept that has only received significant attention in the past decade, said Houlton. Now, scientists are looking to soil for its carbon-sequestering properties. A three-day meeting on the climate-mitigating power of soils begins today at the U.N. Food and Agriculture Organization's headquarters in Rome.

"For decades, it's been very carbon-centric," said Houlton. "All of the global carbon models assumed that plants just needed water and could grow whatever they want."

Agriculture, he added, has understood the need for nitrogen, the key ingredient in fertilizer.

"You need to fertilize the heck out of a farm field for a plant to grow," said Houlton. Climate modelers, on the other hand, did not accurately account for the element.

"It means we're underestimating climate change," he said. "It could be a degree Celsius warmer than has been expected because nitrogen hasn't been very abundant."

Hot weather accelerates rock weathering
As climate change warms the Earth and creates more erratic rainfall, bedrock will erode at a faster pace, as well, added Houlton. He and his colleagues are building a global model that that links the rates at which rocks erode to climate change, a tool that could streamline carbon trading by showing which sinks could be worth more credit.

"If these carbon markets become more widespread, we're going to have to start understanding where the carbon will be [kept]," he said. "We're not quite sure how widespread this phenomenon is, but it's one new way to think of how markets come online in the future."

Reprinted from Climatewire with permission from Environment & Energy Publishing, LLC. www.eenews.net, 202-628-6500

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