Nitrogen Study Could 'Rock' A Plant's World

All plants need nitrogen to grow, and a new study says some plants get this nutrient from a surprising source. Conventional wisdom believes nitrogen for plants starts in the air and is converted by microbes in the soil into a usable form. But a report in the journal Nature finds that some nitrogen comes directly from rock. This discovery has implications for global warming.

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Anyone with a green thumb will tell you that plants need nitrogen. Gardeners might use fertilizer, but in nature, the conventional wisdom is that air is the ultimate source of nitrogen for plants. It's converted to plant food by microbes underground. A new study, though, finds that conventional wisdom is wrong. NPR's Richard Harris explains that this could have some big implications for the planet.

RICHARD HARRIS: It's been such a central fact in botany that for many decades, nobody has bothered to question whether plants rely on the air for their nitrogen. Yet, there have been hints that the conventional wisdom is wrong.

Mr. BENJAMIN HOULTON (University of California, Davis Professor): If you go into most forest ecosystems for a long time it's been recognized that there's way too much nitrogen accumulating in soils and plants than could be explained by the atmosphere.

HARRIS: Benjamin Houlton, at the University of California, Davis, says you can add up all the nitrogen captured by microbes and you can add in the nitrogen particles that fall in the forest in the rain, but you still come up short.

So where else could forest nitrogen be coming from? Houlton was chatting about this with a colleague one day, and they started mulling over an idea that had been discarded long ago. That's the notion that significant amounts of nitrogen could leach out of sedimentary rocks. It seemed like a long shot to revisit this idea, but what the heck.

Mr. HOULTON: So we took a trip out to this mountain in Northern California, the Klamath Mountain Range, and it's underlain by this sedimentary rock that was laid down at the time of dinosaurs and has a lot of nitrogen in it and took a couple of samples. And my graduate student Scott Morford(ph) brought those samples back. And I'll never forget it. He came running into my office and I was absolutely shocked.

HARRIS: The trees contained twice as much nitrogen as the scientists expected. And chemical tests showed that the nitrogen was coming directly from the ancient rock. In contrast, trees growing nearby on rocks that didn't contain nitrogen contained a lot less of the stuff.

For scientists like Houlton, who are trying to understand nutrient cycles on a global scale, it's a huge deal to realize that sedimentary rocks are so important for plants.

Mr. HOULTON: It would increase the amount of nitrogen we think is available to terrestrial plants by about a third.

HARRIS: This affects how the forests of the world could react to climate change. Trees are gobbling up a fair amount of the carbon dioxide that we pour into the atmosphere, but scientists have been worried that in the next few decades, they'd start to run out of the nitrogen they need to keep fueling this growth spurt. It turns out maybe not.

Mr. HOULTON: This could act as a big source of fertilizer that allows plants to continue to pull some of our human CO2 emissions out of the atmosphere.

HARRIS: Houlton and his colleagues make their case in the latest issue of the journal Nature. And assuming what they found in California is true around the world, everyone who studies natural plant nutrition is going to have to change their thinking.

Ecologist Kathleen Treseder at the University of California, Irvine, says she's focused much of her effort on understanding the microbes that capture nitrogen from the air and deliver it to plants.

Ms. KATHLEEN TRESEDER (University of California, Irvine Professor): But now there's a whole other source that I need to consider. I need to think about how that might change under different environmental conditions. So it's not just the microbes that are operating.

HARRIS: Does it sort of take the microbes down a notch?

Ms. TRESEDER: Well, I study microbes, so I don't want to say that necessarily. But it just adds an additional level of complexity.

HARRIS: And she adds, it means that the biology textbooks that students use everywhere are just plain wrong.

Ms. TRESEDER: Textbooks are always being outdated. So I'll have to add an extra slide or two to my lectures.

HARRIS: But this isn't just a little sort of tweak of a fact on page 17. I mean, this is kind of a central idea, isn't it?

Ms. TRESEDER: Yeah, it is. It really is. So it's very exciting.

HARRIS: And it's a reminder of the rich reward that can occasionally come from challenging your assumptions.

Richard Harris, NPR News.

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