The science of carbon banking: Explained

A woman points to a metal box
Caley Gasch assistant professor of soil health for North Dakota State University in her Fargo laboratory in April.
Dan Gunderson | MPR News

Some U.S. farmers are hoping to invest in a new cash crop, deep within the soil: Carbon.

The Biden administration has said it wants agriculture in the U.S. to become “carbon neutral” — and it hopes to get there, in part, with carbon banking.

What is carbon banking?

The idea behind carbon banking is that, by changing their farming practices, farmers can store carbon in the soil, helping to reduce carbon dioxide gas in the atmosphere and slow global warming. 

But it’s complicated, and understanding the science is important.

Where does the carbon come from?

Caley Gasch is a soil health researcher at North Dakota State University. In her Fargo lab, she holds a handful of black crumbly soil, collected from a farm field.  

"We study all the organisms that live in the soil and how nutrients and organic matter cycles through the soil," she said.

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In her hand, mixed in with the soil are bits of roots and decomposing leaves. The plant material and the black soil particles are all part of the carbon cycle.

It starts with carbon dioxide. It’s a gas, and it’s all around us. We exhale it every time we take a breath. Power plants, cars and trucks also exhale carbon dioxide when they burn fossil fuels — and that CO2 contributes to global warming.

No-till dirt
David Legvold demonstrates how dirt in a no-till field holds together and encourages growth of nutrients at a farm in Greenvale Township, Minn., in 2011.
Jeffrey Thompson | MPR News file 2011

But plants take in that carbon dioxide as they grow — and, along with sunlight, use it to make the sugar they need for food. 

"Then when that plant dies, that contributes to the organic matter," said Gasch. The plants converted that carbon dioxide into carbon through their life cycle. "And they're releasing carbon as they continue to decompose." 

How does the soil hold on to the carbon?

Dead plants are food for billions of tiny microbes in the soil, which eat the carbon in leaves, stems and roots.

“The microbes need some of the carbon for themselves,” said Gasch. “They want to build more microbes, and then they also need carbon.”

And as the microbes reproduce, they process that carbon. But they don’t retain it all.

“Let’s just say about 50 percent of it goes into their biomass,” Gasch said. “And then 50 percent of it goes into the atmosphere through the process of respiration."

Those microbes in the soil exhale carbon dioxide.

In fact, when carbon dioxide is released from the soil, it’s a sign that the soil is healthy — because there are lots of microbes digesting organic carbon. 

But isn’t the goal to take carbon from the atmosphere and lock it away in the soil?

Yes — but here's where it gets complicated.

When there are more plants growing on the land — like cover crops on farm fields — it means there’s more carbon in the soil, for the short term

But all that carbon will get eaten by microbes. There's no way around that natural process. 

That’s why real, long-term storage of carbon in soil requires patience. 

"Storing carbon is possible, it just takes a very long time," said Gasch.

Remember: All those tiny microbes exhale about half of the carbon they eat, back into the atmosphere.

“The carbon has to go through that microbial processing in order to be stabilized in a form where it will stay in the soil for a very long time,” said Gasch.

“It turns out that a very small amount each year, less than 1 percent, is processed to the point where it becomes that black material that coats the soil particles,” she said. “And that's the stuff that stays in the soil for a very long time." 

How can farmers aid this process?

You can't change the natural carbon cycle, of plant biomass being broken down by microbes.

But farming practices can affect how quickly that process happens.

For instance: Stop tilling the soil, and microbes break down the bits of dead plants more slowly. 

Add more biomass — more plants growing on the land, for as long as possible throughout the year — and you can increase the amount of carbon in the cycle.  

Besty Jensen said she is pleased with the rye cover crop.
Rye grows as a cover crop on Betsy Jensen's farm in northwestern Minnesota.
Dan Gunderson | MPR News file

“The more biomass we produce on an acre of land, the more likely we'll have more carbon ultimately being stored for the long term,” said Gasch.

But that takes time, investment and consistent practices, she added.

"We have to be patient. And we have to think about longer-term trends rather than change from one year to the next,” she said.

Is it possible to measure how much carbon is being stored?

It takes at least five to 10 years to measure a trend in carbon stored in the soil. 

And while scientists can very accurately measure the volume of soil carbon in a sample in a lab, it's more complex to accurately measure across an entire field, because soil conditions can vary widely. 

There are programs that pay farmers to store carbon, as part of a carbon credits system — and many of them rely on computer models to estimate the amount of carbon that could be stored by various farming practices. 

That's still a developing area of science, said Gasch, and models can be inaccurate.

“Even though we've been at this for a very long time, we're still learning how the combinations of these various practices kind of interact across different geographies and climates,” she said.

"To just go to a part of the country or a single field and say, ‘These practices will increase the carbon this much and in this amount of time,’ is still a little bit hard for us to do."

But carbon in soil and the microbes that process it are getting a lot of attention and a lot of new research.

For farmers considering carbon storage, the best approach might be to just make life easier for those carbon-munching soil microbes by adapting regenerative agriculture practices: reducing tillage, adding cover crops and diversifying crop rotations.

Those practices create healthier soil, increase water storage, decrease pollution and allow for more carbon storage. 

"We just need to kind of step back a little bit and let the soil do what it does best,” said Gasch.  “The microbes are very well adapted to our climate, to our soil conditions, and they just want to be there doing their thing. And so we just need to provide the safe environment for them to do that. And then we'll see the beneficial results.”