For all its plant and animal life aboveground, the Amazon rainforest’s soils are surprisingly poor in nutrients necessary for growing food. Thousands of years ago, the region’s Indigenous peoples solved this problem by creating “terra preta” from table scraps and charcoal and tucking it away in the hostile soil.
Today, that ancient bit of ingenuity is a powerful climate solution. As biomass like trees and crops grow, they sequester carbon in their leaves and branches. Heat that biomass up without fully consuming it and it turns to nearly pure carbon known as biochar, which farmers soak in compost or fertilizer to “charge” it with nutrients, then add to their soils. (In 2023 the global biochar market was worth $600 million, and is expected to grow to $3 billion this year.) That simultaneously improves crop yields and better retains water, all while locking carbon away from the atmosphere. Rising demand from farmers and big business is expected to push the global market for biochar from $600 million two years ago to $3 billion this year.
The nagging question, though, is exactly how long that carbon stays in the soil. A new study adds to a growing body of evidence that scientists have been underestimating the staying power of biochar, meaning the technology is actually an even more powerful way to store carbon than previously thought. “I’m talking about over 90 percent very easily surviving multi-thousands of years,” said Hamed Sanei, a professor of organic carbon geochemistry at Denmark’s Aarhus University and lead author of the paper published in the journal Biochar. The research suggests that biochar is much more resilient than currently calculated by researchers. “The current model that we’re talking about is saying 30 percent of almost all biochar that’s being produced will be gone in 100 years.”
Nailing down exactly how long biochar can hold onto carbon is crucial for the carbon-removal credit industry, where companies like Microsoft and Google fund projects to draw carbon out of the atmosphere. These credits reached 8 million metric tons of carbon in 2024, a 78 percent jump from the prior year. So scientists have been running experiments monitoring how microbes degrade biochar over a few years in soil, then extrapolating that over longer time scales. Doing that sort of modeling, the U.N.-backed Intergovernmental Panel on Climate Change and other research groups have reckoned that after a century, between 63 and 82 percent of the biochar will stay in the ground.
The critical clue for Sanei was a naturally occurring material called inertinite, a stable form of organic carbon in Earth’s crust, formed when wildfires char forests, and the burned vegetation fossilizes. Biochar is just the result of humans replicating that process: If the biomass is exposed to sufficiently high temperatures — over 1,000 degrees F is ideal — the carbon should transform into a material that soil microbes struggle to digest, which is how the charred plants in inertinite were able to last long enough to fossilize. Much as humans eat food off dishes instead of eating the dishes themselves, bacteria and fungi choose to eat organic matter like leaves over biochar. “It’s kind of like if you have a nice piece of cake and they bring it to us on a plate, we’re going to eat the cake,” Sanei said. “If we are very hungry, we eat it much faster. But still, we’re not going to eat the plate.”
Much as inertinite survived over vast stretches of geologic time, biochar should be able to last for millennia, Sanei and his coauthors calculate. The fact that scientists are finding intact biochar in the Amazon’s ancient terra preta suggests that it’s happening. “Biochar is already a compelling solution,” said Thomas A. Trabold, a sustainability scientist at the Rochester Institute of Technology and CEO of Cinterest, a company developing biochar technology. “This data just suggests that the benefits are even greater than we already assumed.”
Not all biochar is created equal, though. For one, woody biomass turns to better biochar because it has a higher carbon content than leafy material or grass. And the higher the temperatures used in the manufacturing process, the better chance that carbon will stay in the soil. The local climate matters too, as warmer soils lead to more microbial activity that can degrade biochar.
Still, by carefully controlling the production of biochar, companies can produce a material that they know contains a given amount of carbon. This becomes a carbon removal credit, which companies buy to show they’re investing in removing carbon from the atmosphere (even if they’re not doing all they can to reduce their own emissions). Most carbon removal credits have a standard time frame of 100 years, according to Erica Dorr, who leads the climate team at Riverse, a carbon crediting platform in France. But if scientists are now talking about biochar lasting for thousands of years instead of centuries, that makes it more appealing for corporations buying credits, Dorr said.
“It wasn’t very interesting to issue a 500-year or 1,000-year biochar removal credit, because the model would tell us that there’s not much remaining after that long,” Dorr said. “Now, the new research is really unlocking this 1,000-year argument.”
That would put biochar on par with other carbon removal techniques like direct air capture, in which giant machines suck carbon out of the air and pump it underground. But direct air capture remains expensive, and the technology is nowhere near widespread enough to put a meaningful dent in carbon emissions. Biochar, on the other hand, is a proven technique that’s been used for thousands of years, capable of improving agriculture and, according to this new research, locking carbon away for millennia.
This story was originally published by Grist with the headline This ancient bit of ingenuity keeps carbon trapped for thousands of years on Mar 3, 2025.
Turning plant waste into biochar captures loads of carbon. New research suggests it stays trapped for much longer than scientists thought. Food and Agriculture, Science Grist