How long does biochar last?

This is an article from Grist, and taken from another website, a facebook group about biochar.  It has intriguing implications about how long biochar will last, and the policy implications of government subsidies of large scale biochar:

John S
PDX OR

“This story was originally published by Grist and appears here as part of the Climate Desk collaboration
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. 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. “

I'm very glad to read this, having previously read the articles that said it was much shorter lasting than that.  I hope that more studies prove this out and we really are doing something that can last thousands of years.  What a great legacy to leave behind.

That's great!!!

I've sort of assumed that it has to be able to last very long, since it obviously has in the Amazon (where the climate is just about ideal for degrading just about anything) but it's nice that it's backed by research...

Some of the carbon in coal seams is pyrogenic. In other words, the peat deposits and swamp vegetation that eventually became the coal formations of the Carboniferous period had fires go through them from time to time, and the charcoal produced in those fires didn't go anywhere. So there is biochar embedded in coal that is hundreds of millions of years old, and that biochar may have sat in the swamps for millions before the coal formed.

Now, whether biochar in a more aerobic environment would hang around that long is probably pushing the extremes, but at those timescales the environment itself changes to the point that it might no longer be anything like how it started out. So the "old" pyrogenic carbon in dark prairie soils, which has been accurately dated to well over 10,000 years, might still be there in a few million but it won't be prairie anymore...it might be a scrubland or a forest atop a rising set of mountains, or a layer of sediment under a new sea.

The general consensus is that as long as the treatment temperature is high enough to form aromatic ring structures (over 400 C), most of the carbon in biochar will resist breakdown for hundreds to thousands of years. The trick has been figuring out what "most" means...I think that the 30% loss assumption over 100 years was introduced more as a hedge against uncertainty and a strategy so the emerging industry wouldn't get attacked for overpromising. There are protocols for doing accelerated weathering tests on biochar to simulate longer timescales, and there's even a standard that assigns a 100-year expectation based on the ratio of carbon to hydrogen in the material.

It's an example of how humans like to turn everything into financial models and one benefit I see from the cautious framing is that it ultimately helps the argument for making biochar production a normal part of business as usual that we just need to do for the long term.

This is heartening news. It underscores that my efforts to make char (which is difficult) instead of ashes (which is easy) are worthwhile and make a genuine contribution. I shall persevere.