In an unexpected twist of nature, certain fig trees in Kenya appear to do more than just produce fruit – they also capture CO₂ from the air and convert it into stone.
Scientists discovered that these trees can form limestone deposits in their trunks and surrounding soil, locking carbon away long-term in a stable, mineral form. This largely unknown carbon pathway, previously studied mainly in non-fruit-bearing trees, could play a valuable role in climate solutions – especially within agroforestry systems.
Fig Trees That Form Stone
New research shows that some fig trees in Kenya possess a remarkable ability: they can take carbon dioxide (CO2) from the air and convert it into calcium carbonate – a process that literally forms stone in their trunks and surrounding soil.
The research, conducted by an international team of scientists from Kenya, the US, Austria, and Switzerland, was presented at the Goldschmidt Conference on Geochemistry in Prague.
These fig trees are among the first known fruit-bearing species to use a process known as the oxalate-carbonate pathway. This means they store CO2 not only in organic structures like wood and leaves, but also in the form of solid minerals deep in their trunks and surrounding soil.
A Hidden Carbon Pathway
All trees use photosynthesis to pull CO2 from the air and convert it into organic material like wood, leaves, and roots. That’s why forests are often cited as a natural climate solution.
But some trees go a step further. They convert CO2 into tiny crystals of calcium oxalate. When parts of the tree die and decay, microbes and fungi convert these crystals into calcium carbonate. That mineral stores carbon much longer than organic material, while also making the soil more alkaline and nutrient-rich.
Dr. Mike Rowley, lecturer at the University of Zurich (UZH), presented the research at the Goldschmidt Conference. He explains: “We’ve known about the oxalate-carbonate pathway for some time, but the potential to lock away carbon this way remains underappreciated. If we plant trees for agroforestry because of their ability to store CO2 as organic matter and provide food, we could better select trees that also store inorganic carbon — in the form of calcium carbonate.”
Insights via Synchrotron and Microbes
The team, from UZH, Nairobi Technical University, Sadhana Forest, Lawrence Berkeley National Laboratory, University of California Davis, and the University of Neuchâtel, studied three fig tree species in Samburu County, Kenya. They mapped how far from the tree calcium carbonate formed and which microorganisms were involved. Using synchrotron analysis at the Stanford Synchrotron Radiation Lightsource, they discovered that calcium carbonate formed both on the bark and deeper in the wood.
According to Dr. Rowley: “As calcium carbonate forms, the soil around the tree becomes more alkaline. Formation occurs both on the outside of the trunk and inside the wood tissue – likely because microorganisms break down crystals at the surface and also penetrate deeper into the trunk. It suggests that inorganic carbon is locked away deeper than we previously thought.”
Of the three fig species studied, Ficus wakefieldii proved most effective at capturing CO2 as calcium carbonate. Researchers now want to further investigate this species’ suitability for agroforestry by examining water requirements, fruit yield, and the amount of CO2 that can be stored under different conditions.
Potential for Agroforestry and Fig Tree Performance
Until now, most research on the oxalate-carbonate pathway has been conducted in tropical regions, and mostly on trees that don’t produce food. The first species where this pathway was actively demonstrated was the iroko (Milicia excelsa) – a tree that can store up to one ton of calcium carbonate in the soil over its lifetime.
Calcium oxalate is one of the most common biominerals and is formed by many plant species. The microorganisms that convert it to calcium carbonate are also widely distributed.
Toward a Global Tree Solution
“Calcium carbonate is easier to detect in dry environments,” says Dr. Rowley. “But carbon can still be locked away in wetter regions too. We’ve now identified multiple tree species capable of forming this mineral – and we suspect there are many more. That opens the door to a largely untapped, natural pathway to reduce CO2 emissions, especially when planting trees for food production or forestry.”
Verified Sources
Conference: Goldschmidt 2025
The Goldschmidt Conference is the world’s leading congress on geochemistry. It’s a joint initiative of the European Association of Geochemistry and the Geochemical Society (US), with an expected attendance of 4,000 participants. The 2025 edition takes place in Prague, Czech Republic, from July 6-11.
Thanks to SciTechDaily.com
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Frequently Asked Questions
How do fig trees lock away CO₂ as stone?
Certain fig trees, like Ficus wakefieldii, use a process where CO₂ from the air is converted into calcium oxalate. When this mineral breaks down, microbes form calcium carbonate – a solid that stores carbon long-term in trunks and soil.
What is the oxalate-carbonate pathway?
The oxalate-carbonate pathway is a biochemical process where plants form calcium oxalate, which is later converted by microorganisms into calcium carbonate. It offers an alternative way to lock away carbon long-term besides wood formation.
Why is calcium carbonate important for climate?
Calcium carbonate is a highly stable mineral form of carbon. Unlike organic material, it doesn’t decay quickly, making it a durable way to remove CO₂ from the atmosphere.
Are there other trees that use this process?
Yes, the iroko tree was the first known species with an active oxalate-carbonate pathway. Researchers suspect many more tree species, including fruit trees, possess this ability.
Can these trees be used in agroforestry?
That’s exactly what researchers are investigating further. Trees like Ficus wakefieldii combine food production with effective carbon storage, making them attractive for sustainable farming systems.
