Fig Trees Turn Atmospheric Carbon Into Stone

Kenyan fig trees are doing something remarkable: they’re converting carbon dioxide from the air into limestone-like rocks stored in their trunks and surrounding soil.

This natural alchemy, discovered by an international team of scientists, could reshape how we think about carbon capture through agriculture.

The research, presented at the Goldschmidt Conference in Prague, reveals that certain fig species native to Kenya use what scientists call the oxalate carbonate pathway. Think of it as nature’s version of a carbon storage facility—but instead of burying CO2 underground, these trees literally turn it into stone.

How Trees Make Rocks

Most trees capture carbon through photosynthesis, turning atmospheric CO2 into the organic matter that builds their wood and leaves. But these fig trees go a step further. They create calcium oxalate crystals, which specialized bacteria and fungi then convert into calcium carbonate—the same mineral found in limestone and chalk.

Dr. Mike Rowley, a senior lecturer at the University of Zurich who led the research, discovered something unexpected when his team used synchrotron analysis to peer inside the trees. The calcium carbonate wasn’t just forming on the surface—it was penetrating deep into the wood structure itself.

“As the calcium carbonate is formed, the soil around the tree becomes more alkaline,” Rowley explained. “The calcium carbonate is formed both on the surface of the tree and within the wood structures, likely as microorganisms decompose crystals on the surface and also, penetrate deeper into the tree.”

Key Findings from the Study

• Ficus wakefieldii proved most effective at carbon sequestration among three fig species tested
• Calcium carbonate forms both on tree bark and deep within wood structures
• The process makes surrounding soil more alkaline while increasing nutrient availability
• Inorganic carbon storage lasts much longer in soil than organic carbon

The discovery matters because calcium carbonate has staying power. While organic carbon from decomposing leaves and branches eventually returns to the atmosphere, this mineral form can persist in soil for much longer periods. It’s like the difference between storing water in a leaky bucket versus a sealed container.

Beyond Carbon Storage

What makes this finding particularly intriguing is that fig trees produce food while simultaneously turning carbon into stone. The Iroko tree, the first species identified with this pathway, can sequester one ton of calcium carbonate over its lifetime—but it doesn’t feed people.

Rowley sees potential for what he calls a triple win: “If we’re planting trees for agroforestry and their ability to store CO2 as organic carbon, while producing food, we could choose trees that provide an additional benefit by sequestering inorganic carbon also, in the form of calcium carbonate.”

The research team, spanning institutions from Kenya to Switzerland, is now quantifying exactly how much CO2 these fig trees can capture under different conditions. They’re also assessing the trees’ water requirements and fruit yields to determine their viability for large-scale agroforestry projects.

The Bigger Picture

This oxalate-carbonate pathway isn’t limited to fig trees. Calcium oxalate crystals are among the most common biominerals produced by plants, and the microorganisms that convert them to calcium carbonate are widespread in nature. The researchers suspect many more tree species have this ability—we just haven’t been looking for it.

The implications extend beyond individual trees. If agroforestry programs could prioritize species with this rock-forming ability, they might capture significantly more carbon than previously calculated. It’s a reminder that nature often has more tricks up its sleeve than we realize—sometimes literally turning thin air into stone.