Fig trees turn atmospheric carbon into stone

Kenyan fig trees are doing something extraordinary: They are converting carbon dioxide from air to limestone-like rocks stored in the trunks and surrounding soil.

This natural alchemy, discovered by an international team of scientists, could reshape the way we capture carbon capture through agriculture.

The study, presented at the Goldschmidt conference in Prague, revealed that some native fig species in Kenya use what scientists call carbonate pathways. Think of it as a version of a natural carbon storage facility, but these trees actually turn it into stone instead of burying carbon dioxide underground.

How trees make rocks

Most trees capture carbon through photosynthesis, turning atmospheric carbon dioxide into organic matter that builds wood and leaves. But these fig trees took a step further. They produce calcium oxalate crystals, which then convert their specialized bacteria and fungi into calcium carbonate – minerals found in limestone and chalk.

Dr. Mike Rowley, a senior lecturer at the University of Zurich, led the study and discovered something unexpected when his team used synchronous accelerator analysis to stare inside trees. Calcium carbonate not only forms on the surface, but also penetrates deep into the wood structure itself.

“As calcium carbonate forms, the soil around the tree becomes more alkaline,” explains Rowley. “Calcium carbonate is formed both on the surface and within the wood structure of the tree, probably as the microorganisms break down crystals on the surface and penetrate deeper into the tree.”

Major findings from the study

• ficus wakefieldii It turns out that among the three fig species, the most effective in carbon-solidation exchange
• Calcium carbonate forms wood structures in the bark and deep
• This process makes the surrounding soil more alkaline while increasing nutrient availability
• Inorganic carbon is stored in soil for much longer than organic carbon

This finding is important because calcium carbonate has endurance. Although the organic carbon that breaks down leaves and branches eventually returns to the atmosphere, this mineral form can last longer in the soil. It’s like the difference between water and leaky reservoirs compared to sealed containers.

Beyond Carbon Storage

What makes this discovery particularly interesting is that fig trees produce food while turning carbon into stone. The iRoko tree is the first species identified using this pathway, which can isolate a ton of calcium carbonate throughout its lifetime, but will not feed people.

Rowley sees the potential for what he calls a triple victory: “If we are going to plant trees for agroforestry and plant trees and their ability to store carbon dioxide as organic carbon, while producing food, we can choose trees that provide the added benefit by isolating inorganic carbon in the form of calcium carbonate.”

The team, which spans institutions from Kenya to Switzerland, is now accurately quantifying how much carbon dioxide these fig trees can capture under different conditions. They also evaluate the trees’ water demand and fruit yield to determine their viability for large agroforestry projects.

Larger pictures

This oxalate carbonate pathway is not limited to fig trees. Calcium oxalate crystals are the most common biolipids produced by plants, and microorganisms that convert them to calcium carbonate are inherently common. Researchers suspect that more species have this capability and we are just looking for it.

These meanings go beyond the individual trees. If agroforestry plans can prioritize species with this rock formation capacity, they may be more carbon than previously calculated. This reminds people that nature usually has a lot more tricks than we realize, and sometimes actually turns thin air into stone.

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