Some species of fig trees store calcium carbonate in their trunks—essentially turning themselves (partially) into stone.

This ‘auto-petrification’ may offer a strange new way to reduce human carbon emissions, as the mineral created by the trees has a much longer lifespan than organic carbon absorbed and deposited in its root system.

An international team of scientists found that three species of Ficus that grow in Kenya were able to draw carbon dioxide (CO2) from the atmosphere and store it as calcium carbonate ‘rocks’ in the surrounding soil.

The figs are one of the first fruiting trees shown to have this ability, known as the oxalate carbonate pathway. One species in particular, Ficus wakefieldii, was especially prolific.

All trees use photosynthesis to turn CO2 into organic carbon, which forms their trunk, branches, roots and leaves, but certain trees also use CO2 to create calcium oxalate crystals.

When parts of the tree decay, these calcium oxalate crystals are converted by specialized bacteria or fungi into calcium carbonate—the same mineral as limestone or chalk. This increases the soil pH around the tree, while also increasing the availability of certain nutrients.

The inorganic carbon in calcium carbonate typically has a much longer lifetime in the soil than organic carbon, making it a more effective method of CO2 sequestration.

“We’ve known about the oxalate carbonate pathway for some time, but its potential for sequestering carbon hasn’t been fully considered. 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,” said Dr. Mike Rowley, a senior lecturer at the University of Zurich (UZH) who presented his study on the subject at the Goldschmidt Conference in Prague.

Rowley and his team identified how far from the tree the calcium carbonate was being formed and identified the microbial communities involved in the process.

“As the calcium carbonate is formed, the soil around the tree becomes more alkaline,” Dr. Rowley said. “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. It shows that inorganic carbon is being sequestered more deeply within the wood than we previously realized.”

They are now planning to assess F. wakefieldii’s suitability for agroforestry by quantifying its water requirements and fruit yields and by doing a more detailed analysis of how much CO2 can be sequestered under different conditions.

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Most of the research into the oxalate-carbonate pathway has been in tropical habitats and focused on trees that do not produce food. The first tree to be identified as having an active oxalate-carbonate pathway was the Iroko (Milicia excelsa). It can sequester one ton of calcium carbonate in the soil over its lifetime.

Calcium oxalate is one of the most abundant biominerals and the crystals are produced by many plants. The microorganisms that convert calcium oxalate to calcium carbonate are also widespread.

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“It’s easier to identify calcium carbonate in drier environments,” explained Dr. Rowley.

“However, even in wetter environments, the carbon can still be sequestered. So far, numerous species of tree have been identified which can form calcium carbonate. But we believe there are many more. This means that the oxalate-carbonate pathway could be a significant, underexplored opportunity to help mitigate CO2 emissions as we plant trees for forestry or fruit.”

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