Trees are known to absorb carbon dioxide from the air for photosynthesis, and then convert it into the substances they need. Research has found that certain types of trees go even further by transforming excess carbon dioxide into rocks, storing them in the soil, offering an alternative method to reduce carbon emissions.
The European Association of Geochemistry stated in a press release that a research team composed of scientists from Kenya, the United States, Austria, and Switzerland discovered that fig trees in Kenya can absorb carbon dioxide from the atmosphere and store it in the surrounding soil in the form of calcium carbonate, a major component of rocks.
These trees in Kenya are among the first fruit trees confirmed to possess this capability. This process, known as the oxalate carbonate pathway, involves converting atmospheric carbon dioxide into calcium carbonate, a natural occurrence.
All trees on Earth convert carbon dioxide into organic carbon through photosynthesis to build their trunks, branches, roots, and leaves. This is why tree planting is seen as a potential method to mitigate carbon dioxide emissions.
Some trees also use carbon dioxide to produce calcium oxalate crystals. When parts of the trees decompose, specific bacteria or fungi convert these crystals into calcium carbonate. This raises the pH of the soil around the trees, while also increasing the availability of certain nutrients. Inorganic carbon in calcium carbonate generally has a longer lifespan in the soil than organic carbon, making it a more effective carbon sequestration method.
Mike Rowley, a senior lecturer at the University of Zurich in Switzerland, mentioned that while the oxalate carbonate pathway has been understood for some time, its potential for carbon sequestration has not been fully appreciated.
He suggested that planting trees for agroforestry and utilizing their ability to store carbon dioxide as organic carbon while producing food could lead people to select trees that can sequester inorganic carbon as calcium carbonate for additional benefits.
In their research, scientists used synchrotron radiation analysis techniques and found that calcium carbonate forms both externally on the tree trunk and deep inside the wood.
They studied three varieties of fig trees and discovered that Ficus wakefieldii showed the strongest effect of storing carbon dioxide in the form of calcium carbonate.
Researchers plan to quantify the water requirements and fruit yields of these fig trees and conduct further analysis on carbon sequestration under different conditions to evaluate the suitability of these trees for agroforestry.
Most research on the oxalate carbonate pathway focuses on tropical regions and trees that do not produce food. The first tree confirmed to actively utilize this pathway is the African iroko (Milicia excelsa), which can sequester one ton of calcium carbonate in the soil during its lifetime.
Calcium oxalate is one of the richest biominerals produced by many plants. Microbes that convert calcium oxalate to calcium carbonate are widespread.
Rowley mentioned that calcium carbonate is more easily identified in drier environments but can still be sequestered even in wetter conditions.
He emphasized that while many tree species capable of forming calcium carbonate have been identified, there are likely more. This suggests that the oxalate carbonate pathway could be an important and unexplored opportunity to help reduce carbon dioxide emissions when planting trees for reforestation or fruit production.
These research findings were presented this month at the Goldschmidt geochemistry conference in Prague, the capital of the Czech Republic.