To combat global warming, it is said to plant many trees. Trees grow through photosynthesis, which absorbs carbon dioxide (CO₂), a greenhouse gas, and converts it into carbohydrates. A tree that is optimal for fixing such greenhouse gases has been discovered in Africa. This tree not only absorbs carbon dioxide to create leaves and stems but also buries it underground as stone. By preventing global warming and bearing fruit, it achieves two goals with one effort.
An international research team led by Switzerland's Zurich University announced this research finding on June 6 (local time) at the Goldschmidt Conference, the world's largest geochemistry conference, held in Prague, Czech Republic. Kenya's Nairobi University of Technology and the U.S. Lawrence Berkeley National Laboratory also participated in this study.
The research team found that three species of fig trees in the Samburu region of Kenya store carbon dioxide in the form of calcium carbonate (CaCO₃) in the soil. Calcium carbonate is the main component of limestone. Stalactites seen in limestone caves are formed when calcium carbonate dissolved in groundwater precipitates and grows like icicles as it falls from the ceiling.
The process by which fig trees store carbon dioxide involves a microbial 'oxalic-acid-carbonate pathway.' When the tree absorbs carbon dioxide and converts it into fine crystals of calcium oxalate for storage, microorganisms such as bacteria or fungi decompose the crystals and convert them into calcium carbonate.
The research team stated that fig trees are better suited for fixing greenhouse gases than other plants. The organic carbon produced through photosynthesis returns to the atmosphere when the leaves and stems decay, but inorganic carbon, like that in calcium carbonate, can remain in the ground for a long time.
Calcium carbonate was generated not only on the surface of the fig tree but also deep within its stems. The research team explained, "This means that microorganisms penetrate into the tree's interior and convert carbon dioxide into calcium carbonate, demonstrating that much deeper levels of carbon fixation are possible compared to the transfer method."
Among the fig trees investigated this time, the Ficus wakefieldii species was the most effective at storing carbon. Indeed, small calcium carbonate crystals were found in the branches, the interior of the stems, and the surrounding soil.
Mike Rowley, a researcher at Switzerland's Zurich University who led the study, noted, "While we knew that the oxalic-acid-carbonate pathway existed in trees, its potential as a means of carbon storage has been underestimated. Moving forward, choosing trees that can store inorganic carbon while producing food could be key to a carbon-neutral strategy."
Until now, the oxalic-acid-carbonate pathway has primarily been studied in non-edible trees that grow in tropical forests. African iroko wood (Milicia excelsa), which is used for timber, has been reported to store up to 1 ton of calcium carbonate in the soil over its lifetime. The research team stated that the recently discovered fig trees can satisfy both food production and climate response.
The research team mentioned, "We plan to investigate the water usage, fruit yield, and carbon storage capacity of the fig trees to see if they can be utilized in actual agricultural sectors, and we expect there will be many more edible trees that can produce calcium carbonate."