Photo :EPSRC
In this Canadian prairie soil, a brown organic layer overlies gravel containing natural calcium-bearing carbonates, which could aid the fight against global warming.

Three mounds of soil, covered in wild grasses in an abandoned corner of the quarry. Apparently, nothing could be more ordinary. Unless you are David Manning, professor of geology at Newcastle University (United Kingdom), for whom they were the source of a very big surprise. These tumuli concretize an experiment that the scientist has been conducting since 2002 in this quarry for road works materials.

    There, David Manning tested a mixture of compost and quarry dust in order to evaluate whether it would make a good ground cover after the quarry's closing. In 2007, when he returned to the site, he observed that crystals of calcium carbonate - i.e., chalk - had formed in totally unexpected proportions. A subsequent analysis showed that the carbonates came from the plants and not from the stone.

    That observation led the geologist to think that there was a completely new way of absorbing carbon gas: stimulation of plants' natural carbon fixation process. We know that plants absorb CO2 through photosynthesis. But they secrete part of it in the form of organic acid. Why? "It's the result of stress," explains David Manning. "When they lack nourishment, they release an acid that dissolves the underlying stone and frees nutritive elements, such as phosphorus."

    In most soils, that carbon returns to the atmosphere. But in calcium-rich soil, the acid, which contains carbon, reacts with the calcium by forming calcium carbonate deposits around the roots. Isotopic analyses have shown that this carbon-trapping is significant: it could reach 150 kg [330 lbs.] per year per hectare in a field of wheat.

    The carbonates in the soil remain stable for very long periods and consequently constitute a virtually permanent geological carbon sink. "They could be used passively, the same way reed beds are used in lagoons to grab heavy metals out of polluted water," David Manning asserts.

    By enriching soils with calcium, we could, in fact, stimulate this process. Such calcium could come from volcanic rock quarries, which produce great quantities of it through the dust they generate. Demolition sites could also constitute another source of calcium, as could steelmaking.

    A Computer Model

    According to David Manning, the 2.5 million hectares of wheat cultivated in England could, in this way, absorb 14 million tons of CO2, or close to 3 percent of the country's emissions. But carbon could also be sunk on restored quarry or construction sites planted with vegetation.

    The validity of the method still has to be verified by a network of laboratories. The researchers will make up artificial soils, highly enriched with calcium, and grow wheat, lupin and sedum in order to measure the amount of carbon fixed. That will allow a computer model to be developed that will define the speed and the quantities of calcium carbonate formation in soils with different compositions. "This method of fighting climate change could be almost painless," David Manning believes.

    This experimentation is highly illustrative of the new interest soil arouses as climatologists discover it is a major actor in the carbon cycle. The planet's soils contain more carbon gas that the atmosphere and terrestrial vegetation: 1,500 billion tons in organic soils and 720 billion tons in carbonated soils, versus 500 billion tons in all vegetation.