Lowering the amount of carbon dioxide in the atmosphere requires not only reducing emissions, we also need to capture and store the excess carbon that has already been emitted. In a perspective paper published September 21 in the journal Trends in Plant Science, a team of plant scientists suggests that arid lands such as deserts may be one answer to the carbon capture problem. The researchers propose using dry lands combined with specific plants and soils to create efficient carbon capture systems, providing a solution that does not compete with agricultural land.
The annual growth rate of the atmospheric carbon pool (blue arrow) is the difference between fossil fuel emissions (96 billion tonnes of carbon), land use change (12 billion tonnes of carbon), and the absorption of carbon by terrestrial (31 billion tonnes of carbon) and oceanic (29 billion tonnes of carbon) carbon pools. Only terrestrial carbon fluxes are shown here. Image source: Trends in Plant Science, Hirtetal.
The authors believe we can transform arid ecosystems into efficient carbon capture systems with improved soil health, increased photosynthetic efficiency, and increased root biomass by engineering the ideal combination of plants, soil microbes, and soil types to promote a naturally occurring biogeochemical process called the oxalate-carbonate pathway to create a subsurface carbon sink.
"Regreening deserts by restoring ecosystem functions, including carbon sequestration, should be the preferred approach," wrote the team, led by senior author Heribert Hitt, a plant scientist at King Abdullah University of Science and Technology. "The advantage of reclaiming arid areas for regreening and carbon sequestration is that they do not compete with land used for agriculture and food production."
The role of oxalate in carbon sequestration
This method takes advantage of drought-adapted plants' ability to produce oxalate—an ion containing carbon and oxygen that you may be familiar with if you're unfortunate enough to have kidney stones or gout. Some soil microorganisms use oxalate as their sole source of carbon and excrete carbonate molecules into the soil. Carbonates usually break down quickly, but if these plant-microbe systems are grown in alkaline and calcium-rich soil, the carbonates react with the calcium to form stable calcium carbonate precipitates.
Carbon naturally cycles between the atmosphere, oceans and terrestrial ecosystems, but human actions have led to the accumulation of excess carbon dioxide in the atmosphere. "...even if we are able to reduce CO2 emissions, the climate impacts of rising CO2 will remain irreversible for at least 1,000 years unless CO2 can be sequestered from the atmosphere," the researchers wrote.
Carbon capture from dry land versus trees
Trees are considered an ideal system for carbon capture, but afforestation directly competes with agriculture for arable land. In contrast, drylands cover about one-third of the land area but are not used for agriculture.
Currently, arid ecosystems support few plants, with water scarcity being the biggest limiting factor. However, some plants have adapted to life in drought by evolving different mechanisms to cope with water shortages and extreme temperatures. Some plants adapted to arid environments have specialized root systems that dig deep into the soil to tap hidden sources of water, while others use different forms of photosynthesis to minimize water loss during the hottest parts of the day. There are also plants, so-called "oxalate" plants, that produce large amounts of oxalate, which can be converted into water during times of drought. When oxalate plants grow under certain conditions, some of the carbon in these oxalates will be deposited underground as carbon deposits, and the authors hope to exploit this mechanism for carbon sequestration.
"Overall, in this form of carbon fixation, one in every sixteen photosynthetically fixed carbon atoms may be fixed into the carbonate," the authors write.
Expanding this naturally occurring biogeochemical process in arid regions could transform these currently unproductive and degraded ecosystems into carbon sinks, making soils and plants healthier, the authors say. They recommend starting with "fertility islands," small patches of regreened habitat from which plants and microorganisms can spread, forming a carpet of vegetation.
The authors estimate that these methods could significantly increase the amount of carbon sequestered by plants and soil in less than 10 years. However, they note that the success and speed of the proposed method will depend on the growth rate of the plants (which tend to be very slow under water-scarce conditions) and "... will also depend on the financial and political means to apply this technology in various arid countries".