Storing carbon in the sand

Deserts worldwide could be important CO2 sinks

By Victoria Schlesinger

If you’ve ever walked through a national or state park in the southwestern United States, you’ve probably noticed the ground cover. Tiny castle-shaped lumps and mini mountain ranges stipple the sandy earth, and signs posted everywhere warn visitors not to trample the soil. Holding the sand in place are biological soil crusts—a ‘skin’ of microscopic cyanobacteria, algae, lichens, mosses, fungi, and bacteria—that can take from decades to centuries to form.

Recently, interest in this thin, organically rich layer has grown, along side increasing concern over climate change. Scientists worldwide are studying how carbon enters and leaves ecosystems, ranging from tropical rain forests and grasslands to glaciers. Long neglected, deserts are now getting a closer look, too. These arid landscapes account for some 30 percent of the earth’s land surface, potentially making them an important player in the globe and atmosphere’s carbon cycle.

“The more we know, the more we can understand CO2 in the atmosphere, and desert ecosystems have been ignored,” says Jay Arnone, an ecologist with the Desert Research Institute in Nevada. Arnone and his colleagues recently published a paper about carbon dioxide uptake in the Mohave Desert, where they tested how much of the gas enters and leaves the ecosystem; they found the in flux to be considerably higher than anticipated.

“It’s a surprising amount of CO2 over two years and why that is could mean a lot,” says Arnone. Other studies have not documented as great an uptake of CO2, but all found higher levels than expected. Arnone says the next step is to determine how the ecosystem is absorbing the gas.

These findings would seem to suggest that deserts naturally sequester a good deal of carbon. But according to one veteran in the field of arid soils, that’s not the case. “If it was a huge sink, we would see it in the soil,” says USGS biologist Jane Belmap, “and there’s just not much there.”

In keeping with the rules of organic chemistry, if the carbon is not in the soil, it must be released or stored in some form, and Belmap says, “It’s hard to imagine what that would be.”

To Belmap it’s a matter of accounting for ingredients. For example, take a bowl of spiked fruit punch. If after tasting, smelling, and examining the punch under a microscope, there’s no sign of alcohol present one has to assume the alcohol evaporated, broke down, or was never added in the first place. For Belmap, carbon is like the alcohol. Since she can’t find it in the soil or explain the disappearance of volumes as great as Arnone’s, she concludes the carbon couldn’t have been absorbed by the desert soils in the first place.

But another group of scientists trying to explain just that—why and how desert soil takes up CO2—has been studying the sands of the Kalahari in Botswana. There, the top millimeters of desert crust soil are chalk full of cyanobacteria, a group of bacteria, also known as blue-green algae, that obtains its energy through photosynthesis.

Cyanobacteria are present in almost all desert crust soils and provide them with several crucial ingredients. Surrounding the bacteria is a gelatinous sheath that once shed acts as a glue holding together grains of sand and organic material to create the crust. This, in turn, holds the soil in place, helping other plant species to take root. Cyanobacteria also fertilize the desiccated soil by fixing carbon and nitrogen. Disrupting soil crusts is like pulling out the foundation beneath a house (hence all those warning signs in parks).

Biologist Andrew Thomas and his colleagues at Manchester Metropolitan University monitored samples of the Kalahari cyanobacteria for two years and found that under certain conditions they are responsible for a rapid uptake of CO2. Sensitive to moisture, cyanobacteria lie dormant during dry periods and then come to life and begin photosynthesizing during light rains.

“Under certain conditions the cyanobacteria are taking CO2 out of atmosphere, but it’s really ephemeral and under very specific conditions,” says Thomas. “We’re trying to pin down what they are. If we better understand them, maybe there’s a way to optimize them.”

As for the release of the CO2 from the soil, Thomas’ team found that during the rainy season, when the moisture reaches a meter deep, “it switches on a different bacteria.”

Heterotrophic organisms living in the lower layers of desert soil begin consuming organic matter and expiring CO2 as waste. Thomas found a net loss of the gas from the soil.

Perhaps scientists will soon be able to explain the mechanisms by which deserts sequester and release carbon, but it seems unlikely that the net difference will be large. Nonetheless, both Arnone and Thomas maintain that even a small change in the balance of an ecosystem that occupies a third of the world’s land mass could have large implications for global CO2 concentrations.