Human Survival Depends on Plant Roots
How plant-driven chemical weathering balances the carbon in soils, oceans, and the atmosphere
On the way to his office, Kent Keller, a geologist at Washington State University, must drive across a stretch of highway that is “hell on the suspension of every car that goes across it.” The stretch is lined with sweet-smelling ponderosa pines. Their roots have grown under the highway and begun to break it up from below.
As they grow, plants physically weather soil around their roots and the things on top of it, such as highways. They also weather it through chemical reactions. Chemical weathering impacts the availability of many nutrients in the soil, but its influence on carbon is especially important. It changes where carbon is and the forms it is in. In doing so, it maintains the conditions humans need to survive.
Chemical weathering can begin two ways. Through photosynthesis, plants convert carbon dioxide, water, and light into sugars and oxygen, removing carbon dioxide from the atmosphere in the process. Plants then convert those sugars and oxygen to carbon dioxide, water, and energy through cellular respiration. They release this carbon dioxide into the soil through their roots. Thanks to cellular respiration, carbon dioxide concentrations can be ten to 100 times higher in the soil than in the atmosphere. Without rooted plants, much of that carbon dioxide would instead be in the atmosphere. When roots and the microorganisms that live in and around them release carbon dioxide, it dissolves in the soil water and becomes an acid, which breaks down the soil. “If they didn’t do that, we wouldn’t be here,” says Keller, who studies chemical weathering.
Chemical weathering also happens because plants need nutrients, such as calcium and iron, to grow. Nutrients in the soil are often bound to other particles. To access them, plants release organic acids that free the nutrients. They use the nutrients and leave the rest, including forms of carbon. In both pathways, the carbon that was previously bound to other particles is freed to be transported to other places.
Wherever the carbon is, it will end up in the ocean. Plankton, corals, and other marine organisms use calcium carbonate, a bi-product of chemical weathering, to create their skeletons and shells. When they die, some of that calcium carbonate gets buried in the seafloor. The process described so far removes carbon dioxide from the atmosphere and releases it to the seafloor. If this process went unchecked for about 10,000 years, there would be no carbon dioxide left in the atmosphere.
However, there is a check. After humans, the main source of carbon dioxide to the atmosphere is volcanoes. When two tectonic plates collide, one slides under the other, taking the calcium carbonate from skeletons and shells with it. When a volcano erupts, that is burned and released above the Earth’s crust as carbon dioxide. Chemical weathering removes carbon dioxide from the atmosphere, and volcanoes release it back.
Humans, however, are changing how chemical weathering works. We are replacing complex ecosystems, such as prairies full of deep-rooted golden grasses and bright flowering plants, with rows of identical shallow-rooted crops like corn and soybeans. These crops do not release carbon dioxide as deep into the soil, and they’re harvested all at once, leaving little carbon dioxide in the soil post-harvest, which could slow chemical weathering down.
Chemical weathering works well over long time scales. Humans, however, are burning fossil fuels in the blink of a geologist’s eye. As atmospheric carbon dioxide rises rapidly, it is difficult to predict how chemical weathering by plants and the balance of carbon will change. Regardless, it will not be enough to stabilize atmospheric carbon dioxide in the time humans need it to. The good news is, in thousands of years, Earth’s carbon balance will be restored. Whether humans are there to witness it is a matter of how we change our carbon emissions.
Information provided by Washington State University graduate student Summer Lockhart and professor Kent Keller
