A few weeks ago several news outlets publicized a new carbon-capture method tested in Iceland, but there’s also a low-tech way of storing carbon in the ground that people can consider, which is restoring degraded lands that once held large amounts of carbon and could become fertile again if we follow certain practices. Stephen Wood reports for Cool Green Science:
Soils have twice as much carbon as the atmosphere. Which means there’s a lot of interest in figuring out if soil can hold even more carbon—to help fight climate change.
Sequestering carbon in soil is like saving money in your bank account—simple in theory, but challenging in practice. If you’re frugal enough you may end up fighting climate change. Spend too much and you could make the situation worse.
Carbon is deposited into the soil in a few steps. First, plants convert carbon dioxide into types of carbon that are used to build cells. Plant carbon then ends up in soil when plants die or release carbon through their roots. Soil microbes—bacteria and fungi—turn this carbon into forms that can be locked away by magnet-like soil particles that can bind certain forms of carbon for decades to millennia.
But microbes also carry out withdrawals when they use carbon for energy, respiring some to the atmosphere and returning some to the vault when their cells die. Since the amount of carbon held in soil is a balance of deposits and withdrawals, building up soil carbon requires both adding it to the soil and reducing its loss to the atmosphere.
The Soil Carbon Equation
To limit global warming to 2 degrees celsius, we will need to limit emissions to no more than 565 billion metric tonnes (gigatons) of carbon dioxide. Because carbon dioxide isn’t all carbon, that comes to about 100 gigatons of carbon. Soils currently contain 15 times that much carbon. And that carbon can easily be lost, which means that soils can easily worsen the climate change problem.
Up to half of soil carbon can be lost when humans convert ecosystems—from, say, forest to agriculture. Because soils currently contain twice as much carbon as the atmosphere, extensive conversion of ecosystems could lead to drastic increases in the amount of carbon dioxide. Keeping soil carbon below ground by halting land conversion is potentially the most important way to use soils to avoid climate change.
In addition to avoiding losses, soils can build up—or sequester—more carbon. But the analogy to a bank account is not perfect, and this is where it breaks down.
If you add to your bank account without spending, you can continue to build up your assets. Soils, however, can only hold a certain amount of carbon, which is limited by the physical properties of the soil, such as the amount and type of clay minerals that are present.
Like a Glass Holding Water
Clays have a strong capacity to bind carbon—the more clay the more carbon soils can hold. In this sense, a soil’s ability to sequester carbon is less like a bank account and more like a glass holding water: you can add as much water to the glass as you like, but you can only hold onto the volume of water that matches the physical size of the glass.
Soils also differ in their sequestration potential because some glasses are emptier than others. Soils with low levels of carbon have more to gain than soils that are closer to their physical holding capacity. Because of this, restoring degraded lands—lands that have lost large amounts of carbon—should be priorities for building up carbon.