Just what is soil carbon?

Maia Welbel

Carbon on Earth

Carbon atoms are the building blocks of life. Because of its ability to form complex, stable bonds with itself and other elements, carbon can be the basis of an almost unlimited range of molecules. All living things contain carbon in some form, and it is the primary component of macromolecules, including proteins, lipids, nucleic acids, and carbohydrates. It is also the fourth most abundant element in the universe.

Like water, carbon is always cycling in various forms through the living and non-living parts of the environment. The carbon cycle refers to the element’s circulation through land, ocean, and atmosphere, with transitions occurring in a vast range of timescales. While some forms of carbon are transferred via respiration and photosynthesis in the span of hours or days, others can be stored for hundreds of thousands of years.

The carbon cycle is constantly moving towards equilibrium — a mutual exchange of CO2 between the atmosphere and storage reservoirs that keeps biological processes viable on this planet. But humans have disrupted this balance by releasing more carbon into the atmosphere than can be naturally reabsorbed. CO2 is a greenhouse gas, which means an excess of atmospheric carbon traps heat, leading to exponential upset in global climate patterns.

Where is carbon stored?

A carbon sink is the accumulation and storage of carbon dioxide from the atmosphere into any natural or artificially made reserve including soil, oceans, and forests. The majority of Earth’s carbon exists in the ocean. But on land, soils account for about 75 percent of carbon storage. This ecosystem service is one reason it’s imperative that we work to protect and repair global soils.

Soil organic matter (SOM) is the primary keeper of soil carbon. This includes decomposing plant and animal tissue, microbes, and soil minerals. Some carbon remains stable there for millennia, and some is released back into the atmosphere shortly after it lands on the ground. Soils that support healthy, productive ecosystems contain large quantities of the latter type. It is also this enduring mode of storage that can be employed as a climate mitigation strategy.

In short, getting more carbon into the soil and keeping it there for the long haul is in the best interest of us humans. There are many ways to make this happen, and a diversity of solutions must be adopted to do so sustainably.

Supporting Soil Carbon Sinks

Regenerative agriculture, a system of farming practices that replenishes the surrounding ecosystem and its inhabitants, inherently facilitates carbon sequestration. By prioritizing soil health, minimizing chemical inputs, and increasing biodiversity, regenerative farmers drastically increase their land’s potential for storing CO2.

Conservation tillage means limiting how much farmers turn over their soil while growing crops. The rapid release of carbon from soil organic matter occurs mainly at the very top layer (or O horizon), so preventing the deeper layers from being displaced allows their carbon to stay intact.

Cover cropping is the cultivation of plants (including native species, legumes, small grains, etc.) between cash crop seasons. This reduces erosion by buffering the soil from wind, enhancing soil structure, and introducing soil nutrients and organic matter. All of these effects help keep carbon sequestered underground.

Intercropping + crop rotation involves growing a diversity of crops (either at once or in succession, respectively) on the same area of land. This helps keep soil nutrients balanced and abundant, preventing depletion of the soil ecosystem from repeated planting of a single plant species, which eventually leads to erosion and carbon release.

Forest conservation and reforestation are crucial to carbon reabsorption. Trees and other plants can sequester massive amounts of CO2, incorporating it into their biomass and eventually depositing much of it into the soil. Researchers cite global tree restoration as one of the most effective carbon drawdown solutions we have to date. But like all climate strategies, it’s not as simple as planting legions of new trees.

Emerging research shows promise in the sequestration potential of enhanced rock weathering.  When silicate rocks (the most abundant rock type on earth) are exposed to water, heat, or acidity and begin to break down, a chemical reaction occurs which removes carbon dioxide from the atmosphere and stores it as carbonate minerals. We can speed up this reaction, thereby catalyzing carbon drawdown, by grinding the rock into small particles and distributing it strategically on soils, bodies of water, and other carbon sink environments.

We know that soils all over the world have been compromised by human exploitation (see: threat of a modern day Dust Bowl), but by supporting soil’s natural role in the carbon cycle, we could nudge the process back towards balance. More carbon in the soil leads to more vibrant ecosystems, which leads to more carbon sequestration, and so on.