How Deep Is Organic Carbon Stored in the Soil?

This article is adapted from “Soil organic carbon sequestration calculated from depth distribution” by Alan J. Franzluebbers, first published in the Soil Science Society of America Journal on March 2, 2021.

The short answer: Most organic carbon is stored in the upper 30 cm (1 foot) of soil because the surface is where biological activity is most active. Soils do contain carbon below this depth, but deeper carbon is generally of inorganic nature in aridic soils.

Organic carbon enters the soil ecosystem though plants and biota, with the greatest activity and carbon storage in the top 30 cm of the profile. Illustration by Marie Johnston.

Break it down: Soil organic carbon is typically found in the surface layers, or horizons, because this is where the soil receives the largest amounts of organic matter.

• Land management practices that supply plant and animal residues to the soil surface with minimal soil disruption can be effective at building soil organic carbon relative to “baseline” soil carbon located deeper in the profile.

First, the surface – Increases in soil organic carbon will first occur nearest the surface.

• This means that an impactful change in land use may require decades for soil organic carbon levels to increase within the upper 30 cm.
• This also means that if the site has a history of carbon accrual, then the surface may already have high levels of organic carbon that could be difficult to increase further.

Then, at depth – Organic carbon may be stored below the top foot of soil, but the processes of accrual and storage require even more time than near the surface.

• The effects of land management on soil organic carbon stored at depths greater than 30 cm are likely to be extremely small.
• It is estimated that measurable changes in organic carbon deeper than 30 cm occur over centuries.

In short: Organic carbon accrual and storage in soil takes time, but differences due to land use might be detected by comparing soil carbon near the surface to what is stored at depth. Focus on promoting soil organic carbon storage in the upper 30 cm, and these impacts may slowly lead to changes at depth.

Photo by Tracy Robillard, NRCS.