Challenges in Accurately Measuring Soil Carbon Sequestration

One prominent proposed climate solution is soil carbon (C) sequestration, the process of removing carbon dioxide (CO2) from the atmosphere and storing it as organic C in soil. Focusing on soil C, however, must not come at the expense of increasing emissions of other greenhouse gases. This blog post addresses the scientific difficulties of assessing the impacts of conservation agriculture practices on net greenhouse gas emissions.

At first glance, the total C storage potential in soils is quite high, since nearly 80% (2500 Gt) of C found in terrestrial ecosystems is stored in soils. Estimates of historical soil C losses due to agriculture range between 42 and 133 Gt C, two-thirds of which is theoretically able to be recovered.

However, the maximum biophysical potential for soil C sequestration is realistically much lower than these values due to the dynamics of soil C stocks, which tend toward an equilibrium state and thus usually change only temporarily. Estimates of global soil C sequestration potential in agricultural soils range from 0.4 to 5.5 Gt CO₂ per year and are generally agreed to last from two to three decades before decreasing significantly as soil C levels approach a new equilibrium.

These approximations assume near-perfect adoption of best management practices, ignoring numerous social, economic, and environmental constraints. Framing the adoption of important conservation agriculture practices as reliant on C sequestration benefits thus risks ignoring the holistic importance of these practices while failing to achieve the desired climate mitigation.

Two agricultural practices viewed as having a large potential impact on greenhouse gas emissions are cover cropping and conservation tillage. Cover cropping is the practice of planting a non-commodity crop along with or ahead of a commodity crop with the purpose of improving soil health. Conservation tillage refers to a suite of practices meant to preserve crop residue on the soil surface, reducing soil disturbance, and thus, soil erosion and degradation. These two conservation practices are being promoted on a global scale as climate mitigation strategies.

There are numerous obstacles to effective research on soil C sequestration, however:

• Accounting for other greenhouse gas emissions. Cover cropping and conservation tillage increase emissions of non-CO2 greenhouse gases, with one study finding that alternative management in arable soils enhanced nitrous oxide emissions enough to offset 75-310% of sequestered C.
• Assessing long-term change. Even if cover cropping and conservation tillage lead to soil C sequestration in the short term, the permanence of this benefit is suspect, with a recent meta-analysis finding no-till benefits leveled off at zero net sequestration from 0-60cm after fourteen years.
• Incorporating soil C redistribution. Most of the increases in soil C found in studies investigating no-till are a result of the redistribution of existing soil C and not additional sequestration. This error comes from measuring organic C concentrations rather than stocks.
• Measuring soil bulk density. The above redistribution challenge requires measuring soil bulk density using equal soil mass and not equal soil depth—without this step, sequestration estimates can be highly inflated.
• Sampling error. Soil C changes at scales so small that they are often within standard error, and equipment capable of measuring small changes in soil at sufficient depths to avoid redistribution error is often prohibitively expensive.

Addressing these difficulties in soil C sequestration research is critical to providing policymakers with accurate data, allowing them to incentivize agricultural practices that simultaneously promote healthy soils and climate mitigation.

Jack Miklaucic

Student, Penn Carey Law School

Jack Miklaucic is a second-year student at Penn Carey Law School, where he is on the boards of the Student Public Interest Network, Environmental Law Project, and American Constitution Society. He hopes to build a career in public interest environmental and energy law.