Direct measurements can reduce uncertainty in soil carbon credit markets

The researchers also found that using appropriate study designs applied to the real-world scale of agriculture allows for feasible verification of how much carbon is being stored in soil through climate-smart practices, including growing crops to cover soil and not disturbing soil through tillage. The direct measurement method, when coupled with study designs that are common to fields such as epidemiology, can also be used for quantifying how climate-smart practices affect soil health and other desired outcomes, the authors noted.

"The study suggests that we can do direct measurements at scale, taking its application beyond carbon markets to other greenhouse gas accounting efforts like those conducted by countries to report their national emissions," said co-author Mark Bradford, the E.H. Harriman Professor of Soils and Ecosystem Ecology, who was involved in the work through the Yale Applied Science Synthesis Program, an initiative of the Yale Center for Natural Carbon Capture and The Forest School at YSE.

Natural carbon solutions, which include managing croplands to enhance carbon storage, is considered essential by the Intergovernmental Panel on Climate Change (IPCC) to help mitigate the impacts of the climate crisis, Bradford noted. Soil organic carbon (SOC) accounting and crediting primarily uses measure-and-model approaches, relying on predictive biogeochemical models informed by small field trials and limited direct measurement, but their real-world accuracy for predicting outcomes on commercial farms is uncertain, he said. Instead of relying on predictions from models, the researchers found that a "measure and remeasure" approach using soil samples collected across hundreds of fields can provide reliable evidence of how much carbon is being stored.

Measuring changes in the amount of carbon stored in soils accurately is challenging, Bradford said. Soil carbon changes slowly against a large background stock, meaning that measuring change requires collecting and analyzing many samples. At small scales, this method has long been considered too costly.

The research team found that sampling 10% of fields across many farms -- up to tens of thousands of acres -- over longer time frames can provide reliable data. By directly measuring and remeasuring soil carbon using causal study designs, the credits sold are much more likely to reflect actual carbon storage, helping buyers feel confident their money is supporting real climate benefits and, at the same time, shrinking costs as projects are scaled up, the authors conclude. The approach also could be useful for validating the suitability of predictive models currently used for other cropland greenhouse gas accounting purposes, such as the internal accounting that companies do to meet their net-zero goals, they added.

To help farmers calculate the cost-benefit of soil management projects, Eric Potash, a research scientist with the Agroecosystem Sustainability Center (ASC) at the University of Illinois who led the study, developed an open-source web app that allows users to explore the costs and profitability of soil carbon projects based on specific parameters: project size, duration, analysis costs, and sampling strategy.

"This study suggests that people may be able to reliably quantify how much soil carbon is changing due to adoption of climate-smart agricultural and regenerative practices," Bradford said. "If you can address the measurement and verification concerns around how soil carbon stocks are actually responding, it will help prioritize policies and investments that achieve soil restoration and protection, leading to improved water and nutrient retention in soils, aeration, and soil biodiversity. Such healthy soils will be more resilient to extreme weather and build food security."

The study was also co-authored by Emily Oldfield '05, '11 MESc, '19 PhD, a soil scientist at the Environmental Defense Fund, and Kaiyu Guan, director of ASC.