Breaking down active soil carbon with Dr. Jennifer Wells

What is active soil carbon and what does it mean for soil health and the carbon market?

What is active soil carbon?
Active soil carbon, also known as permanganate-oxidizable carbon (POXC) is the portion of carbon in the soil that is actively decomposing.  It is important because it is the food source for microbial activity and releases nutrients into the soil. Active soil carbon contains nutrients that are easy for microbes to digest and use for their metabolism. Higher active carbon scores indicate a trend toward more organic matter building up in the soil through biological activity.^1,3

How is it different from total organic carbon?
Total organic carbon is the carbon that is stored in soil organic matter.  It enters the soil through the decomposition of plant and animal residues, root exudates, living and dead microorganisms, and soil biota.  Soil organic matter is made up of approximately 58% total organic carbon.  Active soil carbon is only a small part, making up about 1 to 4% of total organic carbon.  You can think of active carbon as the portion of organic carbon that is acting as the food source for the microbes that are decomposing plant and animal material into soil carbon.²

What does it mean for soil health?
Studies have shown that active carbon is a leading indicator of soil health.  The active carbon soil test is a quick and repeatable test that is scalable for production agriculture.  Due to its role in providing available food and energy sources for the soil microbial community, active carbon can be positively correlated with better soil health through increased organic matter, biological, physical and chemical properties.  Studies have shown that changes in active carbon, due to a practice change, can be detected in a much shorter period of time (often years sooner) than changes in organic matter.  Therefore, monitoring changes in active carbon year over year can be particularly useful to farmers who are making a practice change (such as no-till or cover crops) with the goal of building up soil organic matter and carbon.³
 
What does it mean for the carbon market and carbon payments?
Carbon markets offer payments to growers based on the addition of (or positive change) in total organic carbon made by converting from old practices to new practices, such as no-till and cover crops. Payments are not made on active carbon or total organic carbon currently in the field.  While evaluating your active soil carbon tests year over year can give you an indication of how your field is responding to a particular practice change, it does not directly measure the fraction of carbon that the carbon payments are based upon. Measuring this change in carbon is very complex and utilizes both organic carbon soil tests, bulk density soil tests and soil and environmental models. The bottom line is that while active carbon tests are a great tool to help growers navigate their soil health journey, it is not the current measure to calculate carbon payments.

For more information about the Truterra carbon program, visit here.

Sources
¹Weil, R.R., K.R. Islam, M.A. Stine, J.B. Gruver, and S.E. Samson-Liebig. 2003. Estimating active carbon for soil quality assessment: A simplified method for laboratory and field use. Am. J. Altern. Agric. 18(1):3–17.
²https://www.nrcs.usda.gov/Internet/FSE_DOCUMENTS/nrcs143_019177.pdf
³https://cpb-us-e1.wpmucdn.com/blogs.cornell.edu/dist/f/5772/files/2016/12/CASH-Soil-Health-Series-Fact-Sheets-One-through-Sixteen-plus-cover-072717-1-2m3lquo.pdf