What Is the Soil Carbon Pool?

This article was adapted from “Assessing Soil Health: Soil Carbon Cycling and Storage” first published on Nov 3, 2020 in Crops & Soils magazine. 

The short answer: A “carbon pool” is any part of the climate system with the capacity to store, accumulate, or release carbon, according to the European Union. The soil carbon pool includes all the carbon in the soil, but the size of the soil carbon pool can be changed depending on management.

The soil carbon pool is important because it contains more carbon than the plant and atmospheric carbon pools combined.

Break it down: Soil microbes (bacteria and fungi), plant matter, decaying animal tissue, and fecal material all contribute to the soil carbon pool. This pool is constantly changing as new carbon is added. Some carbon is converted by soil microbes back to carbon dioxide or partially decomposed into microbial waste products that remain in the soil.

• You can divide up the soil carbon pool based on chemical characteristics or biological availability. For example, some of the soil carbon can be extracted by water and some will react with enzymes produced by microbes in the soil.
• These different ways to analyze the types of carbon in the soil can provide insight into how soils might be changing that might not be as easily detected by analyzing total soil carbon. You can read more about measurements and methods for understanding total soil carbon here.

Why does it matter? Measurements of total soil organic carbon and soil organic matter quantify the entire soil carbon pool, but other carbon measurements can potentially indicate how soils will respond to management. These other carbon indicators may be able to give growers a more complete understanding of carbon cycling in their fields by showing the soil’s response to adoption of soil health practices (see table 1, from Liptzin et al, 2022).

Table 1. Significant responses of soil carbon storage and cycling indicators to management from the data set in the North American Project to Evaluate Soil Health Measurements. “Yes” means that the indicator changed significantly (α = .05) as a result of the soil health treatment listed.

Two common soil health tests are used to quantify distinct types of carbon in the soil carbon pool, and both are reported in milligram of C per kilogram of soil.

• Permanganate oxidizable C (POXC) is the fraction of soil organic carbon that is oxidizable by a weak potassium permanganate solution.
• Water-extractable organic C is the fraction of soil organic carbon extractable by water.

These measurements have been suggested to reflect the carbon compounds that are available to microbes. Microbes are the engine that drives carbon sequestration—increasing the amount of food available to microbes can, over time, increase soil carbon.

Other common measurements of soil carbon cycling include tests for carbon mineralization potential (or soil respiration measured in the laboratory) and Beta (β) Glucosidase enzyme activity.

• Carbon mineralization potential measures the amount of carbon dioxide given off by a sample of dried soil which is rewetted, then incubated (for 24 hours in the Haney test, or 96 hours in the Cornell test). Laboratories document the amount of COC per kilogram of soil per day emitted by the sample. This indicates the pool of carbon that is readily respired by the microbes and is correlated with microbial biomass.
• The Beta (β) Glucosidase test measures the soil enzyme activity responsible for the decomposition of plant cell walls, given in milligrams of p-nitrophenol (pNP) per kilogram of soil per hour. The activity of this enzyme is thought to be representative of overall microbial activity.

So, what does it all mean? The Soil Health Institute has been conducting a long-term, wide-scale study: The North American Project to Evaluate Soil Health Measurements. Based on the results of this study and the responsiveness of soil carbon storage and cycling indicators to management, SHI recommends measuring soil organic carbon concentration and carbon mineralization potential get a sense of carbon cycling in response to management.

Photo by Vitor Favoretto.