Soil Carbon

glomalin and soil c

Prairie et al. (2023) “Restoring particulate and mineral-associated organic carbon through regenerative agriculture” Macroaggregation sequesters more carbon. ( ( Labile organic carbon (LOC), particulate organic carbon (POC), microbial biomass carbon (MBC), and dissolved organic carbon (DOC) are active SOC fractions that are easily mineralized, sensitive to vegetation succession, and affect nutrient supply (Ramírez et al., 2020, Rocci et al., 2021). Moreover, the changes in active SOC fractions can reflect the practices of tillage and land management, making them an important indicator for assessing the quality of SOC pools quality (Nie et al., 2019). In contrast, recalcitrant organic carbon (ROC) and mineral associated organic carbon (MAOC) are stable SOC fractions (Lavallee et al., 2020). Because of its stability, stable SOC fractions can reside in the soil for thousands of years, strongly affecting the terrestrial C sink (Xiang et al., 2022).

Since tillage-based farming began, most agricultural soils have lost 30% to 75% of their soil organic carbon (SOC), with industrial agriculture accelerating these losses (Delgado et al. 2011).

By converting land for agricultural use over recent millennia, but especially over the past 200 years, humanity has consumed large amounts of SOM by accelerating its rates of mineralization and erosion over those of organic matter inputs into the soil and soil formation, resulting in a global estimated loss of 133 Pg carbon (C) from the top 2 m of soils (Sanderman et al., 2017) (Cotrufo and Lavallee, 2022)

In the end, roots help plants transfer carbon in all three states of matter: liquid (plant exudates in solution), solid (plant root material) and gas (such as CO2 or CH4). All three states have an impact on the ability for us to sequester carbon in our soils by targeting the various soil carbon fractions.

Disposition of 100 g of organic carbon in residues one year after they were incorporated into the soil. More than two thirds of the carbon has been oxidized to CO‍‍2, and less than one-third remains in the soil—some in the cells of soil organisms, some in forms still readily accessible for further breakdown, but a larger component as relatively stabilized soil humus. The amount converted to CO‍‍2 is generally greater for aboveground residues (litter) than for belowground (root) residues. Courtesy of Brady and Weil (2017).

Carbon Cycle

If you couldn’t already tell, getting living roots in the soil is the foundation of regenerating diversity in the soil and promoting succession of the landscape. Soil microbes (and every living organism on the planet) rely on plants turning useless sunlight energy and carbon dioxide into useful building material and energy. So the more carbon and energy captured by plants and pumped into the soil, the more soil microbes can improve its structure and cause the land to produce more than it did in the past. Another way in which the land becomes more productive and succession rolls on is through the mixing of all the emitted carbon dioxide and water. All microbes are respiring carbon dioxide which mixes with water to make a weak acid called carbonic acid. This acid slowly mines out nutrients from soil minerals, so the more microbes respiring, the more nutrients become available. CO2 levels in the air pockets in soil are very high (3,000-10,000 ppm!) and a lot of it floats out of the soil profile where it can be vacuumed in by growing plants and land becomes more productive over time. More nutrients mined out of the rocks, more water, more energy in the total pools available for plant growth.

Carbon Sequestration

There’s an important lesson to emphasize at this point that often gets forgotten in most conversations about carbon sequestration. The lesson is that the bucket of usable carbon in the soil slowly builds up over time despite the fact that carbon is extremely dynamic and never stops moving. When microbes consume carbon-rich material as food, they respire most of the carbon back out as carbon dioxide because it’s a natural by-product of metabolism. We do the very same thing with every exhale. This means a small percentage of carbon gets incorporated into their bodies, into quickly consumed molecules or into stable humus compounds. Check out the image below from the Nature and Properties of Soils textbook that illustrates this point.


The point is that as more C is pumped into the system and microbes respire the vast majority of C, one would think no C would ever be gained. That’s where good soil structure and aggregation comes into play and why it’s so important. Broken down plant material and other dead organism material gets physically hidden in places smaller than even bacteria can reach. Some also gets chemically protected, so over time a little C gets socked away with each cycling of C and it slowly accumulates over time. ( This slowly increases OM in the soil and all of the benefits that that brings. “The results showed higher the application rates for each organic amendment, higher the CO2 emissions from the soil.” “Organic matter buffers the soil from chemical, physical, and biological changes.” Higher organic matter soil and systems farther along the succession path are more resilient to external conditions.