Soil Carbon Sequestration
Soil sequestration involves the management of land to enhance the passage of carbon dioxide through plants and into soils and the subsequent storage and protection of that carbon in the soil. The potential of this method is driven by the vast storage capacity of soils.
Introduction
Changing our agricultural practices to recapture CO₂ in soil is an essential part of green transition. With limited land and an increasing need for agriculture due to rising populations, changing practices to make this land usable for climate mitigation is a win. Alongside this, methods of soil sequestration, like regenerative agriculture, are scalable and can be implemented all over the world today due to the vast history of knowledge surrounding this practice.
This important and adaptable method is a climate solution just beneath our feet and applicable to the food we eat every day!
The Steps
The diagram below displays how CO₂ is transported from the atmosphere, through plants, and into the soil.
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CO₂ Absorption
Carbon dioxide is absorbed through small openings, called stomata, on the surface of a plant’s leaves.
Photosynthesis
Using energy from the sun, a green pigment (chlorophyll) in the leaves converts water and CO₂ to oxygen and carbon-rich carbohydrates.
Carbon Exudation
The carbohydrates are transported down into the roots in the phloem tissue and are released into the surrounding soil. This is root exudation.
Carbon Storage
The carbon-containing root exudates and plant biomass can be directly stored within soil aggregates and/or on soil particle surfaces or they can be taken up and processed by soil microorganisms first.
How it works
Soil Carbon Incorporation
Living plants release 10-40% of their photosynthetically fixed carbon into the soil around their roots in the form of exudates. Alternatively, carbon is introduced to the soil as necromass when the plant dies and decomposes. The exudates and the necromass act as food substrates for the soil microorganisms, which in turn become another source of soil organic carbon (SOC) when they die. This SOC generally makes up a small fraction (0.5-5%) of the content of agricultural soils, but it plays a disproportionately important role in the fight against climate change.
Carbon Storage
The incorporation of carbon into the soil is not good enough, it also has to be securely stored, as soils can release CO₂ back to the atmosphere during respiration. Carbon is only securely stored once it is incorporated into soil aggregates or absorbed onto the surfaces of soil particles (sand, silt, and clay). Soil aggregates are simply made of soil particles clumped together and bound by clay, fine roots, and exudates. They create tiny pores in which the carbon-containing material is safe from microbes and oxygen, so it does not decompose and release CO₂.
Soil Sequestration & Regenerative Agriculture
A primary goal of regenerative agriculture is to increase SOC levels and maintain them. One of the movement’s main principles is to keep the ground covered year-round, for example, with cover crops. This also ensures that there are living roots present at all times, which increases the transfer of CO₂ from the atmosphere into stable forms of carbon in the soil. Another foundational principle is to minimise soil disturbance (for example, by reducing or completely avoiding tilling, especially ploughing) to protect the soil aggregates and microorganisms.
The climate change solution right beneath our feet
As there are some issues regarding the carbon permanence of soil sequestration, the positive impact from its co-benefits will likely outstrip its climate impact in the future. However, very few carbon dioxide removal (CDR) technologies are ready for deployment at the scale that soil sequestration is, making it one of the most feasible solutions in our toolbelt right now. Soil sequestration’s most important role in climate change mitigation is to buy us some time to develop and scale more permanent CDR methods for the future.
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The jury is still out regarding the scale of soil sequestration’s impact. Some parties believe that global CO₂ emissions can be completely counteracted by converting from conventional to organic regenerative farming practices, but a more realistic estimate is that 5-15% of annual global fossil-fuel emissions can be sequestered in soils. Soil sequestration’s climate impact is difficult to define as large questions remain about the permanence of the CO₂ sequestered and measuring SOC is an expensive and time-intensive process.
However, as a lot of the knowledge to implement regenerative farming practices already exists, soil sequestration is a method that can be rapidly implemented and at a massive scale.
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Increasing SOC levels improves many of the ecosystem services (worth up to 6420 USD/ha ) that soils provide. Therefore, it is clear that soil sequestration can have a widespread impact outside the boundaries of climate change mitigation. As SOC levels increase, so too does crop productivity, and therefore soil sequestration contributes to improved food security.
Organic carbon also acts as the ‘spongey’ part of soils, which allows them to regulate water movement and reduce the incidence of catastrophic flooding events. Amazingly, soils are home to about 25% of all species on Earth, and it is the organic carbon that makes soils such a desirable place to be!
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