Soil Carbon Sequestration: What Is It And How Does It Work?

Taking care of our soil can help curb global CO2 emissions and mitigate the effects of climate change—this is a bold statement that, while true, is vastly oversimplified. While there is a general consensus that healthy soil is central to sustainable farming, less is known about the practice of using soil organic matter to store carbon as a climate solution. Conversations about the “soil solution” do not always dive into the details of how soil sequesters carbon, leaving room for skepticism about its effectiveness when compared to flashier technological carbon-capture solutions. In this article, we’ll break down how soil absorbs carbon and why it is an amazing natural phenomenon worth getting excited about.

What is Soil Carbon Sequestration?

Also called carbon farming, soil carbon capture is a suite of land management practices that promote soil health and are used in both organic and regenerative agriculture. This includes farming without the use of agrochemicals as well as practices such as cover cropping, controlled grazing, companion farming and limited or no-till agriculture. 

All of these practices have been proven to help promote soil health and fertility by rebuilding soil organic matter. 

Soil organic matter (SOM) supports key soil functions such as stabilizing soil structure, storing nutrients and allowing water infiltration and retention. Soil organic matter is made up of three components: Living organic matter (bugs, earthworms, fungi, microorganisms etc.), dead organic matter (recently dead plant matter, animal matter etc.) and humus (long dead and decomposed matter, which is what gives SOM-rich soil its characteristic dark colour). 

SOM (particularly humus) is responsible for converting atmospheric carbon into soil bonded carbon reserves, so building soil organic matter increases the amount of carbon stored in the soil.

How Does Soil Carbon Sequestration Work?

Carbon is not just a common greenhouse gas, it is also the chemical foundation of life on earth. As a result, most living organisms both draw and release carbon from and into the atmosphere. In our soils, living organisms draw carbon from plants and store it as carbon stocks made up of organic carbon and inorganic carbonate. While organic carbon can be stored in the soil for several decades if left undisturbed, inorganic carbonate can be stored for thousands of years (upwards of 70,000 years to be exact).

The process of soil carbon capture can be broken down into three stages: 

1) Plants take in CO2 from the atmosphere through photosynthesis; 

2) The plants transfer this gathered carbon into plant biomass and store it in their roots;

3) Finally, plants have formed over the centuries a symbiotic relationship with the SOM that resides around their roots. The roots provide the SOM with carbon, which the SOM converts into carbon stock and uses as an energy source, and SOM provides the plant with much-needed nutrients in return.

The more SOM present in the soil, the more nutrients are shared with the plants. Every 1 percent increase of organic soil matter results in the release of 20-30 pounds of nitrogen, 4.5-6.6 pounds of phosphorus, and 2-3 pounds of sulfur per year, along with smaller doses of rare nutrients like potassium and magnesium.  

The SOM also protects the plant from soil-borne diseases and helps to stabilize the soil structure to prevent erosion and improve water filtration.

Globally, more than 95 percent of the food we eat is cultivated on land that is classified as “at-risk” from degradation. This at-risk land lacks the healthy SOM that is needed to draw down and maintain carbon stocks to the soil’s greatest potential.  As a result of this degradation, soil releases carbon into the atmosphere, resulting in the world’s cultivated land having lost between 25 -75 percent of its original carbon stocks.

How Much Carbon Can Soil Sequester?

Soil is the second largest reservoir of carbon on Earth with only the ocean being able to store more. But the question of how much carbon can soil sequester is tricky to answer because soil varies in quality from location to location. There are many factors that can affect the soil’s ability to store carbon, including climate, vegetation, soil composition and time of year. For instance, cool and damp soil may have more SOM activity than hot and dry soil, and in the winter SOM is less active than in the summer.

With these factors in mind, the amount of carbon all global soil stocks could absorb if SOM were improved varies from more conservative estimates (9-23 percent of the total global emissions  from all sectors in 2017) to more optimistic estimates (over 100 percent of all global annual emissions). 

Despite these wide ranges, there is an overwhelming amount of scientific evidence affirming that maintaining current soil carbon stocks and fostering soil carbon sequestration where the potential exists could greatly contribute to mitigating the impacts of climate change. 

Does Soil Release Carbon?

Skeptics of the soil solution mistake the complexity of soil sequestration for futility. Their main argument is that soil also releases carbon, particularly when SOM is oxidized, a process which occurs whenever the soil is tilled. 

Soil carbon sequestration exists as part of a natural cycle of release and drawdown. Current standard farming practices do draw down some carbon but the soil is not absorbing as much CO2 as it could and also serves as a source of emissions. The goal of carbon farming is to tighten up that cycle by maximizing drawdown and limiting release. When this is done correctly, it’s possible to have farms operating with net-negative carbon emissions.

How Can We Improve Soil Carbon Sequestration?

Ten countries hold more than 60 percent of the world’s soil organic carbon stock, and Canada has the second-largest amount of SOC in the world, 12.7 percent to be exact, which puts us in a position to become leaders in the soil solution.  
There are many techniques that can improve soil health and boost SOM, although each farm will often develop unique strategies tailored to their farm’s landscape and climate. Some of these practices include:

• Reducing soil disturbance by switching to low-till or no-till practices
• Returning biomass to the soil after harvesting as mulch instead of burning it
• Planting cover crops instead of leaving fields bare 
• Planting a wide variety of crops
• Integrating trees 
• Greatly reducing or eliminating the use of chemical inputs, replaced with integrated pest management techniques, nutrient management and precision farming.
• Replacing surface flood irrigation with drip, furrow, or sub-irrigation.
• Managed grazing 
• Applying organic compost to fields
• Using biochar to improve soil’s carbon sequestration ability

Final Thoughts

There are many solutions that must be implemented to effectively combat the climate crisis, including reducing emissions, using renewable energies and removing carbon from the atmosphere. Carbon capture and storage technologies are being developed to replicate the natural carbon capture process but these technologies are extremely expensive and their effectiveness may not be sufficient to meet our growing need for carbon capture. Soil carbon sequestration is a natural, viable, cost-effective solution that can be adopted now, and it has great potential to reduce agriculture’s climate impact and contribute to climate solutions. 

Biodiversity loss is as dire a threat to the world as climate change, learn all about how agriculture can help support biodiversity, both above and below the soil in our next article: Biodiversity Loss and Agriculture: Rebuilding What’s Been Lost (Coming Soon)

This knowledge article is part of our Organic Climate Solutions campaign. Check out OCO’s Organic Climate Solutions campaign, funded in part by the Government of Canada, to learn more about how farmers can reduce the environmental impact of agriculture and be part of the climate solution.