Soil carbon sequestration

A guide to soil carbon sequestration, where healthy soils absorb water and carbon dioxide

soil carbon sequestration diagram

What is soil carbon sequestration (meaning/definition)?

Soil carbon sequestration is the process of removing carbon dioxide from the air via plants and trees. Carbon becomes locked up in the soil through many biological processes including the incorporation of leaf and other plant litter by creatures living in the soil. Sometimes this process happens as part of the agricultural cycle and plant remains are incorporated during ploughing and tilling.

Soil carbon sequestration is the process of locking carbon into soils. There are many processes that contribute to sequestration. In agriculture remains from the harvest may be incorporated into the as part of the cultivation process: ploughing and discing for example. Composts and manures applied as top dressing or through injection are other routes used in farming to provide nutrients and carbon rich materials into the surface layers. In forests and uncultivated lands leaves, plant litter and fire debris are all degraded and incorporated into the surface layers through natural processes; actions of fauna and fungi for example. The net effect of such processes is to incorporate carbon into soils and as such remove carbon dioxide from the atmosphere through the previous plant growth that led to the plant, crop litter or animal feed. These processes do not lead to a total permanent removal of carbon dioxide. They are rather part of a dynamic equilibrium between the above examples of carbon incorporations and soil processes that release carbon dioxide and methane (and nitrogen oxides) back into the atmosphere. Where the soil is less disturbed by agriculture carbonaceous rich material does build up in the soil. 

There are concerns that rising global temperatures may disturb this equilibria and release more carbon dioxide than is being sequestered. In turn this may create a feedback loop with yet more warming etc.

The role of biochar in soil carbon sequestration

Biochar is a material with high stability in the living environment. Through incorporation into the soil it can help improve soil performance in agriculture and horticulture. Importantly it remains stable in the soil for decades, centuries and possibly longer. It is a viable means for carbon sequestration and at the same time helps plants grow better in many conditions. This double benefit is in sharp contrast to some other approaches to carbon sequestration that are currently being developed and trialled.

Other natural carbon sinks

Whilst soil and the addition of biochar is one very important carbon sink, there are other natural carbon sink systems to be aware of

The ocean absorbs and stores x50 more carbon than the atmosphere via physical and biological mechanisms.

Carbon dioxide easily dissolves in sea water, which then helps support animal and plant life. The dead material then sinks to the seabed and is then essentially sequestered longer term.

The physical mechanism is the natural process of ocean circulation where dense water drags down sequestered carbon.

Plants and trees depend on the soil for health. They sequester carbon via the process of photosynthesis and become a carbon  sink until they die. At this point the material slowly begin to break down and release the carbon as carbon dioxide back into the atmosphere.

Peatlands are similar to soil but stay very stable due to them being waterlogged, slowing down the decomposition process trapping the carbon as a sink. If they dry out or are mined then the carbon is released as carbon dioxide back into the atmosphere.

In some cases peat builds up in bogs over millennia making deposits metres deep.

Shale contains organic carbon (OC) and carbonates. It is believed to make up ~33% of all rock formations on the earths surface and therefore is the largest total mass of organic carbon on earth. It should not be underestimated as to how important it is.

Limestone is a sedimentary rock composed primarily of calcium carbonate (CaCO3). It essentially locks up the carbon from decaying organic material like shells, algae and coral. The process of turning limestone into concrete is an energy intensive one, which releases the carbon dioxide into the atmosphere.

The benefits of carbon farming and regenerative agriculture

Soil carbon sequestration is the process of removing carbon dioxide from the air via plants and trees. Carbon becomes locked up in the soil through many biological processes including the incorporation of leaf and other plant litter by creatures living in the soil. Sometimes this process happens as part of the agricultural cycle and plant remains are incorporated during ploughing and tilling.

Reduce disturbance of the top soil, make fine drills and plant directly into the ground.

This leads to more water storage, microbe growth and healthier more vigorous plants.

Plant cover crops between seasons, this provides diversity of root exudate, feeding the soil biology, building a healthy diverse soil.

This is the process of combining trees and agriculture together. This can either be planting trees directly into crop fields, growing trees next to agricultural fields or farming in a forest environment that already exists.

This optimises the beneficial interactions between trees and crops under ground, as well as bring in pollinators and biodiversity above ground.

Animals are a great way to improve soils. They eat the top layer of organic material, drop manure/urine full of microbes and nitrogen as well as break up the top layer of material with their hooves, stimulating growth. Its important to move them on quickly and allow that patch of land to rejuvenate. It is worth the extra planning and work involved.

Provide the ground with with material that will breakdown and provide the soil with nutrients and microorganisms.

All of these techniques can lead to more localised rainfall, pull carbon out of the atmosphere through photosynthesis and reduce the risk of desertification – turning the soil into dust, making it vulnerable to wind and water erosion.

Future carbon capture and storage (CCS) technologies

There is also growing interest in capturing carbon dioxide at the end of industrial processes such as power generation, cement or steel manufacture etc. This typically would use a chemical system to capture the carbon dioxide in one step and release it in a second step thus producing a concentrated stream of pure carbon dioxide that can be dried and compressed for piping or shipping to a well and pumped as a super critical fluid into a deep sub-surface formation (rock) where it would be permanently stored. An alternative is to separate the carbon dioxide using a membrane that is selective for transmitting the gas but resists transmitting other gases. The carbon dioxide would then be dried and compressed before being piped or shipped and pumped into the reservoir. These simple sounding steps hide a multitude of challenges that will need to be overcome to make these processes work. The projected costs for these processes are very high. The carbon dioxide in the ground has no value but represents a long term liability w.r.t. the need to maintain monitoring for well and reservoir stability as well as leakage.

Some are seeing advantage in directly stripping CO2 out of the air to offset earlier CO2 emissions eg by Microsoft. Others see commercial advantage in using the recovered CO2 in greenhouse horticulture. Climeworks recently reported costs to strip out the gas at $800/tonne but they aim for $100 per tonne. This would be before costs to dispose of the collected gas eg by sequestration in a sub-surface aquifer. With current approaches the DAC route appears to be very expensive. $800/tonne for capturing CO2 is equivalent to ca $2900/tonne for carbon. On this basis biochar would be a bargain.

There are at least a couple of alternative approaches to CCS. One that has very recently been in the news is the use of basalt or silicate rock flour spread on the land to absorb and react with carbon dioxide in the air to form a mineral carbonate. A paper in Nature claimed there were stockpiles of basalt available as a by-product from mining.
Beerling, D.J., Kantzas, E.P., Lomas, M.R. et al. Potential for large-scale CO2 removal via enhanced rock weathering with croplands. Nature 583, 242–248 (2020)

With all these approaches, the costs are high, there are many technical obstacles to overcome and the scales required are enormous. There is an opportunity for biochar to be used to offset carbon dioxide emissions. Biochar is resistant to degradation in the environment making it a suitable vehicle for offsetting. What’s more it is a product with value and prospectively value at very large scale through its use in agriculture, horticulture, forestry and in the urban environment. The traditional methods for making biochar are themselves carbon dioxide intensive but using electrical heating with renewable electricity this can be radically reduced. The market in biochar needs to be expanded with some urgency to enable this opportunity to flourish.