Carbon Sequestration in Soil (Video)

Our planet’s rich soil can store vast amounts of carbon dioxide (CO2). But, since the Neolithic Revolution 10,000 years ago, agriculture and land conversion have decreased worldwide carbon sequestration in soil by 762 billion tonnes. Many soils under cultivation have subsequently lost 50 to 70 per cent of their original organic carbon. The CO2 that this releases ends up in the Earth’s atmosphere.¹ There, in its gaseous form, CO2 traps heat and causes global warming.² 

Some studies estimate that carbon sequestration in soil could absorb two to five gigatonnes of CO2 per year (GtCO2/y) by 2050.³ The French ‘4 per mile’ initiative proposes increasing the quantity of carbon contained in soils by four per cent annually. It asserts this figure would “halt the annual increase in CO2 in the atmosphere”. 100 nations signed the initiative at the 21st Conference of the Parties in Paris.⁴

Can soil carbon sequestration solve the climate crisis? 

It is true that soil carbon sequestration has the potential to remove large quantities of CO2 from the air. Consequently, it is an important climate change mitigation approach. But this alone cannot solve the climate change conundrum. 

In addition, there are various issues around implementing carbon sequestration in soil as a viable solution. To prevent a catastrophic increase in global temperatures over 1.5°C, humanity must take two crucial steps.⁵

Firstly, we must stop producing fossil fuels. Secondly, we need to protect natural carbon sinks, like forests and wetlands. This requires a fundamental shift in how we approach energy generation, food production and land use.

How much carbon can we sequester in soil?

Estimates for how much carbon we can sequester in soil vary depending on the source. For instance, the Institute for Carbon Removal Law and Policy references a “recent expert assessment” that finds carbon sequestration in soil potential to be between two and five GtCO2/y. By 2100, this could store 104-130 GtCO2. According to the same assessment, the cost per tonne of CO2 would fall between USD $0 and $100.⁶

Similarly, an article in Frontiers in Climate claims that the “majority of studies suggests four to five GtCO2/y as an upper limit for global biophysical potential with near-complete adoption of best management practices”. However, they also argue that over the long-term, the global figure could reach eight GtCO2/y.⁷

Helping the soil absorb an additional two to five GtCO2/y would undoubtedly help mitigate the effects of global warming. Removing CO2 and other greenhouse gases from the atmosphere is crucial in the fight against climate change. However, humans emitted about 39 GtCO2 in 2019.⁸ Emissions from fossil fuels are anticipated to rise over 2021.⁹ Therefore, even if we were able to maximise carbon sequestration in soil to 8 GtCO2/y, it would still capture less than 20 per cent of our current CO2 emissions.

How does carbon sequestration in soil work?

How does carbon dioxide get from the air to the soil?

Carbon sequestration in soil works through the mediation of plants. Plants pull CO2 out of the air in the process of photosynthesis. Then, they use the carbon to form compounds for growth. Any excess carbon exudes through the plant’s roots. 

Therefore, soil carbon sequestration is a natural form of carbon capture and storage (CCS). It feeds soil organisms and renders the carbon stable. Carbon is essential for soil organic matter. It provides it with its water-retention capacity, structure and fertility.¹⁰ 

In this way, soil can hold large quantities of carbon. Soil contains three times as much carbon as what currently exists in the atmosphere. Therefore, improving carbon sequestration is very important for removing CO2 from the air.¹¹ It is one of the three principal ways to remove and store CO2 via the management of terrestrial ecosystems.¹²

How can we increase carbon sequestration?

Carbon sequestration in soil can be enhanced using improved management practices. These approaches may increase the rate by which plants add carbon to the soil. Alternatively, they may prevent the amount of carbon the soil releases back into the atmosphere. Furthermore, improving soil carbon sequestration also enhances soil health and fertility.¹³

Regenerative agriculture

Regenerative agriculture refers to various techniques to sequester carbon in agricultural soils.¹⁴ Some adherents hold the view that these practices have enormous potential to combat climate change. They believe that farms have the potential to be at the “forefront of the fight against climate change”.¹⁵ 

Subscribers to this school of thought argue that agriculture has the largest potential for sequestering CO2 from the atmosphere after forestry.¹⁶ Whilst this may or may not be true, it is important to remember that the fossil fuel industry mainly contributes to climate change. It causes as much as 89 per cent of CO2 emissions.¹⁷ Unless we address this problem, this approach alone cannot save the planet.

On the contrary, others consider these aspirations to be an exaggeration. They find that soil carbon sequestration falls short of the expected climate benefits. Consequently, they see it as poor carbon accounting and a dangerous red herring that does not provide the climate mitigation it appears to offer.¹⁸

Planting more trees and plants

Since plants are the conduit for converting CO2 in the air to carbon in the soil, cultivating more vegetation is key to increasing soil carbon sequestration. One way to do this is to expand the use of cover crops. Cover crops are planted in soil. They are not intended to be harvested. Instead, they are typically planted in between periods of regular crop production.¹⁹

Scientists estimate that if US farmers increase their use of cover crops to 88 million hectares of cropland, it will cut US agricultural production emissions by about a sixth. This is equivalent to 100 tonnes of CO2 per year. Likewise, simply planting trees among row crops can have positive impacts. If ‘alley cropping’ was employed on 10 per cent of US cropland, it could sequester 82 million tonnes of CO2 per year.²⁰

Increasing the numbers of plants growing – particularly trees – has other environmental benefits. They provide much-needed habitats for wildlife, particularly insects and birds.²¹ In addition, trees perform significant climate mitigation by absorbing CO2. 

Through the same process by which they pull CO2 from the air and deposit it in the soil, trees also store large amounts of greenhouse gas in their biomass. For example, a mature tree sequesters about 22 kilograms of CO2 each year using photosynthesis.²² This is why protecting trees is fundamental to combatting climate change.

Reducing soil disturbance 

Minimising soil disturbance in agriculture is another way to increase carbon sequestration. For instance, adopting no-till or low-till practices avoids disturbing the soil.²³ This may mean not ploughing the soil to aerate it and remove traces of the previous year’s crop before planting. Also, it may mean using a planter or seed drill to reduce interference with the soil.²⁴

Alternatively, planting perennial crops can also increase soil carbon sequestration. This is because perennial plants do not require reseeding or replanting every year. Therefore, they do not require annual ploughing. In contrast to perennials, farmers need to spend more time killing weeds that compete with annual crops, especially while still seedlings. Disturbing the soil in this way causes significant quantities of soil carbon loss.²⁵

Can soil be a carbon sink?

A carbon sink is something that captures more CO2 from the atmosphere than it releases. Forests and oceans are both carbon sinks. Soil can also be a carbon sink.²⁶ Through regenerative agriculture and the other practices mentioned above, carbon sequestration can be improved to make soil a better carbon sink.

However, soil can also be a carbon source. As the temperature of the planet rises, the amount of CO2 that the soil releases will increase. Scientists warn that 4˚C of global warming could cause the soil to release as much as 37 per cent more CO2 than usual.²⁷ 

Rainforests and carbon dioxide

Soil can also release carbon in other ways. For instance, the destruction of rainforests results in emissions of vast quantities of CO2. Between 2015 and 2017, deforestation of tropical forests produced 4.8 billion tonnes of CO2 per year. That is about eight to 10 per cent of annual CO2 emissions. A proportion of this CO2 comes from disturbing the soil.²⁸

Is carbon sequestration in soil a viable solution to climate change?

We need to do everything we can to remove CO2 and other harmful gases from the atmosphere. Carbon capture and storage methods, such as soil carbon sequestration, are an important tool at our disposal. They can help us to avoid exceeding global warming by more than 1.5°C.

On the other hand, this approach has limitations. Even the most optimistic estimates find that it can capture less than one-fifth of our current annual emissions. To truly address the issues that we have caused for ourselves, we need to look at the source of the problem.

Almost 75 per cent of all emissions come from energy use, primarily from burning fossil fuels.²⁹ We must simultaneously enhance offsetting opportunities, like soil, whilst stopping the production of fossil fuels immediately.

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