The role of soil carbon in climate mitigation

In this installment of “Climate change 101”, Carolien Samson, head of sustainable banking at Oxbury Bank Limited, looks at soil carbon and its benefits to farmers.

Previously, we looked at how trees are used for carbon removal and what involvement farmers could have. In this article, we unpack the role of soil carbon, a complex and very contentious subject at the moment.

Maybe let’s start with what is soil carbon?

Plants capture carbon dioxide (CO₂) from the atmosphere and temporarily stores (sequester) the carbon inside the plants as well as in the soil. When plant matter decays, the carbon it contains becomes part of the soil for a while until it is broken down by microbes and eventually released back into the atmosphere.

The length of time that the carbon stays in the soil varies depending on for instance climate, soil composition and land use practices. For instance, in the short-term, conventional ploughing may speed up the release of soil carbon compared to no-till practices which leave more of the soil undisturbed.

Soil organic matter versus soil carbon

The terms are often used interchangeably, but soil carbon is one, normally the largest, component of soil organic matter. Soil organic matter is the part of soil that can pass through a 2mm sieve and consists of water, carbon and other nutrients like nitrogen and potassium.

As carbon normally makes up more than half of the sample, it is the easiest to measure and report. The presence of soil organic matter is a good indicator of soil health and assists in water retention and infiltration which is in itself important in areas prone to drought. Soil organic matter also nurtures fungi and bacteria that contribute to overall soil and plant health.

Why is soil carbon important for climate mitigation?

Agricultural production currently emits about 5.0 – 5.8 gigatons of carbon dioxide equivalents (including equivalents like methane) per year. The Intergovernmental Panel on Climate Change (IPCC) estimates that soil carbon has the technical potential to mitigate up to 5.3 gigatons of carbon dioxide equivalents per year by 2030, which theoretically, could make agriculture a net zero sector.

However, it is often highlighted that this is unrealistic as it would require thousands of hectares to be removed from food production and large-scale adoption of different agricultural practices like regenerative agriculture.

Another challenge is that there is a saturation level where soils will no longer be able to absorb additional carbon, although the process to create soil carbon is very slow.

What is the benefit to farmers?

Over and above the substantive contribution that better soil health makes to the resilience of the farm business, it could be used to gain market access.

A farmer who operates in a value chain with net zero commitments could use their current soil carbon stock to contribute to their net zero status.

ALSO READ: Climate change 101: Let’s talk trees and farmers

What about soil carbon offsets?

The Climate Neutral Group launched an agricultural carbon programme in South Africa for farmers practising regenerative agriculture in 2021 which enrolled 65 farms in 2021.

The programme meets the international Verra Carbon Standard and could provide useful income to farmers making the transition to more regenerative practices as income can be earned for five years.

Farmers should compare the price offered to global market prices and consider costs like soil sampling, certification and ongoing monitoring when considering these contracts.

Importantly, farmers should remember that if they sell any soil carbon for offsets, that carbon can no longer be counted towards their own operations which could be important in certain value chains.

There is still legal uncertainty around projects where carbon is removed in one country and buyers of offsets are from other countries.

All countries have committed to remove and reduce carbon emissions in terms of article 4 of the Paris Agreement and submitted a Nationally Determined Contribution (NCD) to the United Nations Framework Convention on Climate Change (UNFCCC).

Article 6 of the agreement sets out a process to trade offsets between countries, but with rare exceptions, this has not yet come into effect. The principle remains that carbon removed and sold as offsets to buyers outside the country, will not count towards the NCD of the country which sold to credits.

Although the voluntary soil carbon market is developing quickly, farmers need to treat this area with caution. There is still legal uncertainty and more than half the revenue from the sale may accrue to other players in the trade rather than the farmer who nurtured the soil for a long time.