Carbon sequestration in agri

Can we really farm with carbon?

Realistically, some greenhouse gas emissions will be hard or even impossible to avoid (for example from livestock, aviation, and soils). So, we need to go one step further and reabsorb or remove greenhouse gases from the atmosphere to reach “net zero”. Fortunately, nature does this already. We know from school biology that plants absorb carbon dioxide (CO2) during photosynthesis (Figure 2) – this is how they “eat”. The

consumed CO2 becomes embedded in new growth. Though not the only greenhouse gas, CO2 is the main driver of climate change. The agriculture sector is well positioned to take advantage of plant-driven, or nature-based CO2 removals and could play a critical role in mitigating climate change. However, the most obvious problem with removing carbon as biomass is permanence – it all breaks down again. When carbon is moved to natural systems (e.g., plants and soil), there is only a benefit for as long as the carbon is held (captured). Croplands are unstable, unlike natural ecosystems, which tend to reach maturity and permanently hold a level of carbon. Products like fruit or grain are quickly consumed, annuals become decomposing residues within a season, and orchards are replaced once they are no longer sufficiently productive. So, while growing plants do absorb CO2, when considering a long-term view, they are more likely borrowing it than removing. Therefore, the goal is to keep the CO2 out of the atmosphere for as long as possible. Plant material that takes longer to break down or decompose will store the CO2 for longer. Decomposition is an active process and occurs when organic carbon is eaten by an organism (animals, insects, microbes, etc.) and converted back into CO2 as a by-product of metabolism. Food that is rotting is also being eaten, but only by bacteria, fungi, and insects. The type of material, moisture content, and nutrients it contains, and how easily it can be broken down into molecules (sugar, starches, proteins, carbohydrates, fibres) will determine how long decomposition will take. This is why lettuce goes bad within a week, but a wooden bench can last for decades – even though both are plant captured carbon. According to the draft guidelines recently published by GHG Protocol on how to account for CO2 removals, any carbon stored in living or dead biomass can be accounted for on condition that the removal is reversed as soon as it is released into the atmosphere again. This means that when it comes to determining the amount of carbon a farm has sequestered, it makes most sense to account only for the CO2 held in perennial woody plants such as fruit trees or grapevines.

For example, several hectares of orchard can account for a lot of carbon, but two things are important to remember:

1. A maximum amount of carbon will be reached where no more carbon can be removed, as all the orchards/vineyards are at their largest. The amount will fluctuate as blocks are replaced, and plants increase in size. At the same time, during production and when agricultural inputs are applied, emissions will be released.
2. Trees consume resources. All farms occupy land, which was originally natural vegetation. Furthermore, irrigated agriculture is a significant user of water, a scarce resource in SA. Alien invasive species are a good example where the overall ecological damage far outweighs their “carbon removal”, leading to their removal and repurposing as firewood.

A second, more important carbon removal mechanism, is soil sequestration – a potentially semi-permanent solution. Capturing carbon in soils is a balance between two processes, carbon flowing into soils (through plant roots, and organic inputs) and carbon being decomposed as a food source (this is also called mineralisation and makes the nutrients available for plants too). Farming practices influence these processes. They can increase carbon flow into soils by increasing the number of active roots in the soil or by adding inputs. Breakdown can be slowed through soil protection and minimising shocks. Like biomass carbon, there is a limit to the amount of carbon soils can securely hold, and this is mainly determined by the soil texture (clay and silt can hold and protect organic carbon). In most areas, croplands are known for degraded soil carbon levels, which means that there is an opportunity to store more carbon. Factors like topography, climate, and moisture availability also impact the amount of carbon in the soil. In dryland systems, it’s good to aim for carbon levels like those of the surrounding natural areas. Since irrigated systems increase the ability of the system to produce biomass, they feed even more carbon into the soils than could occur naturally. In this case it’s possible for soil organic carbon to exceed the “natural amount” (as seen in irrigated dairy pastures in the Western Cape). Farm management activities can be used to remove carbon from the atmosphere, and this is an important way to help mitigate climate change. However, a set amount of carbon cannot be predictably removed each year in the same way that fruits are harvested. They are finite pools that eventually fill up. Removals cannot be used as a replacement for decreasing emissions. So, it’s more important that greenhouse gas emi-ssions are rapidly cut down, and the natural removals pool used to buffer for that which we cannot reduce.

Carbon footprinting and carbon credits

Increasingly, on-farm carbon removals are being recognised within carbon footprinting. Though it is not as simple as subtracting the removals from emissions, because they have different levels of permanence. Increasing numbers of food brands and retailers are focusing on, and setting targets for, removals on the farms of their suppliers. A farm that removes carbon can either choose to report it as part of the carbon footprint, or it can be sold as a carbon credit – but not both. The difference: Carbon removal activities at farm level can be used to positively contribute towards that farm’s own emission measurements and targets. A carbon credit is a valuable, tradable asset that signifies the removal of carbon from the atmosphere or the prevention of its release into the atmosphere. Essentially, a credit represents a unit of carbon that has been either sequestered or avoided through activities considered beyond the usual business requirements.

A carbon credit can be:

• sold to another business or individual to allow that entity to reduce its net carbon emissions
• bought by a business or individual to offset one’s own net carbon emissions.

What should the agriculture sector be doing?

Step 1: Start measuring and managing your carbon footprint to identify where to focus emission reduction efforts. Step 2: Find a different way to do things. Reduce emissions as much as possible. Cutting emissions is more beneficial than carbon removal, and some greenhouse gas types cannot be removed once emitted. Step 3: Increase carbon removals to neutralise emissions that cannot be reduced. Step 4: Repeat steps 1–3. Set an ambitious science-based emission reduction target: Aim to reduce your own emissions by 42% (from a baseline) in 10 years’ time, and eventually reach “net zero”.

For more information, please contact Blue North/CCC at: support@bluenorth.co.za or 063 688 5593.