Can we store more carbon in our soils?
19 December 2024There is a lot of discussion around how we ‘lock up’ more carbon in our soils, however due to natural processes, this is more challenging than just applying more organic matter. The aim of this factsheet is to ‘go back to the basics’ to understand how carbon is stored in soils to provide further understanding of soil carbon that can inform future soil management plans.
To benefit from all the functions soil provides, a more holistic viewpoint is needed to conserve soil carbon and manage soils for overall health and resilience.
Soil carbon is not ‘locked up’ indefinitely
Carbon cycles through soil as part of the global carbon cycle. This means there are naturally carbon inputs to soil and carbon outputs of soil. The net difference in carbon going in or out is a soil’s carbon stock (Figure 1).
Carbon inputs to soils via the addition of organic matter (E.g. residues, manures, roots) decompose over time. The rate of decay depends on complex interplay of soil characteristics, land management and climatic factors (see Figure 2). As organic material breaks down, carbon is lost as carbon dioxide (CO2) through soil respiration and can be transported via soil water movement to watercourses. Some carbon will remain in soil via (1) chemical bonding, (2) protected within aggregates and (3) held within soil biology (who feed on the organic matter for energy) - however the storage of carbon is not fixed as carbon can move within the soil matrix.
How can we control soil carbon stocks?
There will always be carbon losses from soil (Figure 1), but you can manage soils to minimise soil losses to retain more soil carbon.
Figure 2
The different factors affecting soil carbon storage potential. Inherent soil properties (blue), climatic (green) and management (orange).
We have little influence over many of these factors. Those highlighted in orange are the main ways we can affect soil carbon stocks. Suggested management methods to increase soil carbon include reducing soil disturbance, incorporate residues, manage soil pH (liming), adding organic manures, growing catch crops and changes in grazing intensity - many of these are likely to already be practiced and can contribute to your soil health.
What does the evidence show?
A recent report showed no statistical change in carbon content for arable, improved grassland, semi-natural grassland, moorland and bog in Scotland. Many believe soils reach an equilibrium over time, where the decomposition rate of organic matter is such that carbon losses equal that which was added, resulting in no net change of soil carbon. If soils are in equilibrium, high C inputs may lead to high C outputs. An additional theory is that soils have a limited capacity for soil carbon sequestration and become ‘saturated’ over time.
Key take-away messages
- Soil carbon is not fixed. Soils have a complex digestive system just like humans. Carbon that has been gained can also be lost,
- Regular monitoring of soil carbon stocks is essential if you are to know whether your soils are gaining/losing carbon or in equilibrium.
- Carbon storage in soil is vital for climate regulation, but soil serves many more functions including food production, water purification, flood resilience, platform for woodland and forests as well as supporting biodiversity.
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