Soil organic carbon sequestration: scoping study

2 Background

Organic matter (or organic carbon) is important in soils in terms of soil fertility, soil biodiversity and reducing the risk of erosion. It promotes and helps to maintain soil structure thereby enabling the infiltration of excess rainfall into the soil while storing sufficient water for crop growth.

Increasingly there is an awareness that soils may be used to sequester (store) carbon as part of the Scottish Government’s targets to achieve net-zero carbon emissions by 2040. The Scottish Government has prioritised actions to improve carbon sequestration in the land use sector within its current Climate Change Plan.

While much of the focus has been on the ability of trees to sequester carbon above-ground, there is also an acknowledgement that cropland and gazing land management can play a part in in reducing atmospheric carbon dioxide (CO₂).

The Committee on Climate Change (CCC) in their assessment of the first Scottish Climate Change Adaptation Programme (SCCAP) in 2019 stated that ‘changes in carbon stored in soils will have a more significant impact on priority habitats and provisioning of ecosystem services in Scotland than the rest of the UK’ thereby acknowledging the importance of soils but also that soils can both be a sink and a source of CO₂.

Atmospheric CO₂ can be taken up by plants through photosynthesis and converted to organic matter. This organic matter can be deposited into the soil as leaf litter, dead roots, plant material and animal waste. Micro-organisms in the soil decompose this organic matter and, if the rate of accumulation is greater than the rate of decomposition, then the amount of carbon in the soil increases. However, when decomposition rates exceed the accumulation rate, then greenhouse gases such as carbon dioxide and methane are released and the amount of soil carbon declines.

The amount of carbon that can be stored in the soil depends on the type of organic matter, climatic conditions land management practices (both past and present) and, crucially, the amount of carbon already in the soil. The amount of carbon that can accumulate is finite.

This report sets out to critically evaluate the ability of existing datasets, using exemplars, to answer the following questions:

• how much carbon is stored in Scottish soils and where
• how much carbon is stored in soils under different land uses
• how has Scotland’s soil carbon content changed over time
• what is the potential for Scotland’s soils to gain or lose carbon and what are the likely land use drivers of this change.

In addition to answering the above questions, a table of known datasets with soil organic carbon (SOC) data has been compiled (Appendix 1). This table is based on earlier work to review UK-wide soil monitoring datasets (Emmett et al., 2007) and was subsequently modified by Britton (2017) and then by Neilson et al. (2020). Throughout the report, datasets that are in the public domain or easily accessible will be used as examples to show how the data can be used to answer the questions posed. Some of these examples have been slightly updated where appropriate where new data was available. We will also explore how novel Infra-Red spectroscopy can be used to quantify SOC and examine the potential to quantify the role of SOC in water retention, aggregate stability and erosion risk.