Can We Control Soil Carbon? The Challenge of Managing Soil Carbon Stocks in Scotland

Soil carbon sequestration for climate change mitigation has become a hot topic over the last few years, particularly within agriculture, which contributed 7.7 MtCO2e of Scotland’s total 2022 greenhouse gas emissions of 40.6 MtCO2e. Protecting and enhancing soil carbon is highlighted within recent Scottish policies such as the National Planning Framework, Biodiversity Strategy and the Vision for Agriculture. But how can this be achieved in practice? 

What is Soil Carbon? 

Soil is mostly comprised of air, water and mineral particles with a small proportion being soil organic matter (SOM) derived from the breakdown of leaf litter, dead roots, plant material and animal waste. SOM comprises a mixture of key nutrients and carbon (C) constituting approximately 58% of SOM. Soil carbon is a vital component of the global carbon cycle with topsoils containing almost twice as much Carbon as the atmosphere (Figure 1). SOM also contributes significantly to soil health and resilience in terms of supporting physical (e.g. soil structure, stability and aggregation), chemical (e.g. pH buffering, nutrient cycling etc) and biological (supporting the soil food web) components of soils. Good soil health supports soil functions and wider ecosystem services (Figure 2).

Figure 1. BSSS: Science note soil carbon 

Figure 2. FAO: Soil functions 

What Happens to Carbon in Soil? 

Once carbon enters the soil (e.g. root material, root exudates, residues) the organic material (OM) begins to break down and decompose at different rates depending on: 

• The chemical makeup of OM. e.g. woody materials decomposes slower compared to leafy materials.  
• Soil characteristics like pH, texture, moisture and nutrient status influence biological activity.  
• Weather conditions (e.g. rainfall and temperature). 
• Management practices like tillage (disturbance and aeration of soils), nutrient management (fertiliser, residues/mulches and manure additions) and liming (influencing soil pH). 

As OM breaks down, carbon can be temporarily stored or transported through the soil profile.  

How is Carbon Stored in Soils? 

The are three key ways in which C is held within soils: 

1. Organic and mineral particles bound together creating a shield (physical protection) from decomposers preventing access to OM and slowing the breakdown of OM supporting carbon stabilisation and accumulation. 
2. Carbon ‘sticks’ to clays (and iron and aluminium oxides) via chemical bonding (referred to as mineral associated C). 
3. Soil biological activity promotes soil aggregation and the formation of microbial-derived organic matter (MDOM) which is a pool of living and dead microbes (known as necromass). Research indicates that the majority of ‘sequestered’ C is MDOM.  

How is Carbon Lost from Soils? 

Carbon cycling is a natural process with carbon entering and leaving soils as part of the global cycle (Figure 1). As OM decomposes, there are three main ways in which Carbon leaves soil: 

1. As gas emissions mainly in the form of CO2 (soil and microbial respiration). 
2. Transported by water (leaching) to local watercourses where it continues to break down and decompose. 
3. Erosion and physical loss of soil particles. 

Managing Soil Carbon 

This article highlights some of the complexities associated with carbon storage and why it can be difficult to provide robust guidance across Scottish soils. However, there are a range of management practices recommended for improving soil health which will also contribute to protecting and/or enhancing soil carbon. For example, maintaining soil cover can help prevent soil erosion as well as provide natural source of Carbon through root exudates and OM from residues. Optimising grazing and tillage management can help reduce compaction and soil disturbance while providing manure and residue incorporation to the soil.  

5 top tips for managing soil carbon (which generally apply to all): 

1. Know your soils: Understanding your soil characteristics makes it easier to match effective management practices for your natural soil type. 
2. Protect SOM: Although there is currently significant attention focused on increasing soil carbon stores, there are many advantages to conserving current stores of SOM and carbon for climate change: mitigation and the promotion of good soil health.  
3. Restore and replenish SOM: Scotland has many carbon rich soils where it may be more challenging to further increase soil carbon sequestration. However, identifying areas that are degraded or depleted in SOM offer opportunities for future carbon accumulation.  
4. Enhance SOM: Explore the list of agricultural reform measures to enhance soil carbon and overall health which will have co-benefits of supporting wider ecosystem services. 
5. Monitor: Use soil testing to benchmark progress towards better SOM management which will support future resilience of your soils. 

Sarah Buckingham, Senior Environmental Consultant, Sarah.Buckingham@sac.co.uk

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Posted by Unearthed News on 12/05/2025