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Soil carbon and construction
On construction sites, minimising compaction and retaining as much soil as possible in a healthy condition can go a long way to protecting the soil’s carbon storage and processing abilities. Photo by CatLane on iStock
Soil is the second largest carbon store on earth, embedded in a complex carbon cycle that has been devastated by human activity. The construction industry is a prime culprit, making it vital that soil is put first and foremost in any development.
Soil is a complex system of organic matter, minerals, water, air and organisms that has developed over hundreds of thousands of years, forming the thin outer layer of the earth that supports life. For this reason, our soil systems are considered a non-renewable resource.
Soil, water and air are core components that define our climate and foundation of life on earth as we know it today by supporting complex living organisms and habitats and enabling humanity to thrive. Due to human impact, the current rate of change to these systems has accelerated beyond change that would naturally occur, thereby accelerating climate change and biodiversity loss.
Soil carbon storage and the carbon cycle
The carbon cycle is complex, and after our oceans, soils are the second largest carbon store on earth. Yet they bear a high proportion of that human impact upon our natural resources – through activities such as agriculture, deforestation, mining, and construction.
The Office of National Statistics (ONS) published experimental data on UK biocarbon stock in 2016. This demonstrated the importance of carbon stored in soils, since it vastly exceeds carbon stored in the vegetation growing on these soils. Wetlands are very effective carbon sinks, and forests also store significant amounts of carbon both below and above ground.
The ONS states:
“There was an estimated 4,266 million tonnes of carbon (MtC) of recorded biocarbon in the UK in 2007, of which 94.2% (4,019 MtC) was contained in soil stocks and 5.8% (247 MtC) in vegetation stocks.
Between 1998 and 2007, UK biocarbon stocks declined by approximately 19.9 MtC (-0.5%), on the back of a fall in the volume of carbon stored in soil stocks. Carbon contained in UK vegetation rose by 1.3 MtC (+0.5%) during the decade to 2007, driven by an increase in categories of forest tree cover.”
What is soil carbon?
Soil Organic Matter (SOM) describes the organic matter content created by plants and animals, such as roots, leaf matter, and animal detritus, in various stages of decomposition. SOM also includes carbon, referred to as Soil Organic Carbon (SOC). SOC is created through physical, chemical and biological processes, and we can assess how much carbon is stored in a particular soil.
Soil carbon in the form of organic matter is highly beneficial for soils and future planting, as it contains nutrients, improves water storage, binds polluting elements, enables microorganisms to live in soil, and supports root growth.
SOM is key to a soil’s role in climate mitigation and is often negatively affected by construction projects. If soils are removed from site, the SOM will be removed along with it. Compaction and storage affect SOM and construction-related activities can significantly reduce or lead to loss of carbon storage capabilities. Damaging soils in this way may also result in the release of GHG back into the atmosphere due to changed or interrupted decomposition processes. This removes stored carbon from a site, thereby negatively affecting a project’s carbon credentials.
In 2013, soil carbon losses due to development were estimated at 6.1 million tonnes of CO 2; this is greater than the emissions of GHG from other damaging industries such as concrete production (6 million tonnes) and the chemical industry (5.2 million tonnes).
Soil carbon is stored in both topsoil and subsoil. While we often concentrate on topsoil when discussing soils, subsoil contains stabilised carbon, indicating that both soil systems must be considered. Researchers have stated: “The subsoil has a large influence on ecosystem productivity and the supply of ecosystem services. Carbon stored in subsoils generally contributes to more than half of the total stocks within a soil profile.”
Soil carbon can be measured as part of any soil tests undertaken but would need to be specifically included in the suite of requested tests. Results might then inform consideration of how soil carbon could be improved, or decisions about how much carbon would be lost from the site if soils were to be removed.
Healthy soils: key to maximising carbon storage potential
A soil must be in a healthy condition to maximise its carbon storage potential. Key factors are the soil type, texture and structure, as these dictate the soil’s physical and chemical properties. All this impacts a soil’s ability to bind, transport and store nutrients, including organic matter, air and water. In compacted soils, there is insufficient air and water to support healthy decomposition processes, as well as less physical space to store any organic matter that may manage to make its way into the soil.
Compacted soils can result in anaerobic soil conditions and limited soil functionality, neither of which are supportive of healthy carbon creation or storage processes in the soil. Compaction also affects a plant’s ability to grow. For example, if a new woodland is planted into compacted soil, tree roots may not develop or may grow very slowly, spreading less wide and deep, with poor above-ground growth too. Over time, this means that much less organic matter both below and above ground is created, thereby substantially impacting the carbon storage potential of that new woodland and the soils beneath it long into the future.
Look after soils: minimise compaction and soil handling
On construction sites, minimising compaction and retaining as much soil as possible in a healthy condition can go a long way to protecting the soil’s carbon storage and processing abilities. This will also ensure its suitability for future planting.
Thoughtful design helps to minimise soil sealing and compaction, for example through early consideration of site-specific soil and leaving areas undisturbed and protected (including in conjunction with tree protection areas and biodiversity areas). Other considerations are designing levels to achieve a zero cut and fill balance on site, using permeable surfaces, and planning construction access routes and areas with soils in mind.
How soil is stored on a construction site for reuse after stripping will affect its structure, nutrient and organic matter content, and therefore both its carbon content and ability to store carbon in the future. The key is to minimise handling and storage of soils from the outset. Soils that have been stripped and stored should be re-tested and improved if necessary before placing them in their final location. A Soil Management Plan will help to ensure that soil stripping, or cutting, storage and spreading, and fill operations, are carried out in a more sustainable way.
How soils and green and blue infrastructure work together
Soils and green infrastructure (GI) are inextricably linked – there can be no green infrastructure without soils. The type and quality of soils dictates the type and quality of GI, and soils will also dictate the drainage properties and water quality in landscapes and broader developments. This profound interaction influences both the carbon storage potential of soils and the vegetation they will support.
Wetlands as a type of blue infrastructure have significant carbon storage potential if they are designed to act as carbon sinks. They must be able to accumulate large amounts of decomposing plant material so need to be permanently wet and both large and deep enough to allow this to happen.
The Secretary General of the Ramsar Convention on Wetlands, Martha Rojas Urrego, said in 2019: “The science is clear. Wetlands are the most effective carbon sinks on our planet.”
Landscape architects are ideally placed to raise awareness of the importance of soils with their clients and project teams, and to consider soils in their designs from the start.
The Soils in Planning and Construction Taskforce report, ‘Building on soil sustainability: Principles for soils in planning and construction’ provides further key facts and figures in relation to soils and development, including guidance on soil surveys and soil management plans.
Birgit Höntzsch is a Chartered Landscape Architect with over 25 years’ experience. Her focus is on green and blue infrastructure delivery to support sustainable development, including climate mitigation, soils, biodiversity and trees. She currently works for Cornwall Council as Portfolio Lead for the emerging Langarth Garden Village.
