This report examines the potential of nature-based solutions to contribute to Scotland’s net-zero emissions target.

Scotland is facing the twin challenges of a climate emergency and biodiversity crisis. Changing the way we use the land and sea is now essential to both store carbon and help society adapt to climate change. Doing so can also help to improve the state of nature, which is experiencing unprecedented threats. 

Nature-based solutions feature prominently in the global biodiversity agenda. Vegetation growth and healthy soils, as well as sea floor integrity, provide a crucial way of locking away carbon emissions. However, it is the additional multiple benefits unique to nature-based solutions – addressing biodiversity loss, and adaptation to locked-in climate change – that makes them such a crucial part of a net-zero strategy. These are widely regarded as ‘no-regret’ actions to address climate change, but the evidence base to support their direct impact is complex. As such, further work is required to understand their practical application in Scottish circumstances.

This study assesses evidence for the greenhouse gas (GHG) mitigation potential of four nature-based solutions in Scotland (agroforestry, hedgerows, un-cultivated riparian buffer zones and the restoration of species-rich grasslands) and how these can help mitigate the impacts of climate change and reduce biodiversity loss. In addition, we provide a synthesis of the strength of evidence for including these as part of net-zero policy objectives and carbon codes.

This project set out to review the current state of knowledge on the potential for carbon sequestration in key Scottish upland open habitats. Upland soils play a vital role in regulating greenhouse gas (GHG) emissions in our environment. Scotland’s soils contain 2500-3500 Mt of carbon, much of which is located in upland soil environments. This is equivalent to more than 200 years of Scotland’s annual greenhouse gas emissions. The management of uplands and their soils will therefore be critical to achieving Scotland’s ambitious net-zero emissions target.

Despite the well-known potential of soils to store carbon, however, there is uncertainty as to the long-term stability of this carbon pool. Increasing temperatures, altered patterns of rainfall distribution, and changes in land use all influence this process and threaten to reduce soil carbon stocks.

This review identifies the key drivers of change and covers three upland habitats: upland dry heath, upland wet heath and upland grasslands, defined by vegetation communities. It assesses potential GHG fluxes and the impact on biodiversity within these habitats.

It found very limited information regarding impacts on soil carbon stocks or GHG emissions; studies giving a full balance sheet of ecosystem stocks and flows of carbon in response to environmental or management change were particularly scarce.

Key findings include:

• Scotland’s soils contain around 2,500-3,500 Mt of soil organic carbon. The various mineral, organo-mineral and organic soils found under moorland, montane, and rough grassland contain around 45% of total Scottish soil organic carbon stock.
• Soil organic carbon accounts for 90% of the carbon stocks in these habitats. Therefore, studies which only consider changes in carbon held within the vegetation severely under-estimate changes in total carbon stocks.
• GHG emissions in open upland habitats in Scotland occur as a result of emissions of carbon dioxide, methane and nitrous oxide. 
• There is some evidence from Scotland that Molinia grasslands contain large carbon stocks within the vegetation, which are reduced by grazing.
• When upland soils are left bare after excessive grazing or burning, there is a significant increase in the risk of soil carbon loss due to erosion.
• The impacts of future climate change on carbon stocks are complex and are likely to depend on current and future management, soil type and vegetation communities. They have not been well researched in the Scottish context: this is a substantial gap in knowledge.
• The review found an important knowledge gap on the interactions of drivers on GHG emissions, carbon stocks and biodiversity.

This report was commissioned to analyse the indicators available to monitor Scotland’s soil health. Soil health is essential: the benefits range from food production to filtering water, reducing flood risk and regulating climate.

The second Scottish Climate Change Adaptation Programme (SCCAP) identifies soil health as a priority research area, following concerns over a perceived lack of data or gaps in understanding Scotland’s soils. This study summarises previous work on Scottish soils, explores existing datasets, and identifies metrics to support the monitoring of soil health and the vulnerability of Scottish soils to climate change.

 Key findings

• Scotland has a significant, world-leading soil knowledge base and a broad data resource portfolio. However, the existing evidence base does not contain tools identified as appropriate for monitoring change in Scottish soils.
• Thirteen indicators with potential to measure soil vulnerability to climate change in all soil types were identified.
• A total of 41 existing datasets that contain baseline and/or resurvey data for Scottish soils have been identified. Resampling of some of these long-term national datasets has potential to support further development of the 13 identified indicators (Table A10).
• A critical knowledge gap exists regarding the dependencies of the 13 identified indicators (i.e. factors they are reliant on), their interactions and hence whether a reduced core set of indicators could be identified at a future stage. This is compounded with critical gaps in our understanding of the interactions between soil properties. This knowledge gap has a major impact on soil biological diversity and therefore functioning of the soil system.
• No single indicator measures the full range of relevant properties encompassing all soils or climatic conditions.

Soils are one of the world’s biggest stores of carbon. The level of carbon storage depends on several factors, including the type of organic matter, climatic conditions and land management practices, both past and present. This report explores how the level of storage over time could be measured, and how this could help improve land management practices through a payment system.

Key points

• Agricultural soils (across pasture and arable) account for more than 10% of Scotland’s estimated soil carbon. Changes in land management practices affect the balance between soil carbon accumulation and loss, with conversion from grassland to cropland as the largest single change that releases soil carbon on Scottish agricultural land. 
• Evidence suggests there is large potential for increasing carbon storage in agricultural soils through changes in management practices. Any increase in carbon in the soil is likely to have a positive impact on soil quality, whilst the climate change mitigation benefit may be modest but positive in the longer term.     
• Mechanisms for support through payments exist, but they are largely focused on wider benefits such as preventing soil erosion and there are none that currently specifically enable  soil carbon sequestration.

Sustainable soil management is a particular challenge as Scotland adapts to a changing climate, and has been highlighted by the Adaptation Sub-Committee, in its UK Climate Change Risk Assessment 2017. Soil compaction and erosion have been identified as being important, particularly in exacerbating flooding impacts and decreasing soil carbon storage.

This report collates the current state of confident knowledge for Scotland – what we know, what we don’t know and what is under active debate.

Key findings

• Much of what we know about erosion rates on agricultural land in Scotland comes from a few, individual studies of erosion events, but there is a growing body of evidence that can be used to examine the role of land use (both current and historic), soil type and slope on erosion susceptibility. Other factors such as antecedent moisture content, ground cover and presence of tramlines also play a role, making it difficult to be certain when, or if, erosion will occur.
• Soil erosion models with sediment yield as an output seem to exaggerate the amount of soil loss and are difficult to validate, although they do offer a way to examine the relative changes in erosion rate under different land uses and changing climates.
• There is a link between soil compaction and erosion; soils that become compacted have a restricted capacity to store rainfall and generate overland flow more quickly than soils that are not compacted. This overland flow can then cause erosion.
• The greatest driver of soil compaction is machinery weight, which has been increasing over the past few decades, although using wide tyres, dual wheels and low pressure tyres can reduce the impact.
• We have a better understanding of field level effects with evidence gathered in Aberdeenshire following storm Frank (December 2015) suggesting erosion seemed more prevalent in areas that were more intensively managed.