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.

Soil carbon, or the content of carbon housed within soil, plays a significant role in the release and absorption of global greenhouse gas emissions. Changes in land use contribute to release of this carbon, and Scottish Government is interested in the potential implications of such change across several policy areas.

Land drainage is essential to maintaining the productivity of Scottish agriculture. It also has significant potential to mitigate greenhouse gas emissions.

Wet soils can produce large quantities of Nitrous Oxide, a powerful greenhouse gas. Much of these emissions may be avoided through adequate agricultural drainage.

The Scottish Government has historically provided funding for the drainage of wet agricultural land. However, there is evidence that the condition of Scotland’s drainage infrastructure has deteriorated in the past 50 years, with little investment currently taking place.

As such, improving agricultural drainage infrastructure could represent a cost effective greenhouse gas mitigation measure.

Through a review of existing data, this report identifies:

• the current state of Scotland’s agricultural drainage infrastructure;
• the potential magnitude of greenhouse gas abatement from installing or improving drainage; and
• the impacts that such activity may have on other climate change mitigation policies.