« Previous | Contents | Next »
Listen
CHAPTER 4 : SOILS IN THE CONTEXT OF CLIMATE CHANGE
Climate and soil formation
4.1 Soils and climate are intimately linked. Climate has a direct influence on soil formation and across many areas of Scotland cool, wet conditions and acidic parent material have resulted in the accumulation of organic matter, and, in many locations the development of peat. Blanket peat covers large areas of Caithness and some of the Western Isles and Highlands. Peat contains the partially decomposed remains of plant litter, which varies according to vegetation type and the degree of decomposition it has experienced.
Soil carbon stocks
4.2 Carbon is the main building block of all vegetation and plant material, which accumulates in soils as partially or fully decomposed organic matter- making all soils that contain organic matter natural stores of carbon. Although the degradation of fresh plant material can be a fast process, organic matter accumulates relatively slowly in soils.
4.3 In many areas of Scotland, however, cold and wet climatic conditions have led to the accumulation of soil organic matter. Scottish soils are therefore particularly rich in organic matter, making them an important carbon store. Scottish soils are estimated to contain approximately 3000 MtC, which is the majority of the soil carbon stock of the whole of the UK. Scottish peat soils are also significant both at European and global scales (Fig.4.1).
Impact of climate change on soils
The future climate
4.4 Climate change scenarios indicate increased rainfall intensity in winter and hotter, drier summers in Scotland 13. Climate change scenarios and emission trends in Scotland are summarised in the consultation for the Scottish Climate Change Bill 14. A changing climate with prolonged periods of dry weather followed by intense rainfall could be a severe threat to Scotland's soil resource.
Figure 4.1 Organic carbon content in topsoils In Europe 15 (after Jones et al., 2003)
Peat decomposition and erosion
4.5 Peat is inherently susceptible to erosion, desiccation and decomposition, for which temperature and rainfall are important triggers. The climatic conditions predicted for Scotland over the next decades are likely to stimulate the desiccation and decomposition of peat. Desiccated peats are vulnerable to erosion and 'disrobed' or bare peats are susceptible to landslides or bog bursts during periods of intense rainfall. Atmospheric nitrogen deposition is likely to accelerate the rate of decomposition of organic matter under a warmer and drier climate 16. Increased rainfall intensity and periodicity may accelerate erosion rates in peat soils in the winter and the risk of wind erosion could be high during dry summers, not only in peat soils, but also other soils with poor vegetation cover.
Changes in land capability
4.6 A changing climate could influence the land capability for agriculture. Whilst large parts of Scotland are currently classed as less suitable for agriculture, a future climate might increase the area of soils classed more favourably, potentially allowing more intensive agricultural production into areas of Scotland currently only managed extensively. This could conflict with soils currently supporting sensitive habitats. (Figure 4.5)
Impacts on soil management
4.7 A changing climate could also impact the workability of mineral soils and susceptibility to poaching, erosion, compaction and water holding capacity. Many soils in the north and east of Scotland could be at increased risk from wind erosion given their sandy texture and use for arable farming. Soil management would need to change under a changing climate to reduce soil loss and reduced fertility. If, as suggested, land capability for intensive agriculture increases in the future, taking into account the above mentioned increased susceptibility to threats would be important for soil management.
Greenhouse gas emission & carbon sequestration
4.8 Not only does climate influence soil properties, but soil regulates climate via the uptake and release of greenhouse gases such as carbon dioxide, methane and nitrous oxide. This is a complex relationship with many feedback mechanisms, shown in figures 4.1 and 4.2. Soil can act as a source and sink for carbon, depending on land use and climatic conditions. Land use change can trigger organic matter decomposition, primarily via land drainage and cultivation. Restoration and re-creation of peatlands can result in increased methane emissions initially as soils become anaerobic, whereas in the longer term they become a sink for carbon as organic mater accumulates.
4.9 There is great uncertainty as to how Scotland's soils, especially peat soils will respond to a changing climate. Given they are sensitive to moisture and temperature conditions, once erosion or decomposition is triggered this can lead to a catastrophic loss of carbon. Climatic factors have an important role in peat formation and it is thus highly likely that a changing climate will have significant impacts on this resource.
Soils and renewable energies
4.10 Biofuels and biomass are part of a portfolio of renewable options that can help Scotland reduce carbon emissions. The Wood Fuel Task Force assesses the potential supply of wood fuel, identifies barriers to its exploitation, and advises on how the resource could best be used. Their recently published report sets out a range of recommendations for developing the wood fuel energy sector. A series of energy forestry trials by Forestry Commission Scotland will evaluate the potential of short rotation coppice and short rotation forestry to increase the supply of wood fuel. The trials will also evaluate the environmental impact of growing energy forest crops, including the longer-term sustainability of soil carbon and nutrient removals alongside the potential for emissions reduction in energy use. 17, 37
4.11 Many peatland sites in Scotland are ideally suited to wind farms given their location and lack of competition from alternative land uses. The Scottish Government's planning policy for renewable energy ( SPP6) recognises that in some instances soil disturbance may lead to the release of carbon stored in soils, which is particularly relevant for soils rich in carbon like peat. It recognises that soil disturbance should be minimised during construction and maintenance of turbine bases, roads, tracks and others infrastructure. A Scottish Government study is currently considering the issue of the carbon trade-offs when developing carbon-saving energy systems on landscapes that store large quantities of carbon.
Climate Change and Food Production
4.12 As a result of increasing pressure from climate change on current key areas of food production, there might be a rising need for increased food production in Scotland. It is widely assumed that agriculture in Scotland might be favoured compared to other countries under future climate change scenarios. The production of food more locally is also being promoted in an attempt to reduce food miles. This means the nutritional quality of food intake by local populations is much more dependent on nature and properties of local soils. Because Scotland does have some localised areas of trace element deficient soils, this deficiency could have implications for the nutritional value of the food produced on such soils.
4.13 At the same time, there is pressure to provide the necessary infrastructure (especially around major urban centres) required for sustainable economic growth. To meet food production and security objectives, there might be the need to afford prime agricultural land more protection.
The Scottish Greenhouse Gas Emission Inventory
4.14 Scotland as part of the UK is required under obligations under the Kyoto Protocol to report on total greenhouse gas emissions in an annual inventory, which includes the Land Use and Land Use Change & Forestry ( LULUCF) sector 18. This is the only sector in the inventory that can be both a source and a sink of greenhouse gases.
4.15 The LULUCF format for reporting is "land based": all land is classified into 6 categories and changes in land use between set time periods have an emission or removal factor associated with them. Net emissions/removals are estimated for activities within the sector, e.g. the activity of land conversion to cropland produces a net emission to the atmosphere, but this figure includes both emissions and removals (from the atmosphere). The sector has a high degree of uncertainty because our scientific knowledge of some parts of the LULUCF sector is still in development.
4.16 Changes in land use disturb the soil and also change the volume of soil inputs, producing a change in soil carbon stocks over time: a gain in carbon stocks draws down carbon dioxide from the atmosphere, whereas a reduction in carbon stocks produces an emission of carbon dioxide into the atmosphere or as dissolved organic carbon to water. This is important for Scotland where many soils have high levels of organic matter, and thus the potential for large emissions. Losses of soil organic matter also occur far more quickly than gains.
4.17 For example, the 2005 Scottish Greenhouse Gas Inventory estimated that soil carbon stock changes in land converted to cropland emitted 6.5 Mt carbon dioxide (15% of Scotland's net carbon dioxide emissions) (Figure 4.4) . In comparison, soil carbon stock changes in land converted from arable to grassland gave a sink of 2.8 Mt carbon dioxide even though the area was larger (1200 kha compared to 1000 kha for land converted to cropland). Many of these emissions/removals are due to historical land use change (pre-1990).
4.18 The above sets out the linkages betweens soil and climate change, and the role of soil management in climate change mitigation and adaptation.
Question 4. Do you agree with our analysis of soils in the context of climate change?
Figure 4.4 Total net GHG emission in 2005 by sector and target for 2050.
Figure 4.5 Location of Prime agricultural land ( LCA class 1,2 and 3.1)
a) current b) prediction 2050's UKCIP02 Med-High Emissions (Macaulay Institute, work in progress)
« Previous | Contents | Next »