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Chapter 11 Discussion and Conclusions

Individual chapters of this report have focussed on the major threats to soil function outlined in the introductory chapter and reviewed the evidence for the significance of that threat or group of threats to soil functioning in Scotland. In this chapter we summarise the key findings from each chapter and then attempt to assess the relative importance of each threat in the context of the soil resource of Scotland.

11.1 Summary of main threats to functions of soils in Scotland

11.1.1 Loss of soil organic matter

Loss of soil organic matter is potentially a very serious threat to the soils of Scotland. Evidence from a large number of field and laboratory studies indicates that organic matter influences many soil properties, particularly linked to structure and risk of erosion and compaction, fertility and crop production, soil biota and their diversity and remediation of pollutants. In addition, loss of organic matter directly reduces carbon storage in the soil with potential consequences for climate change. While losses of organic matter from agricultural soils can be reversed by addition of organic wastes, loss from upland peaty soils will be much more difficult to reverse.

Effects of any loss are likely to be seen in all soils, but there are at present few data from which effects can be determined. Carbon and organic matter concentrations in Scottish soils are available from national data sets, but there are uncertainties linked to method of data collection and soil heterogeneity and there is also a lack of any trend data. A recent study in England and Wales, based on an extensive programme of resampling soils at sites on the National Soils Inventory, suggested that there have been significant decreases in soil organic matter concentrations in these countries recently. It is therefore strongly recommended that similar data on soil organic matter in Scotland are collected so that any changes can be quantified.

11.1.2 Climate change

The consequences of climate change are difficult to predict, but potentially serious in the context of current scenarios. Increased rainfall may increase flooding, erosion and compaction, while decreased rainfall may lead to drought and loss of inherent fertility. Climate change may also reduce organic matter concentrations. Changes in temperature and moisture may increase emissions of greenhouse gases, particularly from upland peat soils. The level of uncertainty over climate change effects is high, both in terms of the predicted changes and their impacts. This is primarily due to uncertainties over predictions of climate change as effects of climate on soil are known with more certainty.

Direct effects of climate change are likely to be seen in all soils. Indirect effects such as effect on water holding capacity or workability are most relevant to agricultural soils, although effects of changed rainfall intensity could increase erosion risk in all soils. Although the global effects are only likely to be reversible in the long term, local management strategies are important in minimising greenhouse gas emissions and in mitigating effects such as increased droughtiness or erosion hazard.

11.1.3 Reduction in biodiversity

Reductions in soil biodiversity constitute a reduction in the inherent value of soil, since one of its key functions is maintenance of biodiversity. The consequences are relevant for all soils, but different soils will be impacted in different ways depending on the specific threat. There is generally a low level of understanding and relevant datasets are scarce leading to a high level of uncertainty over this threat. Protecting soil biodiversity as a component of habitats under conservation protection is for the moment the only practical way of ensuring that such biodiversity is not lost. This is, however, a rapidly developing area of soil science and new microbiological and molecular techniques are constantly adding new knowledge to our understanding of soil biodiversity and its functional role.

Some impacts on soil biodiversity are generally difficult to reverse and involve intervention through habitat restoration projects. Loss of specific species and functions such as nitrogen-fixing Rhizobium may be mitigated by use of commercial inoculum, but at increased cost. Other forms of contamination may be amenable to remediation or management of pollutant availability ( e.g. liming to reduce metal availability).

11.1.4 Structural degradation and compaction

Evidence from case studies indicates that structural degradation and compaction reduces plant growth and yield, reduces large pore space causing loss of habitat for larger soil fauna and potentially increasing N ₂O losses from soil and reduces water holding and infiltration capacity of the soil thereby increasing flooding and erosion risk. The overall extent is largely unknown but data that do exist suggest that it is of localised occurrence and affects only a small proportion of soils.

Most damage can be avoided by appropriate soil management techniques set out in good practice guidance in both the agriculture and forestry sectors. Case studies have shown that compaction of agricultural soils is reversible. Structural degradation and its reversal are closely linked to maintaining organic matter concentrations. The worst effects on forest soils can be avoided by strict adherence to guidelines.

11.1.5 Soil erosion and landslides

On-site effects of erosion on biomass production functions are localised. However, the erosion of peat represents a significant threat to the carbon storage function of upland soils and erosion also represents an irreversible threat to buried archaeological features. The most significant off-site impact of erosion in lowland areas is the increased transfer of sediment and associated nutrients to surface waters, reflecting a reduction in the filtering function of the soil. Erosion events with high magnitude can have serious off-site effects such as road blockages with potentially fatal consequences.

The overall incidence of erosion on lowland soils is not known with any certainty, but case studies have provided a good understanding of the factors triggering erosion. There are a number of best management practices that help to minimise the likelihood of erosion and the transfer of sediment to surface waters. Re-cycling of organic matter to soil can also improve soil physical conditions and reduce erosion in these soils. Studies have also shown that erosion of peat is the most extensive type of erosion in upland areas. Areas where erosion is active are smaller but the damage is difficult to rectify. Given the importance of peat soils, research into the drivers and mechanisms of upland peat erosion is needed for their protection.

11.1.6 Soil contamination

Effects of contamination are variable and depend on the source (diffuse or point) and nature of the contaminant. Impacts vary from total loss of soil function in highly polluted sites to lesser impacts over large geographic areas. Off-site effects on surface waters are also important. Acidification by sulphur deposition of sulphur is relatively well understood and characterised spatially, but impacts of nitrogen deposition are the subject of current research. Our understanding of pesticides is relatively good but there is little information on other organic contaminants. National data sets and research studies have provided reliable information on the background levels of heavy metal concentrations in soil and our understanding of their impacts on soil microbiology is the subject of current research.

There is some evidence that soils are recovering from the impacts of atmospheric deposition of sulphur. However, there is less evidence for nitrogen deposition and any recovery is likely to be long-term (decadal). Heavy metal pollution is essentially irreversible. Legislative protection and precautionary principles can prevent build up but some revision of maximum statutory and guideline limits for metals ( e.g. zinc) may be needed based on emerging research findings. Management by liming and organic matter additions can also reduce the availability of some contaminants. Several remediation options are available for discrete problems which affect small areas of contaminated land.

11.1.7 Loss of soil to development and mineral extraction

Loss of soil to development eliminates all ecological soil functions. However, there is limited evidence on which to assess the scale of this threat. It has a high local impact and is extensive in the existing populated areas particularly in the Central Belt. Based on the available data, loss of land to development is currently happening at its fastest rate within the last 25 years; current rates are double those in the early 1980s and early 1990s. However, sealing is poorly characterised in terms of scale and location. Although the total area developed can be calculated, the proportion of soil sealed is not known accurately as there are no data for how much soil remains in open spaces and gardens. The quality of land lost to development is also not recorded. Since loss of land to development is largely irreversible as far as natural soil functioning is concerned, it is essential that a better system of recording such losses is put in place.

11.1.8 Threat to soils as a cultural resource

These arise from a number of the other threats including loss of organic matter, soil erosion, soil sealing and climate change. Damage to our cultural heritage diminishes society's appreciation of the past and may impact on Scotland's cultural identity.

Cultural soils occur in small areas and archaeological sites although large in number are not extensive. However, the precise extent of cultural soils is not known and although individual areas are small, it is becoming clear that they occur around a larger number of Scotland's settlements than first thought. There is evidence from case studies that our archaeological record has been reduced in number and extent over the last 150 years with most loss and damage in arable areas. These losses must be regarded as serious because, once lost or damaged, archaeological features, artefacts and cultural soils cannot be restored.

11.2 Comparing the relevance of threats to Scotland

To determine the overall importance of each threat for each function we scored the consequence, extent, uncertainty and reversibility on a simple three point scale. Details of the methodology we adopted and the results of the analysis are given in Appendices E1 and E2. Table 11.1 is a summary of the relative ranking of the threats, with the scores normalised to 1.

Table 11.1 Comparison of threats across all soil functions

Table 11.1 represents the general ranking of the state of knowledge about each threat in relation to each function. More information is given in Appendix E2. The scores are subjective but are based on the expert judgement of the project team. Summation of scores across the different functions implies that all functions are equally important. This is clearly not the case and importance will vary for different stakeholders. However, when the biomass, environmental interaction and ecosystem support functions were given an increased weighting, the general ranking of functions did not change significantly (Appendix E2).

Among the key findings in Table 11.1 are:

• Climate change and loss of organic matter emerge as the most significant threats to the functioning of Scottish soils. Both affect most soil functions with impacts which are national in their spatial occurrence and which are difficult to reverse. However, there are generally greater levels of uncertainty associated with these threats.
• Loss of land to development (sealing), loss of biodiversity and acid deposition also represent significant threats to soil function. Sealing affects almost all soil functions whereas the impacts of loss of biodiversity and acid deposition mainly affect the ecological functions of the soil.
• The threats most commonly associated with cultivation (erosion, loss of structure, compaction) were not judged to pose high risks at the national scale. However, these do pose a threat locally and can have significant impacts such as loss of peatland habitat or damage to subsurface archaeological features.
• Threats from contamination by heavy metals or by land management are also significant but again the overall risk is assessed as smaller and more localised.
• Most of the threats are relatively close together in terms of their assessed impact; only compaction and salinisation have low normalized scores.
• There is no current threat from salinisation to Scottish soils.

11.3 Overall conclusion

The adopted risk methodology and framework is preliminary but provides the first systematic assessment of the relative importance of threats to soils in Scotland. Whilst qualitative in approach, it provides a structure for further debate and a focus for potential policy development around specific key issues. We believe that it provides a robust method to assess threats at the national scale, based on our current understanding, and has the potential to be developed and refined as more data become available.

A common thread in the assessment of every threat was the lack of systematic baseline data in some cases and a lack of trend data for nearly all cases. Long term baseline data from field sites and/or national datasets are clearly essential for the assessment of future change and trends. Many states in the western world (Canada, USA, as well as England and Wales) have fully functioning Soil Information Systems to report on soil monitoring that serve the needs of various stakeholders such as environmental and heritage agencies. Scotland is fortunate to have one of the best soil databases in the world but this has not yet been developed into a soils information system to meet the increased demands for evidence and environmental reporting by regulatory and conservation agencies. There is a clear need to review this issue and we recommend that Scotland develops its own Soils Information System. It is vital that the development of such a system is widely discussed among stakeholders to gain a clear view of its purpose, structure and internal components. It should also bring together the appropriate evidence from a range of sources and provide access to make it available to stakeholders including researchers and policy makers to enhance our understanding and ability to protect Scotland's invaluable soil resource. Such consultation may also afford an opportunity to join up data on our environment with data on the health of Scotland's population.

Scotland's soil supports first class agricultural, horticultural and forestry industries and in addition underpins its beautiful landscapes and habitats of national and international renown. It also makes the biggest contribution to terrestrial carbon storage in the UK. The status of Scotland's soil is good but we have insufficient evidence to assess recent changes and therefore predictions of change are equally uncertain. This remains the greatest challenge in the coming years for our community of scientists, policy officers, land owners and managers and regulatory agencies.

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