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Scotland's Soil Resource - Current State and Threats

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SCOTLAND'S SOIL RESOURCE - CURRENT STATE AND THREATS

Executive Summary

Objectives

The overall aim of this report is to collate information about the state of Scottish soils and the pressures that affect their ability to support the range of vital functions upon which we rely. These include:

  • Providing the basis for biomass production from our agricultural and forestry industries.
  • Present and future existence of nationally and internationally valued habitats.
  • Regulating our water supply and protecting it from contamination.
  • Storing carbon.
  • Sustaining biodiversity.
  • Providing a foundation for buildings and roads.

The review is a critical assessment of the current evidence. The report sets out to identify important information gaps and make suggestions and recommendations as to how these might be addressed.

Scotland's Soil Resource

Soil is essentially a non-renewable resource and is fundamentally one of Scotland's most important assets. Its most widely recognised function is in supporting plant growth but sitting as it does at the interface between the atmosphere, biosphere and underlying rocks it is increasingly recognised for other environmental and ecosystem benefits. It is against this background that the concept of soil quality has been developed. Here soil quality is viewed in the wider context of 'fitness for purpose' for the range of functions that we expect soils to perform.

Scotland's soils are diverse and differ markedly from those in the remainder of the UK. The majority have acidic and organic-rich surface layers including large areas of blanket bog up to 8 metres thick. Such soils are often not managed intensively and play important roles in nature conservation, biodiversity and carbon storage and make a highly significant contribution to landscape value. In contrast, soils suitable for arable cropping are limited largely to eastern Scotland. Although relatively small in extent this land has produced some of the highest yields of wheat and barley in the world. Lowland soils in the west of Scotland support very productive pastures and a successful dairy industry.

Based on existing information, Scottish soils are generally of good quality. Only a few soils have high levels of contamination and levels in the remainder are generally low. There is little evidence to suggest that serious soil erosion, compaction or other problems related to land management are occurring widely. Some soils are particularly sensitive to acid inputs, but there is some evidence that this problem is less now. The validity of these statements is however reliant on good, recent soils data, and many of the existing national datasets are around 20-30 years old. Work to determine whether key soil properties (or indicators) are changing with time has begun. A systematic resampling of Scottish soils will start in 2007 but will not be completed until 2009. This initiative together with existing data will give an indication of whether soil properties in Scotland are actually changing. These data are components of the soil information system recommended to augment the data and evidence base.

Key findings and recommendations

This report has systematically and objectively reviewed the available evidence of the current status of Scotland's soils and the threats to these soils. On the basis of this review we have identified both potential and actual threats to Scottish soils and issues pertaining to how we might collate information describing these. The key findings related to specific threats and on some wider issues are given below.

Loss of organic matter

There is some evidence that levels of organic matter in Scottish soils may be declining. If the findings of a large study in England and Wales are replicated here, this could represent a very significant reduction in the UK stock of terrestrial carbon.

Recommendation

The status and change in soil organic carbon content in Scottish soils should be determined as a priority. SEERAD have already committed funding to achieve this within their research programme, but opportunities to expand this should be explored.

Climate change

It is very difficult to predict what might happen to our soils given the uncertainty attached to climate models. The effects might range from direct impacts on key soil properties, for example, soil organic matter content to indirect ones that affect soil management.

Recommendations

1. The status of and change in soil organic carbon in Scottish soils should be determined as a priority.

2. Any proposals for irreversible change of use to agricultural land should be carefully considered; where possible land of poorer quality should be substituted.

Loss of Biodiversity

The diversity of life (invertebrates and microorganisms) in soils is vast and un-explored. Soil biodiversity is therefore a true scientific frontier. The major impediment to evaluating any loss in biodiversity is the lack of systematic data that describes its current status, how it varies spatially and temporally as well as the key links between biodiversity and function. Work has already identified strong relationships between rare or valued habitats and rare and valued soils in Scotland and in the short term, this offers the best opportunities of filling this knowledge gap. There is evidence that contamination by heavy metals may alter and reduce specific components of the microbial community.

Recommendations

1. Further work is recommended to increase both our knowledge of soil biodiversity on designated sites and to increase our awareness and understanding of the role of soils in valued habitats.

2. Where there is evidence of biodiversity loss due to contamination or invasive species a full ecological risk analysis should be undertaken and a precautionary approach be adopted

3. Consideration should be to explicitly include soils in site designation criteria and in habitat action plans.

Structural degradation and compaction

Although compaction and structural degradation does occur on cultivated soils the incidence is localised and there is no clear evidence that these pose serious threats to soil quality nationally. In most circumstances the problem can be readily reversed.

Recommendation

Consideration should be given to the establishment of protocols to determine structure and compaction as part of the assessment procedures for farm monitoring of the GAEC requirements. The guidelines established for the Forestry sector provide a useful starting point. Such protocols would help fill the gaps in knowledge on the extent and impact of compaction.

Soil Erosion

Although soil erosion does occur on cultivated mineral soils and the impacts can be very visible and damaging, single events are confined to small areas. There is no clear evidence that it poses serious threats to soil quality and can be readily rectified. Erosion of organic soils is more evident and potentially could increase in frequency and severity under certain climate change scenarios.

Recommendations

1. Consideration should be given to the establishment of protocols to determine the threat and incidence of soil erosion as part of the assessment procedures required for farm monitoring of the GAEC requirements.

2. More research is required into the mechanisms that trigger peat erosion and into developing mitigation strategies to help reduce its impacts.

Soil Contamination

Based on current evidence most Scottish soils are not heavily contaminated, but there is some emerging evidence that sewage sludge application may be having a negative impact on the long term fertility of some soils. Some historic contamination of soil, for example from acid deposition is showing some signs of recovery.

Recommendation

Given the preliminary status of the evidence, a precautionary approach is recommended with reference to the application of sewage sludges relatively high in zinc to soils.

Soil sealing and mineral extraction

Based on the evidence available, agricultural land is being developed at twice the rate as in the mid 1990s. Soil sealing has a profound effect on the ability of soils to perform other functions and is effectively irreversible. Based on previous data, this development is likely to have occurred on some of our most versatile and productive soils.

Recommendation

In the context of sustainable development and use of our natural resources, it is recommended that information on the area of land developed, its location and the quality of the land be collected systematically.

Cultural Heritage

Cultural soils occur in small areas and archaeological sites, although large in number, are not extensive. The main threats are from erosion and sealing. Recent research indicates that cultural soils occur around a larger number of Scotland's settlements than first thought. There is also evidence that our archaeological record has been reduced in number and extent over the last 150 years. Such losses are irreversible.

Recommendation

The effectiveness of existing policies should be evaluated to provide guidance for the protection of cultural soils, particularly in areas subject to soil sealing. This needs to be supported by a better database for these soils.

Salinisation

The threat from salinisation, which has been identified as significant in a European context, does not currently represent a significant threat to soils in Scotland.

Relative significance of threats to Scottish soils

We used a simple scoring system to rank threats according to their relevance to all soil functions. Threats from erosion, compaction and contamination (other than acidification) were judged to be of localised significance, although they can lead to loss of important functions. They were also assessed as being relatively straightforward to rectify. Sealing, loss of biodiversity and acidification were scored more highly as threats nationally, with sealing affecting almost all soil functions. Climate change and loss of organic matter were identified as the most significant threats to soil functioning, although there is much uncertainty in the evidence here.

The data and evidence base

For a number of the identified threats, there is a lack or absence of data upon which to make robust conclusions. In particular there is a lack of trend data from which evidence of change in, and damage to, our soils might be determined.

Recommendations

1. A soil information and monitoring system should be established for Scotland. This would provide the platform for the integration of existing and new data and the information necessary for environmental reporting.

2. Consideration should also be given to the establishment of a system similar to the Representative Soil Sampling scheme in England and Wales to ensure that the level of soil nutrients in cultivated land is being maintained at levels suitable for crop growth without compromising the environment.

Overall conclusion

The soil resource of Scotland has developed over many millennia and continues to do so under both natural and human influences. Our soils are generally in good health and this, in large part, has resulted from the sustainable management systems employed by land managers over a prolonged period. A series of threats and pressures have been identified within this report and a number of recommendations made. Some of these relate to filling data gaps in our knowledge of the scale of these threats while others identify gaps in our fundamental understanding of processes that contribute to certain threats. Our analysis of the significance of the threats to Scottish soils has identified two linked threats, climate change and loss of organic matter, as the most significant. Notwithstanding the uncertainties associated with this judgement, there is increasing evidence of the need to safeguard our soil resource for future generations.