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GROUP 5 Soil Erosion and Compaction

Working group members and contributors

Soil Erosion and Compaction

Importance of soil erosion and compaction

6.1 The physical deterioration in Scotland's soil resource includes damage through landslides, erosion and compaction. There are both financial costs inherent in such deterioration, including costs such as loss of crop productivity, repair of road network and ecological costs such as reduced carbon storage and damage to high biodiversity habitats. Minimisation of such impacts should be a key driver in any soil strategy.

6.2 Landslides are the mass movement of soil and substrate down slope, usually following saturation by rainfall or rapid snow melt. Landslides and debris flows are common in the naturally over-steepened hillslopes of Scotland (Ballantyne, 1991, 2002) and on engineering infrastructure such as cuttings and embankments. The mass movement of peat soils can occur on more gentle slopes. Although many landslides occur in remote areas, some can disrupt transport networks and can cause injury and death (Scottish Executive, 2005).

6.3 Soil erosion by wind or water is a natural process where soil particles become entrained and occurs in all soils to some extent. It becomes of concern when the rate exceeds "natural" rates although it is often difficult to distinguish between natural and accelerated rates of erosion. Much accelerated erosion occurs where human activities cause the removal of the protective vegetation cover. The material eroded from soils is generally the more fertile or organic rich topsoil and is also a major contributor to the sediment and phosphate load carried by streams and rivers. Footpath erosion can be severe in some popular areas.

6.4 Soil compaction is the loss of soil pore space (soil structure) generally caused by mechanical stresses from farm machinery and animals or in forested soils. The increasing weight of agricultural and forestry machinery worsens compaction, particularly of subsoils where remediation is difficult. The lower porosity of compacted soils restricts the movement of air as well as drainage and storage of water thus restricting crop growth and making soils more susceptible to erosion. Although ploughing can alleviate topsoil compaction to some extent, it can take decades for soil to recover from repetitive damage and compaction of the subsoil.

Impact on soil functions

6.5 Soils do more than grow crops and support the production of food and fibre, they also provide ecosystem services such as filtering and buffering pollutants, support habitats and gene pools, provide a platform for infrastructure like housing, road and rail links, provide raw material such as sand and gravel and protect cultural heritage. Peat and other organic soils are also an important consideration in climate change mitigation as they contain a significant proportion of the UK's carbon stocks. Any physical degradation in Scotland's soil resource will affect its ability to provide these six functions.

Food and other biomass production

6.6 Soil erosion generally involves loss of the fertile topsoil leading to a potentially significant threat to the biomass production of the soil by reducing crop yields although soil losses in Scotland tend to be localized and often eroded soil is trapped at field boundaries and may be used to refill any erosion gullies. Compaction can increase the strength of soil, making it difficult for roots to penetrate and reducing water holding capacity, which may increase the risk of drought in dry years. Compacted soils are also more prone to erosion especially along 'tramlines' used in arable crop production. Landslides often expose large areas of subsoil or bare rock which restricts plant growth.

Environmental Interactions

6.7 Effects of soil erosion and landslides include loss of carbon storage as soil losses generally come from more organic topsoil (Quinton et al., 2006). Grieve (2000) found that carbon losses of almost 50% can occur in peaty soils when the protective vegetation cover is lost. Erosion of peaty soils may be a contributory factor to recent declines in carbon concentration of upland soils (Bellamy et al., 2005) and to increased concentrations of DOC in upland streams. The release of nitrous oxide (a powerful greenhouse gas) from the soil may be greater due to development of anaerobic conditions in compacted soil (Ball et al., 1999). Off-site impacts of erosion and compaction include increased flood risk and runoff of organic wastes and agrichemicals (Jones et al., 2003), contamination from nutrients and pesticides adsorbed on eroded sediment (Ball et al., 1997) siltation and reduced capacity of water-supply reservoirs and the loss of fish spawning areas through the deposition of fine sediment on river-bed gravels.

Biodiversity

6.8 Compaction and loss of large pore space between aggregates leads to a significant loss of habitat for larger soil fauna such as earthworms while breakdown of aggregates increases the accessibility of carbon compounds to bacteria and fungi. Birds and other wildlife find it more difficult to forage when surface soil is compacted. Wind erosion can remove sandy soils and destroy habitats and although some systems are naturally dynamic, the removal of the vegetation cover by overgrazing and atmospheric pollution may contribute to the more recent re-activation of wind erosion.

Provision of a platform, provision of raw materials and protection of cultural heritage

6.9 Erosion can damage the built infrastructure by undermining foundations and depositing sediment, for example in reservoirs. The landslides which were triggered by extreme rainfall during the summer of 2004 caused significant damage to parts of the trunk road network.

6.10 Large scale erosion of peat represents the major threat to the soil's function in providing raw materials. The loss of soil poses a threat to buried archaeological features by exposure. In cultivated soils, water, wind and tillage erosion (downslope movement of soil due to repeated ploughing across slope) increase the potential for plough damage to buried artefacts.

Evidence of physical deterioration in Scottish soils

6.11 There is a considerable body of evidence on soil erosion in Scotland but this evidence lacks cohesion, is often sparse or event driven and dated. There is little evidence available to assess the extent or degree of soil compaction.

6.12 Evidence of soil erosion was recorded at each National Soils Inventory of Scotland ( NSIS) sample at 5km intervals. Although an objective sample, it was spread over a 10 year period and sampling was predominantly done during the summer when evidence of soil erosion on arable soils would be masked by crop cover. Land Cover of Scotland ( LCS88) was based on aerial photograph interpretation and recorded the extent of eroded blanket bog and montane vegetation, but provides no other assessment of erosion ( MLURI, 1993). Grieve et al. (1994; 1995) quantified erosion of upland soils based on a sample of 20% of upland Scotland using the same photographs. All three studies estimated the extent of peat erosion to be 6-7.5% of Scotland and both Grieve et al. (1994; 1995) and the NSIS showed that erosion affected 12-14% of Scotland. However, much of the information from these studies is on the location and extent of erosion with little quantification of soil loss.

6.13 A few case studies reported in scientific papers generally give rates of erosion though these tend to be primarily from lowland, cultivated soils and occur during extreme, localised events. They do quantify the potential severity of the problem. Davidson & Grieve (2004) summarise data from event-based studies from the Borders (Frost & Speirs, 1984; Davidson & Harrison, 1995) and Angus (Duck & McManus, 1987 and 1988; Kirkbride & Reeves, 1993).

6.14 Suspended sediment concentrations in rivers have been measured by the Scottish Environment Protection Agency ( SEPA) as part of the Harmonised Monitoring Scheme at approximately monthly intervals since the early 1970s. However, sampling sites are near river mouths and tend to underestimate total sediment load as high flow events are not adequately sampled.

6.15 The Scottish Road Network Landslides Study (Scottish Executive 2005) provides the most recent analysis of the occurrence and risk of landslides in Scotland although the study was focused on assessing the potential for landslides occurring adjacent to the road network. Ballantyne (2002) reviewed the changing frequency of debris flows over time and updated Innes' earlier mapping of the occurrence of debris flows.

6.16 There is a lack of evidence on the extent of soil compaction in Scottish soils partly driven by the closure of the Scottish Institute of Agricultural Engineering in the 1990s. Apart from the qualitative descriptions collected in the NSIS, there are no national data on compaction. The original NSIS did not measure soil bulk density or quantify soil structure, however, these will be measured in the limited resampling programme now underway.

6.17 A European study (Jones et al., 2003) would suggest that subsoil compaction could be a major problem in Scotland due to a combination of soil and rainfall patterns. Locally, subsoil compaction from machinery traffic was found to be a problem on SCRI's experimental farms. Although not entirely typical of general management practices, it may indicate that subsoil compaction may be becoming a problem

6.18 Approaches to the assessment of soil compaction have been developed, based primarily on geotechnical engineering principles (e.g. Gregory et al., 2006) but more research using a combination of field assessments and laboratory studies are required to identify the risk to soil function by compaction in Scotland. An assessment and/or modelling of compaction similar to that developed by Sparling & Schipper (2004) for New Zealand may be applicable to Scotland to assess the extent of compaction in agricultural and forest soils.

6.19 Landslides, wind and water erosion and compaction are all sensitive to climate change. There are predicted increases in the variability in precipitation patterns across Scotland and in the intensity of rainfall events (Barnett et al., 2006). Erosion by both wind and water may be exacerbated if these predictions are correct (prolonged dry periods leading to wind erosion on vulnerable soils and intense rainfall events increasing the risk of soil erosion by overland flow and debris flows). Less predictable rainfall patterns may mean that soil compaction becomes more likely if farmers and other land users are forced to cultivate, spray or harvest when soil moisture contents are unfavourable.

6.20 Soils on river floodplains may also be vulnerable to erosion through increased frequency and intensity of flooding (Werritty et al., 2002; Werritty & Chatton, 2004).

Impact of Policies and advice

6.21 There are a number of policies or advice in place to limit soil erosion and soil compaction, some are voluntary, for example, the Farm Soils Plan which offer practical advice to farmers on how to maintain soil structure and limit erosion losses. Other forms of advice include the guide to better soil structure ( NSRI, 2001) and the visual soil structure quality assessment (Ball et al., 2007). Defra have produced a detailed advisory leaflet on the prevention of soil erosion in England and Wales (Defra, 2005) in which much of the advice is also applicable to Scotland. Scotland has a good extension service network ( SAC) though which such advice can be channelled. Policies can be linked to direct payments, for example, GAEC (Good Agricultural and Environmental Condition) ( SEERAD, 2006) where the maintenance of organic matter contents to maintain good soil structure, avoid compaction and the prevention of erosion by management practices are linked to farm subsidies.

6.22 Forests and Soil Conservation Guidelines (Forestry Commission 1998, update in press) and the Forests and Water Guidelines include measures to limit the movement of eroded soil from a site to watercourses and the damage to soil structure by the heavy machinery used in the forestry industry.

6.23 Guidelines for muirburn (designed to regenerate growth of heather) are also available and burning of eroding peat or where the soil is thin is not permitted (Scottish Executive, 2001). GAEC offers guidelines for the prevention of overgrazing thus minimising the risk of soil erosion in upland grazing systems. However, GAEC is difficult to apply in the uplands due to the large area to be inspected and there are few other policies or guidelines to address the issue of upland peat erosion including mitigation and the potential for restoration. Many of these policies only indirectly protect peat from erosion.

6.24 Some policies, such as the Freedom of Access (Land Reform (Scotland) Act 2003) may exacerbate compaction and lead to footpath erosion in some popular areas.

6.25 Despite such policies, several concerns remain:

• No-tillage techniques are highly effective at controlling erosion because the presence of crop residues at the surface reduces the effect of raindrop impact on the soil and the increased continuity of the pore system decreases surface runoff. Although used extensively in the Americas and Australia (Baker & Saxton, 2007) these are less suited to Scotland where the moist climate can restrict the period when the soils are sufficiently dry to bear the weight of the machinery and can thus lead to increased compaction unless exceptionally high levels of management are employed (Ball et al., 1994).
• The off-site effects of the impact of soil erosion on water quality (in particular where nutrients and pesticides are bound to the sediment) are indirectly considered within the Water Framework Directive ( WFD). However, there is no standard for suspended sediment concentrations. This Directive has led to policy initiatives which currently protect soils such as the Forests and Water Guidelines.
• Soils and soils management have a vital role to play in flood risk management. A proposal by the Scottish government to introduce a Flood Bill would produce legislative, process and structural changes in flood management in Scotland such as the involvement of land-use in flood management. However, the role of soils, in particular, compaction, in flood risk is poorly understood.
• The development of windfarms on upland peats results in a significant short-term increase in dissolved organic carbon losses, erosion and sediment loss from the site due to construction of roads and foundations while the longer-term effects in terms of increased erosion, potential for land slides and carbon loss remain unquantified. Such new developments have many parallels with commercial afforestation, it may be that the Forest and Water Guidelines can be applied here to minimise adverse impacts.
• Erosion of agricultural land can occur where there is sufficient run-on either from flow accumulation along roads or from fields upslope. This often happens where road drains and culverts become blocked thus off-site management of run-on may be important in some areas.

References

Baker, C.J. and Saxton, C.E. 2007. No-tillage seeding in conservation agriculture. 2nd Edition, CABI/ FAO, Italy, 320 pp.

Ball, B.C., Batey, T. and Munkholm, L.J. 2007. Field assessment of soil structural quality - a development of the Peerlkamp test. Soil Use and Management (in press).

Ball, B.C., Campbell, D.J., Douglas, J.T., Henshall, J.K. and O'Sullivan, M.F. 1997. Soil structural quality, compaction and land management. European Journal of Soil Science 48 (4): 593-601.

Ball, B.C., Lang, R.W., Robertson, E.A.G. and Franklin, M.F. 1994. Crop performance and soil conditions on imperfectly drained loams after 20-25 years of conventional tillage or direct drilling. Soil & Tillage Research 31: 97-118.

Ball, B.C., Scott, A. and Parker, J.P 1999. Field N ₂O, CO ₂ and CH ₄ fluxes in relation to tillage, compaction and soil quality. Soil & Tillage Research 53 (1): 29-39.

Ballantyne, C.K. (1991) Holocene geomorphic activity in the Scottish Highlands. Scottish Geographical Magazine. 107: 84-98.

Ballantyne, C.K. 2002. Geomorphological changes and trends in Scotland: debris flows. Scottish Natural Heritage Commissioned Report, No. 052 .

Barnett, C., J. Hossell, M. Perry, C. Procter and G. Hughes (2006) Patterns of climate change across Scotland: Technical Report.SNIFFER Project CC03, Scotland & Northern Ireland Forum for Environmental Research, 102pp.

Bellamy, P.H, Loveland, P.J., Bradley, R. I., Lark, R.M and Kirk, G.J.D. (2005) Carbon losses from all soils across England and Wales 1978-2003. Nature. 437:245-248

Davidson, D.A. and Grieve, I.C. (2004) Trends in soil erosion. Scottish Natural Heritage Commissioned Report F00AC106.

Davidson, D.A. and Harrison, D.J. (1995) The nature, causes and implications of water erosion on arable land. Soil Use and Management. 11: 63-68.

Duck, R.W. and McManus, J. (1987) Soil erosion near Barry, Angus. Scott. Geog. Mag. 103: 44-46.

DEFRA (2005) Controlling Soil Erosion. Advisory leaflet.
http://www.defra.gov.uk/environment/land/soil/pdf/soilerosion-lowlandmanual.pdf

Duck, R.W. and McManus, J. (1988) A flood deposit in an emptied Scottish reservoir. Scott. Geog. Mag. 104: 167-170.

European Commission (2006) EU Communication of the Commission, Establishing a framework for the protection of soil and amending Directive 2004/35/ EC. ( COM (2006)232).

Farm Soils Plan:
http://www.sac.ac.uk/mainrep/pdfs/farmsoilsplandec.pdf

Forestry Commission. (2003) Forests & Water Guidelines. Forestry Commission, Edinburgh. i-vi + 1-66pp.

Frost, C.A. and Speirs, R.B. (1984) Water erosion of soils in south-east Scotland - a case study. Research and Development in Agriculture. 1: 145-152.

Gregory, A.S., Watts, C.W., Whalley, W.R., Kuan, H.L., Griffiths, B.S., Hallett, P.D. and Whitmore, A.P. 2007. Physical resilience of soil to field compaction and the interactions with plant growth and microbial community structure. European Journal of Soil Science (in press).

Gregory, A.S., Whalley, W.R., Watts, C.W., Bird, N.R.A., Hallett, P.D. and Whitmore, A.P. 2006. Calculation of the compression index and precompression stress from soil compression test data. Soil & Tillage Research, 89, 45-57.

Grieve, I.C. (2000) Effects of human disturbance and cryoturbation on soil iron and organic matter distribution and on carbon storage at high elevations in the Cairngorm mountains, Scotland. Geoderma. 95: 1-14.

Grieve, I.C., Davidson, D.A. and Gordon, J.E. (1995) Nature, extent and severity of soil erosion in upland Scotland. Land Degradation and Rehabilitation. 6: 41-55.

Grieve, I.C., Hipkin J.A. and Davidson D.A. (1994) Soil erosion sensitivity in upland Scotland. Scottish Natural Heritage Research, Survey and Monitoring Report, No.24.

Jones, R.J.A., Spoor, G. and Thomasson, A.J. 2003. Vulnerability of subsoils in Europe to compaction: a preliminary analysis. Soil & Tillage Research 73: 131-143.

Kirkbride, M.P. and Reeves, A.D. (1993) Soil erosion caused by low-intensity rainfall in Angus, Scotland. Applied Geography. 13: 299-311.

Kuan, H.L, Hallett, P.D., Griffiths, B.S. Gregory, A.S., Watts, C.W. & Whitmore, A.P. 2007. The biological and physical stability and resilience of a selection of Scottish soils to stresses. European Journal of Soil Science (in press).

MLURI. (1993) The Land Cover of Scotland 1988. Macaulay Land Use Research Institute. Aberdeen

NRSI (2001) A guide to better soil structure. National Soil Resources Institute, Cranfield University, Silsoe, UK.

Prevention of Environmental Pollution From Agricultural Activitiy Code ( PEPFAA). SEERAD, 2005.
http://www.scotland.gov.uk/Resource/Doc/158530/0042981.pdf

Quinton, J.N., Catt, J.A., Wood, G.A. and Steer, J. (2006) Soil carbon losses by water erosion: experimentation and modelling at field and national scales in the UK. Agriculture Ecosystems and Environment. 112: 87-102.

Scottish Executive (2001). The Muirburn code.
http://www.scotland.gov.uk/Resource/Doc/158517/0042975.pdf

Scottish Executive (2005) The Scottish Road Network Landslides Study. M G Winter ( TRL Limited), F Macgregor and L Shackman (Scottish Executive) (eds)

Scottish Executive. (2006) Cross Compliance Notes for Guidance 2006.
http://www.scotland.gov.uk/Publications/2005/12/0990918/09199

Sparling, G. and Schipper, L. 2004. Soil quality monitoring in New Zealand: trends and issures arising from a broad-scale survey. Agric. Ecosys. Environ. 104: 545-552.

Towers, W., Grieve, I.C., Hudson, G., Campbell, C.D., Lilly, A., Davidson, D.A., Bacon, J.R, Langan, S.J. and Hopkins, D.A. 2006. Scotland's Soil Resource - Current State and Threats. Report submitted to SEERAD.
http://www.scotland.gov.uk/Publications/2006/09/21115639/0

Werritty, A., Black, A.R., Duck, R.W., Finlison, W., Thurston, N., Shackley, S. and Crichton, D. 2002. Climate Change: Flooding Occurrences Review. Central Research Unit, Scottish Executive, Edinburgh.

Werritty, A. and Chatterton, J. 2004. Future Flooding: Scotland. Foresight Programme, Office of Science and Technology, London.

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