Lizzie Young @FTF2013


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Lizzie Young @FTF2013

  1. 1. Soil Sensitivity to Wind Erosion in theMachair Landscape of South Uist, Outer Hebrides Elizabeth J. Young1,2 Supervisors: Sue Dawson1 & Blair M. McKenzie2 1Geography (School of the Environment), University of Dundee 2The James Hutton Institute, Invergowrie
  2. 2. Introduction Machair soils: • Low aggregate stability • Organic matter content generally < 10% • Sandy texture • Exposed to highest mean winds speeds in UKFigure 1. Field at Cille Pheadair showing cultivationextending to dune crest, and sand deposition.Figure 2. Idealised machair cross-section (from Angus, 2001).
  3. 3. MethodsSamples rotary sieved at 7 rpmfor 300 s , mesh size = 850 µm.•Erodible fraction = soil passingthrough sieve within 40 s•Potentially erodible fraction =soil passing through sievebetween 41-300 s•Non-erodible fraction = soilremaining on sieved after 300s Figure 3. Simplified rotary sieve, after Tisdall et al. (2012).Analysis of soil properties:FTIR, XRF, pH, particle size, water drop penetration time, water repellency.
  4. 4. Results 100 90 80 % of 40 g soil sample 70 60 Figure 4. Graph showing % of soil 50 Erodible samples in each class (erodible, non- 40 Potentially erodible erodible, potentially erodible) with 30 Non-erodible distance from the coast. 20 10 0 0 15 30 45 70 130 185 220 260 290 Distance from the coast (m) 100 90 80 70Figure 5. Graph showing % of 60soil sample which could pass 50 % < 850 µmthrough the rotary sieve 40 sediment passed through sieve(particles < 850 µm), and % 30of sediment which did pass 20through the sieve. 10 0 0 15 30 45 70 130 185 220 260 290 Distance inland (m)
  5. 5. Human resilience toenvironmental change? Ploughing close to the dune crest exposes soil with very low wind abrasion resistance to high wind speeds. Wind erosion in the dunes can be locally severe (Ritchie, 1971), leading to blowouts, agricultural disruption (Seaton, 1968; Angus, 2001) and can threaten archaeological remains (Moore et al., 2005). Figure 6. Photo of exposed machair front at Staoinebrig, where there is no protective dune cordon. Figure 6. Machair profile at Cille Pheadair showing the decrease in elevation from the dune crest (at left of image). Profile from LiDAR dataset © SNH on behalf of the Western Isles Data Partnership.
  6. 6. Conclusions• Machair soils are very susceptible to wind erosion due tolow organic matter content, low aggregate stability, andexposure to high wind speeds.•At Cille Pheadair there is a positive correlation betweenwind abrasion resistance and distance from the coast.•pH, water repellency, and the ratio of carbonate:silicatesand may be correlated with wind abrasion resistance.
  7. 7. References• Angus, S. 2001. The Outer Hebrides: Moor and Machair. The White Horse Press, Cambridge.• Moore, H., Wilson, G., Dawson, A.G., Dawson, S. 2005. Western Isles (South) Coastal Zone Assessment Survey, Grimsay, Benbecula, and South Uist. Commissioned report for Historic Scotland and Scottish Coastal Archaeology and the Problem of Erosion. EASE Archaeological Consultants, Edinburgh.• Ritchie, W. 1971. The Beaches of Barra and the Uists. A survey of the beach dune and machair areas of Barra, South Uist, Benbecula, North Uist, and Berneray. Commissioned report no. 047 for Scottish Natural Heritage. Aberdeen.• Seaton, D. 1968. Bornish blow-out: a record of co-operation in overcoming machair and land erosion. Scottish Agriculture 47, 145- 148.• Tisdall, J.M., Nelson, S.E., Wilkinson, K., Smith, S., McKenzie, B.M. 2012. Stabilisation of soil against wind erosion by six saprotrophic fungi. Soil Biology and Biochemistry 50, 134-141.