Sea spray and land use effects on clay minerals and organic matter of soils on machair (Harris, Scotland)

Abstract

‘Machair’ describes a landscape form that is present along the Atlantic seaboard of Scotland and Ireland that is characterized by a gently sloping coastal plain developed from aeolian carbonate and quartzose sand. Three soil profiles were characterized along a coastal transect on Harris (Outer Hebrides, Scotland) by standard methods (colour, texture, pH, wet-chemical extractions), infrared spectroscopy, X-ray diffractometry, X-ray fluorescence spectrometry and differential scanning calorimetry. The aim was to understand chemical processes and properties of soils that have formed under the influence of sedimentation, erosion, weathering, accumulation of soil organic matter (SOM), and human impact. One of the profiles differed distinctly from the other two, in particular regarding depth, texture, contents of carbonate and SOM, and properties of SOM (relative content of rather labile permanganate-oxidizable SOM, oxidation, interaction with clay minerals). Thermally stable SOM was present in all soil samples, likely pointing to pyrogenic SOM, i.e. black carbon. The mineralogical composition differed among the profiles and reflected the intermediate character of the local rocks. Magnesium-hydroxy-interlayered minerals were identified in all surface Ah horizons. We suggest that sea spray was the likely source of intercalated Mg. The shallowest soil was classified as Hypereutric Leptosol and the others as a Cambic and Calcaric Phaeozem. Results warrant a more detailed survey on the yet less studied interplay of SOM properties and mineral (trans-) formation in highly diverse and dynamic coastal soils that are frequently affected by long-term agricultural use.