Morphological characterisation of humus forms in recent coastal dune ecosystems in Belgium and northern France

The morphology of humus profiles, developed on recent sandy coastal dune ecosystems, is studied on macro- and microscale. Humus and soil profiles were described, sampled and analysed. Undisturbed, oriented samples were taken from the surface horizons for micromorphological study. The soils are situated in the National Biological Reserve of the Opal Coast, Merlimont, France and near De Haan, Belgium. On both sites, the parent material consists of aeolian calcareous sand. Surface horizons, however, are decarbonated due to leaching processes. limont, the sequence consists of two profiles, one under Pinus nigra ssp. laricio and one under Populus nigra ssp. nigra, characterised by a Resimor11According to Green et al. [Green, R.N., Trowbridge, R.L., Klinka, K. For. Sci. Monogr. 29 (1993)]. der22According to Jabiol et al. [Jabiol, B., Brêthes, A., Ponge, J.-F., Toutain, F., Brun, J.-J. 1995. LʹHumus sous toutes ses formes. Ecole Nationale du Génie Rural, des Eaux et des Forêts, Nancy, France]. hizomull1/Mesomull2 humus type, respectively. At the De Haan site, three profiles under Quercus robur, Populus ssp. and P. nigra var. austriaca are investigated. The humus types are respectively Mullmoder1 to Mull/Dysmull2, Mull1/ Oligomull2, Mormoder1/Dysmoder2. ences among the humus profiles are related to presence/absence among the L-, F-, H- and A-horizons, structure, rooting, presence of hyphae, sclerotia and excrements. orphologically, the difference between humus profiles under coniferous and deciduous forest/grassland is evident by thicker L-, F and/or H-horizons under coniferous trees. The microstructure of the L- and F-horizons shows a loose packing of subhorizontally oriented needles. The microstructure of the H-horizons varies from intergrain microaggregate to locally weak crumb. The microstructure of the surface mineral horizon of all profiles is mainly intergrain microaggregate, sometimes single grain and exceptionally bridged grain. Intimate mixing and complexation of organic matter to the mineral fraction into aggregates is very limited in the A-horizons. All A-horizons, irrespective of the humus type, are to some degree characterised by a “pepper and salt” appearance. The horizon symbol “H+E” is introduced to label this morphology. ll humus forms under deciduous trees are indicative of rather intense mesofauna activities. The absence of a crumb structure in the A-horizon of these soils could be due to the fast disintegration of earthworm casts in these sandy soils with no clay or silt fraction.