Humusica 1, Article 6: Terrestrial humus systems and forms – Hydro intergrades

Highlights

• Humipedon hydromorphic properties: description and illustration.

• Diagnostic hydromorphic horizons: definition and presentation with the help of photographs.

• Names and key of classification of Terrestrial hydromorphic humipedons.

Abstract

In this article, we present the Terrestrial intergrades from never or only few days submersed humipedons, without permanent hydromorphic signs, until humipedons with presence of signs of partial asphyxia. Specific terms are defined and diagnostic horizons illustrated, both with the help of text explanations and photographs. The article ends with a table working as a classification field key.

Introduction

Soil pores may be filled with water, air and living organisms. Air carries into the soil oxygen and nitrogen, both very important elements for soil organisms (roots, bacteria, fungi and animals) (Gobat et al., 2004). When water replaces the air in the soil, soil organisms must face a period of anoxia and sometimes asphyxia (Hattori et al., 1976). Aerobic bacteria are most active in unsaturated moist soils, but anoxic hostile conditions will not completely kill bacteria, they will stop growing and get into a dormant stage (Bouthier et al., 2014). As for macro-organisms, earthworms without oxygen will suffocate and try to escape from their galleries. However, worm experts say that earthworms could survive for several weeks in water, providing there is sufficient oxygen in the water to support them. Earthworms do not surface to avoid drowning. We see them out of soil after rain because they can move around without dehydrating (Lee, 1985, Boyle et al., 1997, Edwards and Bater, 1992, Edwards, 2004) but also because the vibrations caused by raindrops mimic those emitted by predators as moles (Catania, 2008). For some species, growth and reproduction are maximal at a soil moisture of 80% (Hallatt et al., 1992). Anecic earthworms influence the soil porosity and have an impact on the soil water and mineral elements availability for plants (Andriuzzi et al., 2015; Bouché and Al-Addan, 1997; Le Bayon and Binet, 2006).

Concomitantly with soil organisms, specific plant communities grow on hydromorphic soils. Considering only Terrestrial ecosystems (not peat or always submersed soils, related to Histo humus forms and soils), plant phytosociologists list along a gradient from wet to fresh soils, forests belonging to Alnion glutinosae (dominant trees: Alnus glutinosa), Alno-Padion (Alnus glutinosa, Fraxinus excelsior, Prunus padus) and Carpinion (Carpinus betulus) phytosociological alliances (For the lists of plants refer to: Zanella, 1990; Géhu and Géhu, 1977; Noirfalaise, 1984; Oberdorfer, 1957). Rivers are also bordered by hydromorphic soils, which support linear plant formations assigned to Alnion incanae (rather mountain, rocky rivers) and Salicion albae (plain) phytosociological alliances. More complete data and geographical distribution in Europe of hydromorphic soil habitats are available on the website of the European Environment Agency (EEA, 2015). In the EUNIS habitat classification, the forests with hydromorphic soils are listed in Annexe 1 at levels G1.1 (Riparian and gallery woodland, with dominant [Alnus], [Betula], [Populus] or [Salix]), G1.2 (Mixed riparian floodplain and gallery woodland), G1.3 (Mediterranean riparian woodland) and G1.4 (Broadleaved swamp woodland not on acid peat). Broadleaved swamp woodland on acid peat Woodland (Histic humus forms or Histo soils) are placed in a separate G1.5 section. Besides a gradient from semiterrestrial forests to humid forests, there is also a gradient from quacking mires, reeds, sedges to half natural wet to humid grasslands and Sphagnum dominated bog-like short vegetation. A LIFE publication titled “LIFE and Europe’s wetlands: Restoring a vital ecosystem” is also available (Silva et al., 2007) and furnishes very useful information about geographical distribution in Europe of these sites, as well as fauna (especially birds and amphibians), flora and human exploitation, with the aim to support their conservation.