Humus Form ERB 2010. A European Reference Base for humus forms: proposal for a morpho-functional classification.

STRUCTURE OF THE CLASSIFICATION The classification has been conceived for forest soils, for which more information and larger datasets are available, as well as for soils of grasslands, pastures and wetland areas, with a negligible to strong human impact. It is not suited to tilled agroecosystems, because tillage destroys the “natural” organization and radically alters the functioning of the surface horizons. The manuals of the FAO (2006), IUSS Working Group WRB (2007) or Soil Survey Staff (2010) are more appropriate for describing and classifying these soils. An ongoing Canadian-German project on topsoil characterization of arable soils will be presented at the IUSS conference 2010. The humus form classification is based on the sequence and morphological characteristics, including morphological evidence of biological activity, of organic and/or organo-mineral horizons observed and described in the field. In some cases a few basic chemical data (pH, organic carbon content) are required. A complete set of diagnostic organic and organo-mineral horizons, which are mutually exclusive, is defined. The classification keys use diagnostic horizons and other complementary topsoil or environmental data. Every mineral horizon cited in this paper has been classed and named using the manual of the IUSS Working Group (2007). The first dichotomy of the classification separates never saturated and saturated (submerged) humus forms: - TERRESTRIAL humus forms: these are never submerged and/or water-saturated, or only for a few days per year. A or AE (non hydromorphic) organo-mineral horizons characterize these forms. In a second and more detailed step of the classification, Terroforms (= typical) are separated from Entiforms (= directly on bedrock or parent material) and Paraforms (= atypical); - SEMITERRESTRIAL humus forms: these are submerged and/or water-saturated. Hydroforms are submerged and/or water-saturated for relatively short periods (less than 6 months per year) and are characterized by Ag or AEg hydromorphic organo-mineral horizons; Histoforms and Epihistoforms are submerged and/or water-saturated for protracted periods (usually more than 6 months per year) and are characterized by organo-mineral Aa or organic H horizons. Within each group of the Terrestrial compartment (Terroforms, Entiforms and Paraforms) and within the group of Hydroforms of the Semiterrestrial compartment, the same five “biological types” are identified on a morpho-functional basis: Mull, Amphi, Moder, Tangel and Mor. Within Histoforms and Epihistoforms, the Tangel biological type is not present, but a characteristic “soil moisture regime” generates the Anmoor biological type. These “biological types” can be considered as the first taxonomic level of the classification. For Terroforms and Histoforms, the most important and best-known groups of humus forms, a second level of classification has been created. Here, each unit of the first level (Mull, Moder, …) is split in two or more biological sub-types (i.e. Eumull, Mesomull; Hemimoder, Dysmoder…). Specific vocabulary listed in the section “specific terms”, and topsoil layers detailed under the heading “diagnostic horizons”, furnish the potential user with the necessary information for his/her field investigation of all Terrestrial and Semiterrestrial humus forms.