A European morpho-functional classification of humus forms
Highlights
► European specialists conceive principles of a new classification of humus forms. ► Only morphological characters with evident functional effects were considered. ► Ten basic humus forms were circumscribed, available in a wide array of ecosystems. ► Environmental factors determine the structure of the classification tree.
Introduction
The humus form is the part of the topsoil that is strongly influenced by organic matter and coincides with the sequence of organic and underlying organo-mineral horizons. During the 19th century, scientists noticed that the type and rate of decomposition of these organic components, as well as the incorporation of organic matter (OM) in mineral horizons, vary according to forest type (review in Jabiol et al., 2005). These observations led Müller, 1879, Müller, 1884, Müller, 1887, Müller, 1889 to define three ‘humus forms’, named Muld (later becoming Mull), Mor and Mullartiger Torf (now Moder), characterized by their climatic, geological and biological conditions of formation in Danish beech forests. From the outset it was evident to Müller that the humus form corresponds to the “expression of life” within the topsoil. Many authors contributed to the development of a classification system of humus forms based on the key role of living components in the topsoil. The most prominent contributions are those of Humusformen, 2004, Hesselmann, 1926, Kubiëna, 1953, Baize and Girard, 1998, Ehrenfeld et al., 2005.
All these concepts still form the basis of modern classifications of humus forms (AFES, 1995, AFES, 2009, Baritz, 2003, Batjes, 1996, Delecour, 1983, Hartmann, 1944, Babel, 1971, Jabiol et al., 2005, Jabiol et al., 2004, Nestroy et al., 2000, Van Delft et al., 2007, Zanella et al., 2009, Zanella et al., 2006). Although Canadian (British Columbian) and French classification systems are frequently used in an international context, none of them covers site and climate conditions worldwide, not even all European forest ecosystems. Moreover, the new national classification systems differ according to the parameters used for describing and classifying humus forms as well as for scaling diagnostic parameters (Batjes, 1996, Schoeneberger et al., 2002). Similar designations of humus forms often have different contents.
With harmonization purposes in mind, a wide range of European specialists met in Trento (Italy) in 2003 and formed a European Humus Group with the aim of improving the compatibility of established national systems of classification and setting out a unified European reference for humus forms. The present synthesis was elaborated during the course of four plenary field sessions held in Alpine (Trento 2003, Vienna 2004, San Vito di Cadore 2005) and Mediterranean (Cagliari 2007) ecosystems. In these workshops, the place of lesser known Terrestrial humus forms such as Tangel and Amphi and that of semi-terrestrial humus forms was discussed and included in a new classification (Jabiol et al., 2009, Zanella et al., 2001). In the meantime the key of humus forms was also tested by non-specialists in order to improve it and to discard interpretative drawbacks. In the future, the proposed humus form classification will also be included in a worldwide topsoil characterization that is currently being prepared (Broll et al., 2006).
This paper focuses on the aims and principles of the proposed European classification, details and keys being found in a more complete review available at: http://hal.archives-ouvertes.fr/docs/00/56/17/95/PDF/Humus_Forms_ERB_31_01_2011.pdf.
Section snippets
Morpho-functional bases and references for the classification
At the end of the first meeting of the Humus group (Trento, Italy, 2003), after numerous field tests and discussions, the members of the group agreed on a very important “principle of classification”: the classification process has to take into consideration only references (objects observable in the field or complexes of objects) which show an evident “morpho-functional” shape or core. This means that: a) each reference must be recognizable thanks to its particular and characteristic
Diagnostic horizons
A minimum thickness of horizons for description, diagnosis and sampling purposes has been established at 3 mm. Below this threshold, a horizon is considered discontinuous if clearly in patches or absent if indiscernible from other neighboring horizons. The vagueness of transitions between organic and organo-mineral horizons (or mineral ones, in the absence of an organo-mineral horizon) is an important diagnostic character. Three types of transition have been adopted: very sharp transition within
Humus forms classification
The European classification of humus forms is based on the sequence and morphological characteristics, including evidence of biological activity, of organic and/or organo-mineral horizons observed and described in the field (forests, grasslands, pastures and wetland areas, to the exclusion of croplands). 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.
Prospects for the use of the European classification of humus forms
Many benefits can be expected from (i) a better characterization of humus forms, in particular for mapping and database purposes, (ii) a common language and vocabulary that could be accepted and used worldwide, (iii) a field instrument for the estimating the effects of global and local changes on soil C pools, (iv) a practical and conceptual tool for planning studies in soil biology, and (v) a better knowledge of factors acting at the interface of soil, vegetation and climate.
The humus forms
Acknowledgments
The authors warmly acknowledge Alain Brêthes (Office National des Forêts, Boigny-sur-Bionne, France), Gabriele Broll (University of Vechta, Germany), Jean-Michel Gobat (University of Neuchatel, Switzerland), Jean-Jacques Brun (CEMAGREF, Saint Martin d'Hères, France), Gerhard Milbert (Geological Survey of North Rhine-Westphalia, Krefeld, Germany), Eckart Kolb (Technische Universität München, Dept. of Ecology/Forest Nutrition and Water Resources, Freising, Germany), Ugo Wolf (University of
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