A European morpho-functional classification of humus forms

Abstract

In Europe an abundance of humus taxonomies exists starting with early approaches in the late 19th century. Frequently used in an international context, they do not cover all site conditions in the European area. Although having basic concepts and general lines, the European (and North American, Canadian) classification systems differ in important parameters used for the description and classification of humus forms. These discrepancies result in incongruities, so they require adjustments when exchanging partially compatible soil data, even between nearby countries. In 2003, 26 European specialists in humus forms met in Trento (Italy) and decided to formulate rules of classification based on morphogenetic descriptions and diagnostic horizons, adapted to European ecological conditions. Taking into account old and new European and North American systems of humus forms classification, six main references (Anmoor, Mull, Moder, Mor, Amphi and Tangel) were defined, each of them further divided into more detailed categories. This inventory assigned a strong discriminatory power to the action of soil animals. Both semiterrestrial (anoxic) and terrestrial (aerated) topsoils were classified. Descriptors of diagnostic horizons were conceived in accordance with recent international soil classifications. Assigning an ‘ecological value’ to each main humus form along a gradient from biologically active forms, degrading and incorporating all organic remains, to those characterized by the accumulation of poorly transformed organic matter, this European system of classification avoids a strong hierarchical structure and allows a flexible approach open to additional ecological contributions and renditions.

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.