A proposal for including humus forms in the World Reference Base for Soil Resources (WRB-FAO)

ABSTRACT: The last delivery of the World Reference Base for Soil Resources (IUSS Working Group WRB, 2006) updated previous texts adopted by the ISSS Council, and was proposed at the 18th World Congress of Soil Science as the official reference for soil nomenclature. It was considered by the entire soil community as the better framework “through which existing soil classification systems could be correlated and harmonized”. As in previous drafts, the humus form, i.e. the part of the topsoil which is strongly influenced by biological activities and organic matter (litter included), was only partially considered, taking into account organic layers only when their thickness was very high, and ignoring many fundamental evidences necessary for a sufficiently precise characterization of forest soils, as well as all soils not periodically ploughed. A modern, biologically meaningful classification of humus forms has been proposed at the European level by Zanella et al. (2011: a. A European morpho-functional classification of humus forms. Geoderma 164, 138-145; b. European Humus Forms Reference Base [http://hal.archives-ouvertes.fr/docs/00/56/17/95/PDF/Humus_Forms_ERB_31_01_2011.pdf]), encompassing a wide variety of humus forms, both in terrestrial and semi-terrestrial environments. This morpho-functional classification, which has been recently updated thanks to users’ feedbacks, is the basis of our proposal to include humus form characterization in the WRB, for the sake of completing and improving this soil classification system. Following WRB specifications, two tiers of categorical detail have been performed: 31 Reference Humus Form Groups or RHFGs (tier 1), and the combination of RHFGs with prefixes and suffixes, detailing the properties of RHFGs by adding a set of uniquely defined qualifiers (tier 2). The architecture proposed for the RHFGs is based on the same principles as WRB: “[RHFGs] are allocated to higher-level groups on the basis of diagnostic characters, i.e. factors or processes that most clearly influence the biological formation of [humus forms]”. The 10 historical main groups of humus forms, elaborated by Zanella et al. (2011a, b) for classifying terrestrial humus forms (Mull, Moder, Mor, Amphi, Tangel) and semi-terrestrial humus forms (Mull, Amphi, Moder, Anmoor, Mor) and their second-level units (terrestrial: dys, eu, hemi, humi, lepto, meso, oligo, pachi; semiterrestrial: fibri, humi, limi, mesi, sapri) were combined to create a first-level classification of 31 RHFGs. Specific prefix and suffix qualifiers are then associated to RHFGs, allowing a wide variety of variants (second-level classification) to be defined according to biological (vegetation) and environmental (geology, climate) context.