Humus forms: visible field characters of main biogeochemical topsoil interacting systems.

P.E. Müller (1879) assigned the word “humus forms” to structured topsoils. Aware of the morphological observations of forest soils made by Hundeshagen (1830) and Hermann (1841), Müller remarked that the natural soil progressively integrates litter and shows a topsoil made of superposed organic and organic-mineral horizons. He described these horizons and proposed a first classification of the forest floor in two opposed main categories, called Mull and Mor humus forms (Muller 1884, 1887, 1889). Mull humus form was observed in climatic and pedological favourable conditions, in correspondence of active topsoils, where the fallen litter disappears in few months; on the contrary, in less favourable conditions, colder, northern expositions, acid substrates, a Mor humus forms shows thick horizons of not decomposed litter (also in Henry 1908). In 1926, H. Hesselmann proposed to classify the organic horizons in three types, coded with the capital letters L, F and H. In 1944, F. Hartmann described other organic and organic-mineral diagnostic horizons. In 1953, completed in 1955, W.L. Kubiena proposed a first classification of all known European humus forms. With Hartmann, they were real soil-ecology precursors, associating to each humus forms a succession of organic and organic-mineral horizons and corresponding actors of biodegradation, as in modern classifications. After the Second World War started a rich period of researches in Europe (citing only the main publications: Hartmann 1951, Wilde 1954, Duchaufour 1956, Manil 1959, Bernier 1968, Babel 1971, Hartmann 1970, Wilde 1971, Delecour 1980, Duchaufour 1977, Klinca et al. 1981). All these authors underlined the importance of humus forms (also called “humus types”) for understanding the forest ecosystems functioning and organic matter/nutrient elements cycles. In the last 30 years, many groups of research proposed more practical national and international references for humus forms classifications (Green et al. 1993, Brêtes et al 1992, Jabiol et al. 1995, 2009, 2013, Zanella et al. 2001). Born in 2003, a international Humus Group begun the standardisation of all these heterogeneous humus forms references and published a first European Humus Forms Reference Base (Zanella 2011ab). A new version of the European Reference is under publication. It corresponds to a field guide, with double purpose: classify the living topsoil and understand its functioning. The content of the book has been presented in Rome (2 December 2015) at the National Congress of the Italian Society of Soil Sciences, and is here down briefly illustrated.