The humus form is the various arrangement of organic and mineral horizons at the top of soil profiles.[1] The term humus form is not the same as the term humus. It can be composed entirely of organic horizons, meaning an absence of any mineral horizons, or of a superposition of organic and mineral horizons, forming the 'humus profile', also called 'humipedon'.[2] Experts worldwide have developed different types of classifications over time, and common forest humus forms are mainly categorized into mull, mor, and moder. The most ancient contribution to the knowledge of humus forms was that of Peter Erasmus Müller, a Danish forester. In his seminal contribution Studier over Skovjord: som bidrag til skovdyrkningens theori,[3][4] later translated in German[5] and French,[6] Müller described muld (later germanized as mull) and mor, two modes of assemblage of organic and mineral matter, which he associated to two opposite classes of high and low productivity and soil fertility of Danish beechwoods. His investigations embraced a thorough analysis of plant communities, and chemical as well as microscopic investigations in various soil horizons. At the same time Charles Darwin, one year before he died, published a detailed study of the formation of mull (called by him mould, reminiscent of the Danish muld).[7]Moder was later added as a third forest humus form by F. Hartmann, midway between mull and mor and previously described as 'insect mull' by Müller.[8]
Classification
Various classifications of humus exist so far. Most of them are national (French, Belgian, German, Canadian, Russian, among many others) and do not embrace the variety of humus forms found over all world biomes, being mostly focused on forest soils and temperate climates. Thereby we will focus on the most recent and complete one. HUMUSICA, a worldwide morpho-functional classification of humus forms, was achieved by an array of soil scientists working under the direction of Augusto Zanella, after numerous field sessions, workshops and exchanges between participants from various countries.[9]HUMUSICA described and classified humus forms from a wide array of terrestrial, semi-aquatic, cultivated and man-made environments and was published in three special issues of the academic journal Applied Soil Ecology.[10][11][12] A freely available iOS called TerrHum was developed in English, Italian and French,[13] with an APKAndroid version as a tool to identify humus forms directly on the field.[14] In HUMUSICA the three current humus forms called mull, moder and mor are considered as humus systems, abbreviation of humus interaction systems, each embracing several humus forms according to variations in thickness of organic and mineral-organic horizons.[15]
One of the key principles of humus form classifications is that humus profiles (humipedons) may evolve at a different rate from soil profiles (pedons). Given the prominent part taken by soil organisms, from bacteria to mammals, passing by plants and invertebrates, in the spatial arrangement and transformation of organic matter, humipedons display pluri-annual variations,[16] while pedon changes take decades[17] to centuries.[18] However, Walter Kubiëna considered that there was a parallelism between humus forms and soil types, hence his common classification of humus and soil profiles,[19] an opinion not shared by the majority of soil scientists who turned to soil classifications based on physical and chemical properties of more stable underlying mineral horizons, like USDA's Soil Taxonomy[20] and FAO's World Reference Base for Soil Resources (WRB).[21] It has been suggested that the pedon could be subdivided in three parts, called humipedon (for the humus profile), copedon and lithopedon, in a decreasing order of contribution of soil biological activity to their formation, and thus of their cycle of change, from decade to millennium.[22]
Diagnostic horizons
Humipedons display a succession of horizons according to decomposition stages of fallen plant litter and its progressive incorporation to mineral matter.[23] They have been characterized on thin soil sections by soil micromorphologists,[24] but their recognition in the field is easy, being aided by the use of a hand lens if necessary.[25] They can be observed along a humus profile cut with a sharp knife along a trench[26] or be successively collected by hand one by one from the top to the bottom of a small soil monolith.[27]
OL horizon
The OL horizon (Oi in the USDA Soil Taxonomy) is made of recognizable leaves or needles without any prominent signs of fragmentation by litter-consuming soil animals. Its colour is currently brown to black according to microbial successions taking place during the first stages of litter decomposition.[28] Bleaching of litter may also occur when leaves or needles are colonized by white-rot fungi.[29] The OL horizon is often seen permeated by fungal mycelia which penetrate leaves and needles and participate to their decomposition.[30]
OF horizon
The OF horizon (Oe in the USDA Soil Taxonomy) is made of fragmented leaf or needle litter, from the feeding activity of soil animals (macrofauna and mesofauna). Litter debris are mixed with feces deposited by litter-consuming animals in the form of dark-coloured pellets of a size varying from 30-50 micrometre (enchytraeids, oribatid mites, springtails) to 1-2 millimetres (epigeic earthworms, millipedes, woodlice, molluscs). Enchytraeid faeces are so small that they appear as a very fine black powder covering decraying leaves. In coniferous forests enchytraeids and some oribatid mites penetrate fallen needles once they have been colonized by fungi and deposit their faces at the inside, making them invisible if needles are not dissected by the observer.
