Mull is the product of the mixing activity of burrowing soil animals (such as earthworms, ants, termites, moles, pocket gophers) which create nests and galleries within the upper part of the soil profile and excavate earth at the soil surface.[4] These disturbances mix organic matter with mineral particles, aerate soil, create and modify ecological niches of all other soil organisms, from microbes to plant roots and passing invertebrates. Some mull-forming animal groups (such as earthworms, millipedes, termites, crane fly larvae) ingest soil and mix it with mucus in their guts or mix it with their saliva to create nests and tunnels (termites). All these disturbances, whether mechanical or biochemical, stimulate microbial activity, hence faster nutrient cycles and uptake by plants. Thus mull is associated with more fertile and productive of ecosystems, with a positive aboveground-belowground feedback process: more nutrients for plants, better plant growth, more nutrients in plant remains, better quality of soil organic matter, better growth and reproduction of soil animals and microbes, less immobilization of nutrients in microbial biomass, and so on.[5]
Moder is a forest floor type formed under mixed-wood and pure deciduous forests.[6][7] Moder is a kind of humus whose properties are the transition between mor humus and mull humus types.[8][9] Moders are similar to mors as they are made up of partially to fully humified organic components accumulated on the mineral soil. Compared to mulls, moders are zoologically active.[10] In addition, moders present as in the middle of mors and mulls with a higher decomposition capacity than mull but lower than mor.[7] Moders are characterized by a slow rate of litter decomposition by litter-dwelling organisms and fungi, leading to the accumulation of organic residues. Moder humus forms share the features of the mull and mor humus forms.[7]
Mor humus is a form of forest floor humus occurring mostly in coniferous forests.[11] Mor humus consists of evergreen needles and woody debris that litter the forest floor. This litter is slow to decompose, in part due to their chemical composition (low pH, low nutrient content), but also because of the generally cool and wet conditions where mor humus is found. This results in low bacterial activity and an absence of earthworms and other soil fauna. Because of this, most of the organic matter decomposition in mor humus is carried out by fungi.
Mor humus is one of three classifications of forest floor humus, along with Moders and Mulls. Each class corresponds to a scale of increasingly colder conditions, decreasing biological diversity and activity, and decreasing nutrient availability.[12] Mor humus ranks at the bottom of this scale and is characterized by very slow decomposition and accumulation of plant material.
Amphi
Amphi forms occur in environments with strong seasonal contrasts. Amphi forms result from earthworms working in the topsoil (in mull fashion) and insects working in the plant litter (as in the moder form). Different ecological factors regulate the peaks of activity in the topsoil compared to the overlying organic surface. The depth of maximum earthworm activity varies seasonally. Activities by depth peak at different times of the year in the amphi humus form, more so than in the other humus forms.[13] The meaning of "Amphi" closely relates to duality and encompasses two environments or sides, as seen in words like "amphibian" (living in water and on land) and "amphitheater" (a space surrounded). Using the term to identify a humus form emphasizes existence or action "on both sides" or "in two manners".
Tangel
Tangel occurs in high elevation, shrub-dominated regions. It features a strongly developed "tangel layer" of brown plant remains above darker mull-like or moder humus.[14]
Definitions
Virtual Soil Science Learning Resources group: "soil horizons located at or near the surface, which have formed from organic residues (separate from or mixed with mineral particles). Horizons that may comprise a humus form include L, F, H, and Ah, but not B or C."[15]
German Soil Science Society: "Order of distinct units defined by organic surface horizons and the first mineral horizon with similar morphology, depths and type of boundary of horizons indicating specific conditions for bioturbation, decomposition, humification, and mineralisation."[16]
Canadian forest ministry: "a group of soil horizons located at or near the surface of a pedon, which have formed from organic residues, either separate from, or intermixed with mineral materials."[17]
History
Early contributions to the knowledge of humus forms were made by Peter Erasmus Müller, a Danish forester. In his seminal contribution Studier over Skovjord: som bidrag til skovdyrkningens theori,[18][19] later translated into German[20] and French,[21] 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. Enrico Mylius Dalgas also contributed.[22] 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).[23]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.[24]
Classification
Most classifications of humus 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. However HUMUSICA, a worldwide morpho-functional classification of humus forms, was created in the 2010s.[25]HUMUSICA describes and classifies humus forms from a wide array of terrestrial, semi-aquatic, cultivated and man-made environments.[26][27][28] 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.[29]
Humus profiles, like soil profiles, refer to a trench through the soil. Humipedons, like pedons, refer to a column of soil. For the sake of clarity they will be synonymized, because both are made of successive layers the age of which increases with depth, more superficial layers being younger than deeper ones because organic matter is mostly deposited from above.[30] 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,[31] while pedon changes take decades[32] to centuries.[33] However, Walter Kubiëna considered that there was a parallelism between humus forms and soil types, hence his common classification of humus and soil profiles,[14] 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[34] and FAO's World Reference Base for Soil Resources (WRB).[35] 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.[36]
Diagnostic horizons
Humipedons display a succession of horizons according to decomposition stages of fallen plant litter and its progressive incorporation to mineral matter.[37] They have been characterized on thin soil sections by soil micromorphologists,[38] but their recognition in the field is easy, being aided by the use of a hand lens if necessary.[39] They can be observed along a humus profile cut with a sharp knife along a trench[40] or be successively collected by hand one by one from the top to the bottom of a small soil monolith.[41]: 29
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.[42] Bleaching of litter may also occur when leaves or needles are colonized by white-rot fungi.[43] The OL horizon is often seen permeated by fungal mycelia which penetrate leaves and needles and participate to their decomposition.[44] The OL horizon is present in all terrestrial humus forms, to the exception of the most active mull humus forms (e.g. Eumull) where it might be seasonally absent because of a fast decomposition rate of recently fallen litter.[45]
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).[46] Enchytraeid faeces are so small that they appear as a very fine black powder covering or intermingled between decaying leaves.[37] In coniferous forests enchytraeids and some oribatid mites penetrate fallen needles once these have been heavily colonized by fungi and they deposit their feces at the inside, making them invisible if needles are not dissected by the observer.[47] In thick forest floors with active animal activity (e.g. moder humus forms) OF horizons are the seat of maximum development of the fine root system of trees and mycelia of their ectomycrorrhizal fungal associates.[48]
OH horizon
The OH horizon (Oa in the USDA Soil Taxonomy) is the product of transformation of plant remains by soil organisms once these remains are no longer visible, but the humus thus formed is still not incorporated with mineral matter. According to the soil animals which contributed the most to the faunal activity observed in the overlying OF horizon, the OH horizon may be seen as an accumulation of still visible fecal pellets (e.g. earthworms, ants, millipedes, woodlice, crane fly larvae for macrofauna, but also oribatid mites for mesofauna) or in the case of enchytraeids as a fine powder further compacted in depth.[37] By their vertical movements[49] enchytraeids play a decisive role in the transition with the underlying A horizon.[50] Fine root systems are also present in OH horizons, together with subterranean organs of heathland plants (e.g. Ericaceae)[51] and their symbiotic fungal associates (ericoid mycorrhizae) which are able to decompose recalcitrant organic matter and transfer its nitrogen to the host plant.[52]
A horizon
The A horizon results from the mixing of organic matter with mineral matter, mostly effected by burrowing soil animals (e.g. enchytraeids, earthworms, termites, ants, darkling beetles, gophers).[53] Some physical processes may also contribute to the mixing of organic matter with mineral matter, such as shrink-swell cycles of vertisols.[54] The A horizon is mainly made of mineral-organic aggregates (peds) of varying size, depending on the size of soil animals which excreted or moulded them in the course of their burrowing activity. Macroaggregates (> 250 µm) are built by macrofauna (e.g. earthworms, ants, termites) and megafauna (e.g. gophers), while microaggregates (< 250 µm) are built by mesofauna (e.g. enchytraeids, microarthropods).[55] Plant roots and Microorganisms (bacteria, fungi) also contribute to the formation and stabilization of aggregates through their excreta (e.g. microbial extracellular polysaccharides, root mucilages).[56][57]
E horizon
The E horizon appears as a white or grey (ashy) horizon, the lightness (Munsell colour value) of which varies with its carbon content, always feeble.[58] Compared to the abovelying organic and mineral-organic horizons,the E horizon displays only poor signs of biological activity,[59] being mainly the seat of leaching of water, solutes (e.g. nitrates, dissolved organic carbon) and colloids (e.g. clay, humus) through a mineral layer.[60] Whether the E horizon is the main seat of mineral weathering, as suggested by the observation of pore networks of fungal origin within weathered minerals,[61] is still a matter of conjecture because highly weathered minerals are present in the E horizon.[62] This suggests that mineral weathering mainly takes place in the overlying A horizon permeated by plant roots and their microbial rhizosphere associates.[63] Like the OH horizon can be considered as the end-product of biological activity taking place in the above A horizon, the E horizon could be the end-product of biochemical processes of mineral weathering taking place in the above A horizon.
Climate change
Climate change significantly affects humus forms through multiple interconnected mechanisms that alter the balance between organic matter inputs and decomposition rates. Rising temperatures accelerate microbial decomposition processes, resulting in humus depletion and a negative carbon balance in many soils.[64] Research in alpine grasslands demonstrates that a 3 °C temperature increase reduces soil humus content and destabilizes soil structure.[65]
The impacts are complex and vary depending on the specific humus form type and environmental conditions. Climate warming drives predictable shifts between humus forms along temperature gradients. Research in France demonstrates that the change from Moder towards Mull occurs from north to south following increasing temperature gradients.[66]
^Bernier, B (1968). "Descriptive outline of forest humus form classification". Proceedings 7th Meeting of the National Soil Survey Committee of Canada. 39: 154.
^Wiesmeier, Martin; Hübner, Rico; Kögel-Knabner, Ingrid (2015). "Stagnating crop yields: An overlooked risk for the carbon balance of agricultural soils?". Science of the Total Environment. 536: 1045–1051. Bibcode:2015ScTEn.536.1045W. doi:10.1016/j.scitotenv.2015.07.064. PMID26235605. Early signs of a reduction in humus stocks due to the stagnating harvest yields can already be observed. Initial indications of humus depletion in arable soil have been observed in almost all EU countries in recent years.
