Humus form

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Humus form is the features of the topsoil and plant litter in a biome, such as mull humus form in deciduous forest or mor humus form in coniferous forest. Dead organic matter, such as leaves, decomposes into humus. Sometimes soil animals mix underlying mineral soil with the humus, this is also part of the humus form.

Contents

Humus form is important in ecosystem management, [1] such as forest management. [2]

Terrestrial humus forms

Terrestrial humus forms are found in forests, woodlands, grasslands, heathlands, steppes, tundras, deserts and semi-deserts. Five humus systems have been described in terrestrial environments: mull, moder, mor, amphi, and tangel. They all have pore spaces filled with air, where soil organisms live, at least temporarily. [3]

Mull

Mull humus profile in an oak forest Mull humus.jpg
Mull humus profile in an oak forest

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 soil biomantle. [4] These disturbances mix organic matter with mineral particles, [5] aerate soil, [6] create and modify ecological niches of all other soil organisms, from microbes to plant roots and passing by invertebrates. [7] Some mull-forming animal groups (such as earthworms, millipedes, termites, crane fly larvae) ingest soil and mix it with mucus in their gut, [8] or mix it with their saliva to create nests and tunnels (termites). [9] All these disturbances, whether mechanical or biochemical, stimulate microbial activity, [10] hence faster nutrient cycles [11] and better mineral uptake at root level. [12] Thus mull is associated with more fertile soils and productive ecosystems, [13] with a positive aboveground-belowground feedback process: more nutrients for plants, thus better plant growth and more nutrients in plant remains, higher quality of soil organic matter, thus better growth and reproduction of soil animals and microorganisms, with a lesser immobilization of nutrients in microbial biomass, and so on. [14] Mull is linked to favorable environmental conditions, [15] more particularly good litter quality, [16] mild climate [17] and availability of weatherable minerals in the parent rock. [18]

Moder

Moder humus profile in an oak forest Moder humus.jpg
Moder humus profile in an oak forest

Moder is the product of the activity of non-burrowing animals, or those only burrowing over a very short distance. These are mainly active in the litter, which they transform into dark-colored excrement, visible in the form of millimeter-sized fecal pellets or, for the smallest (< 100 microns), a fine reddish-brown powder. [19]  · [20] The animals responsible for the fragmentation of litter and its transformation into humus belong to the detritivorous fauna, including microarthropods (e.g. springtails, oribatid mites), macroarthropods (e.g. millipedes, woodlice, insect larvae) but also molluscs (snails, slugs) and worms (epigeic earthworms, enchytraeids). [21] Litter fragmentation and deposition of organic excrement generate an OF horizon which is gradually transformed into an OH horizon as animal excrement accumulates. [22] The small vertical movements of epigeic worms [23] and enchytraeids [24] generate a thin A horizon, with a microgranular or particulate structure (depending on silty or sandy nature of the mineral substrate), with a diffuse transition with the overlying OH horizon. [25] Although the OH horizon is considered a diagnostic horizon for moder (as opposed to mull) it is the intense biological activity within the OF horizon which truly marks the moder. [26] In addition to the detritivorous fauna, and predators associated with it, the moder OF horizon is permeated by the fine (assimilatory) root system of trees, notably conifers (e.g. pine, spruce) and hardwoods with recalcitrant litter (e.g. oak, beech), and the ectomycorrhizal system generally associated with it. [27] Microscopic observation of moder OF horizon shows that ectomycorrhizal fungal hyphae penetrate the excreta of detritivorous fauna, thus creating a functional link between soil fauna and plants. [28] Moder is generally associated with environments where nutrient cycles are slowed down, either due to cold climate (high altitude [29] or latitude), [30] low availability of weatherable minerals in the parent rock, [31] or because of acidifying vegetation (e.g. conifers). [32]

Mor

Horizon levels of Mor humus and the soil below Mor humus Diagram.png
Horizon levels of Mor humus and the soil below

Mor is the product of a slow evolution of organic matter in the absence of significant faunal activity. [15] The absence of OF and OH horizons as defined below and the presence of an abrupt transition with the underlying mineral horizons distinguish mor from other terrestrial humus forms. [33] However, there is considerable confusion in distinguishing mor from moder humus forms with very thick O horizons but intense activity of soil fauna, particularly enchytraeids. [15] The notion of 'raw humus' has long prevailed in pedology, making it difficult or even impossible to separate mor from moder when the litter is very thick, without taking into account faunal activity. [34] In mor humus, litter fragmentation results mainly from physical processes, such as freeze-thaw cycles, [35] and microbial processes, particularly fungal activity. [36] It has been demonstrated that nitrogen transfer occurs directly from slowly decomposing plant remains to vegetation, via the mycelium of ectomycorrhizal [37] and ericoid fungi, [38] in relation to the exploratory capacity of mycelia, at its maximum in mor. [39] Mor is associated with very cold climates, high altitude and latitude, or with very acidifying vegetation as in certain coniferous forests (notably pine) or in ericaceous heath, or even in highly polluted soils. More generally mor is associated with ecological factors that strongly reduce the activity of soil organisms, whether animal or microbial.

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. [40] 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. [41]

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." [42]

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." [43]

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." [44]

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, [45] [46] later translated into German [47] and French, [48] 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. [49] 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). [50] 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. [51]

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. [52] HUMUSICA describes and classifies humus forms from a wide array of terrestrial, semi-aquatic, cultivated and man-made environments. [53] [54] [55] 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. [56]

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. [57] 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, [58] while pedon changes take decades [59] to centuries. [60] However, Walter Kubiëna considered that there was a parallelism between humus forms and soil types, hence his common classification of humus and soil profiles, [41] 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 [61] and FAO's World Reference Base for Soil Resources (WRB). [62] 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. [63]

Diagnostic horizons

Succession of OL, OF, OH and A horizons in a moder humus from a beech forest: from left to right each picture is a top view of a horizon upon discarding the layer immediately above it Moder dans une hetraie.png
Succession of OL, OF, OH and A horizons in a moder humus from a beech forest: from left to right each picture is a top view of a horizon upon discarding the layer immediately above it

Humipedons display a succession of horizons according to decomposition stages of fallen plant litter and its progressive incorporation to mineral matter. [20] They have been characterized on thin soil sections by soil micromorphologists, [64] but their recognition in the field is easy, being aided by the use of a hand lens if necessary. [65] They can be observed along a humus profile cut with a sharp knife along a trench [66] or be successively collected by hand one by one from the top to the bottom of a small soil monolith. [67] : 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. [68] Bleaching of litter may also occur when leaves or needles are colonized by white-rot fungi. [69] The OL horizon is often seen permeated by fungal mycelia which penetrate leaves and needles and participate to their decomposition. [70] 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. [71]

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). [72] Enchytraeid faeces are so small that they appear as a very fine black powder covering or intermingled between decaying leaves. [20] 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. [73] 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. [74]

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. [20] By their vertical movements [75] enchytraeids play a decisive role in the transition with the underlying A horizon. [76] Fine root systems are also present in OH horizons, together with subterranean organs of heathland plants (e.g. Ericaceae) [77] and their symbiotic fungal associates (ericoid mycorrhizae) which are able to decompose recalcitrant organic matter and transfer its nitrogen to the host plant. [78]

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). [79] Some physical processes may also contribute to the mixing of organic matter with mineral matter, such as shrink-swell cycles of vertisols. [80] 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). [81] 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). [82] [83]

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. [84] Compared to the abovelying organic and mineral-organic horizons,the E horizon displays only poor signs of biological activity, [85] 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. [86] 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, [87] is still a matter of conjecture because highly weathered minerals are present in the E horizon. [88] This suggests that mineral weathering mainly takes place in the overlying A horizon permeated by plant roots and their microbial rhizosphere associates. [89] 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. [90] Research in alpine grasslands demonstrates that a 3 °C temperature increase reduces soil humus content and destabilizes soil structure. [91]

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. [92]

See also

Marine sediment

References

  1. ↑ Kõlli, Raimo; Köster, Tiina (February 2018). "Interrelationships of humus cover (pro humus form) with soil cover and plant cover: humus form as transitional space between soil and plant". Applied Soil Ecology. 123: 451–4. doi:10.1016/j.apsoil.2017.07.029 . Retrieved 9 October 2025.
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  13. ↑ Prescott, Cindy E.; Maynard, Doug G.; Laiho, Raija (1 August 2000). "Humus in northern forests: friend or foe?". Forest Ecology and Management . 133 (1–2): 23–36. doi:10.1016/S0378-1127(99)00295-9 . Retrieved 9 October 2025.
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  16. ↑ Muys, Bart; Lust, Noël; Granval, Philippe (December 1992). "Effects of grassland afforestation with different tree species on earthworm communities, litter decomposition and nutrient status". Soil Biology and Biochemistry . 24 (12): 1459–66. doi:10.1016/0038-0717(92)90133-I . Retrieved 10 October 2025.
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  19. ↑ Bal, L. (June 1970). "Morphological investigation in two moder-humus profiles and the role of the soil fauna in their genesis". Geoderma. 4 (1): 5–36. doi:10.1016/0016-7061(70)90030-3 . Retrieved 10 October 2025.
  20. 1 2 3 4 Ponge, Jean-François (November 1999). "Horizons and humus forms in beech Forests of the Belgian Ardennes". Soil Science Society of America Journal . 63 (6): 1888–901. Bibcode:1999SSASJ..63.1888P. doi: 10.2136/sssaj1999.6361888x .
  21. ↑ Schaefer, Matthias; Schauermann, Jürgen (1990). "The soil fauna of beech forests: comparison between a mull and a moder soil". Pedobiologia. 34 (5): 299–314. doi:10.1016/S0031-4056(24)00031-3 . Retrieved 10 October 2025.
  22. ↑ Article ([[Special:EditPage/{{{1}}}|edit]] | [[Talk:{{{1}}}|talk]] | [[Special:PageHistory/{{{1}}}|history]] | [[Special:ProtectPage/{{{1}}}|protect]] | [[Special:DeletePage/{{{1}}}|delete]] | [{{fullurl:Special:WhatLinksHere/{{{1}}}|limit=999}} links] | [{{fullurl:{{{1}}}|action=watch}} watch] | logs  | views)
  23. ↑ Dymond, Pamela; Scheu, Stefan; Parkinson, Dennis (March–April 1997). "Density and distribution of Dendrobaena octaedra (Lumbricidae) in aspen and pine forests in the Canadian Rocky Mountains (Alberta)". Soil Biology and Biochemistry . 29 (3–4): 265–73. doi:10.1016/S0038-0717(96)00052-1 . Retrieved 10 October 2025.
  24. ↑ Springett, Josephine A.; Brittain, John Edward; Springett, Brian Peter (1970). "Vertical movement of Enchytraeidae (Oligochaeta) in moorland soils". Oikos. 21 (1): 16–21. doi:10.2307/3543833 . Retrieved 10 October 2025.
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  29. ↑ Bayranvand, Mohammad; Kooch, Yahya; Hosseini, Seyed Mohsen; Alberti, Giorgio (1 February 2017). "Humus forms in relation to altitude and forest type in the northern mountainous regions of Iran". Forest Ecology and Management . 385: 78–86. doi:10.1016/j.foreco.2016.11.035 . Retrieved 10 October 2025.
  30. ↑ Berg, Björn; McClaugherty, Charles; Virzo de Santo, Amalia; Johnson, Dale (June 2001). "Humus buildup in boreal forests: effects of litter fall and its N concentration". Canadian Journal of Forest Research . 31 (6): 988–98. doi:10.1139/x01-031 . Retrieved 10 October 2025.
  31. ↑ Ponge, Jean-François; Jabiol, Bernard; Gégout, Jean-Claude (15 April 2011). "Geology and climate conditions affect more humus forms than forest canopies at large scale in temperate forests". Geoderma. 162 (1–2): 187–95. doi:10.1016/j.geoderma.2011.02.003 . Retrieved 10 October 2025.
  32. ↑ Labaz, Beata; Galka, Bernard; Bogacz, Adam; Waroszewski, Jaroslaw; Kabala, Cezary (October 2014). "Factors influencing humus forms and forest litter properties in the mid-mountains under temperate climate of southwestern Poland". Geoderma. 230–231: 265–73. doi:10.1016/j.geoderma.2014.04.021 . Retrieved 10 October 2025.
  33. ↑ Zanella, Augusto; Ponge, Jean-François; Jabiol, Bernard; Sartori, Giacomo; Kolb, Eckart; Le Bayon, Renée-Claire; Gobat, Jean-Michel; Aubert, Michaël; De Waal, Rein; Van Delft, Bas; Vacca, Andrea; Serra, Gianluca; Chersich, Silvia; Andreetta, Anna; Kõlli, Raimo; Brun, Jean-Jacques; Cools, Nathalie; Englisch, Michael; Hager, Herbert; Katzensteiner, Klaus; Brêthes, Alain; De Nicola, Cristina; Testi, Anna; Bernier, Nicolas; Graefe, Ulfert; Wolf, Ugo; Juilleret, Jérôme; Garlato, Andrea; Obber, Silvia; Galvan, Paola; Zampedri, Roberto; Frizzera, Lorenzo; Tomasi, Mauro; Banas, Damien; Bureau, Fabrice; Tatti, Dylan; Salmon, Sandrine; Menardi, Roberto; Fontanella, Fausto; Carraro, Vinicio; Pizzeghello, Diego; Concheri, Giuseppe; Squartini, Andrea; Cattaneo, Dina; Scattolin, Linda; Nardi, Serenella; Nicolini, Gianni; Viola, Franco (January 2018). "Humusica 1, article 5: Terrestrial humus systems and forms — Keys of classification of humus systems and forms". Applied Soil Ecology. 122 (Part 1): 75–86. doi:10.1016/j.apsoil.2017.06.012 . Retrieved 10 October 2025.{{cite journal}}: |last35= has generic name (help)
  34. ↑ Chertov, Oleg; Nadporozhskaya, Marina (16 October 2018). "Humus forms in forest soils: concepts and classifications". Eurasian Soil Science. 51 (10): 1142–53. doi:10.1134/S1064229318100022 . Retrieved 10 October 2025.
  35. ↑ Zhu, Jianxiao; He, Xinhua; Wu, Fuzhong; Yang, Wanqin; Tan, Bo (27 March 2012). "Decomposition of Abies faxoniana litter varies with freeze–thaw stages and altitudes in subalpine/alpine forests of southwest China". Scandinavian Journal of Forest Research. 27 (6): 586–96. doi:10.1080/02827581.2012.670726 . Retrieved 10 October 2025.
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