Humus form
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.
Humus form is important in ecosystem management, such as forest management, as it influences a forest's ability to regenerate through the establishment of new saplings and seedlings. Humus form is affected by land use, vegetation dynamics, pollution, and climate change.
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.Mull
Mull is the product of the mixing activity of burrowing soil animals which create nests and burrows within the soil biomantle. 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 by invertebrates. Some mull-forming animal groups ingest soil and mix it with mucus in their gut, or mix it with their saliva to create nests and tunnels. All these disturbances, whether mechanical or biochemical, stimulate microbial activity, hence faster nutrient cycles and better mineral uptake at root level. Thus mull is associated with more fertile soils and productive ecosystems, 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. Mull is linked to favorable environmental conditions, more particularly good litter quality, mild climate and availability of weatherable minerals in the parent rock.Moder
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, a fine reddish-brown powder. The animals responsible for the fragmentation of litter and its transformation into humus belong to the detritivorous fauna, including microarthropods, macroarthropods but also molluscs and worms. Litter fragmentation and deposition of organic excrement generate an OF horizon which is gradually transformed into an OH horizon as animal excrement accumulates. The small vertical movements of epigeic worms and enchytraeids generate a thin A horizon, with a microgranular or particulate structure, with a diffuse transition with the overlying OH horizon. Although the OH horizon is considered a diagnostic horizon for moder it is the intense biological activity within the OF horizon which truly marks the moder. In addition to the detritivorous fauna, and predators associated with it, the moder OF horizon is permeated by the fine root system of trees, notably conifers and hardwoods with recalcitrant litter, and the ectomycorrhizal system generally associated with it. 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. Moder is generally associated with environments where nutrient cycles are slowed down, either due to cold climate, low availability of weatherable minerals in the parent rock, or because of acidifying vegetation.Mor
Mor is the product of a slow evolution of organic matter in the absence of significant faunal activity. 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. 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. 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. In mor humus, litter fragmentation results mainly from physical processes, such as freeze-thaw cycles, and microbial processes, particularly fungal activity. It has been demonstrated that nitrogen transfer occurs directly from slowly decomposing plant remains to vegetation, via the mycelium of ectomycorrhizal and ericoid fungi, in relation to the exploratory capacity of mycelia, at its maximum in mor. Mor is associated with very cold climates, high altitude and latitude, or with very acidifying vegetation as in certain coniferous forests 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, also called 'twin mull', 'amphimull', 'mull-moder', or 'xeromoder', combines the characteristics of mull and moder, with the presence of OF and OH horizons and an A horizon with a granular structure resulting from the seasonal activity of earthworms. The meaning of "Amphi" closely relates to duality and encompasses two environments or sides, as seen in words like "amphibian" and "amphitheatre". Using the term to identify a humus form emphasizes existence or action "on both sides" or "in two manners". This form of humus, undoubtedly more common than it appears in view of the numerous studies restricted to the classical division into mull, moder and mor, is found in environments with strong seasonal contrasts, of or mountain type. Activities by depth peak at different times of the year in the amphi humus form, more so than in the other humus forms. In Mediterranean environments, summer drought forces earthworms to burrow more deeply. In mountain environments, winter frost plays a similar role. The amphi demonstrates that the sole observation of the thickness of litter layers is not sufficient to characterize the humus form; it must always be complemented with the observation of the structure of the underlying mineral horizon. This humus form deserves to be studied more widely, particularly in the context of the effects of climate warming, because it contains two juxtaposed carbon sinks and perhaps constitutes the form of humus with the best carbon storage potential.Tangel
Tangel is found in mountain environments, on hard limestone or dolomitic substrates, in cold and humid climates, and is characterized by a strong accumulation of organic matter, the Kubiëna's 'tangel layer', that can reach up to 1 m thickness, resting on the parent rock, without an A horizon or with a weakly developed A horizon. Tangel and mor constitute two pathways for the evolution of organic matter in cold and humid mountain climates but on alkaline substrates for the former and acidic for the latter. Biological activity, both faunal and microbial, within tangel profiles is still poorly understood because it has been too little studied. However, a study carried out in the northern Alps shows a decrease in microbial activity with depth, in relation to a fall in the abundance of fungi and bacteria, despite increase in abundance of archaea, without any clear hypothesis being formulated as to the possible causal relationships between biological activity and accumulation of organic matter. The probable presence of earthworm activity has been noted by some authors based on the observation of a granular structure within the accumulated organic but sampling of the soil fauna remains to be carried out.Semi-terrestrial humus forms
The humus forms present in peat bogs and marshes, due to their great variety, are subject to complex classifications, taking into account the stage of evolution towards terrestrial forms, depending on depth of the water table, its variations during the year and the nature of the dominant vegetation. For more information, the reader is referred to specialized articles describing diagnostic horizons and humus forms.Definitions
Virtual Soil Science Learning Resources group: "Humus forms are made of soil horizons located at or near the surface, which have formed from organic residues. Horizons that may comprise a humus form include L, F, H, and Ah, but not B or C."German Soil Science Society: "Humus form is an 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."
Canadian Ministry of Forests : "The humus form is defined as 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."