It is not easy to define what soil is 
. Various aspects of this “object” still elude science 
. In a functional context, researchers can say that soil is the “belly” of our planet Earth 
. In fact, soil “digests” everything that falls on it and permanently recycles and stores everything. It is not yet clear how or what all the structures recycled over time become in detail, but the outcomes of this incessant digestion certainly support the natural evolution of the entire planet 
In the case of forests, a short focus can help to define what forest soil is from a descriptive and functional point of view and provide indications related to the needs of proper forest management 
From a physical point of view, in a forest, researchers walk on the ground: a “pabulum” (literally, “absorbable food”) that soil scientists divide into “horizons”. The following are clearly identifiable in the topsoil or Humipedon: organic horizons (of almost intact litter: OL; of fragmented litter: OF; of litter transformed into humus: OH; organic remains in water, almost intact: HF; half decomposed: HM; well decomposed: HS) and organo-mineral horizons (A from earthworms, A from arthropods, A non-zoogenic). Horizons that are very commonly used to describe other underlying parts of the soil include Copedon horizons (mineral, depleted: E; mineral newly formed: B) and Lithopedon (fragmented rock: C; compact rock: R).
Soil horizons are the result of an interaction between the geo-climatic environment in a geographical area on Earth and the organisms that are more or less permanently present in that volume of the biosphere (Figure 1). Many of these organisms complete their life cycle only in this component of the ecosystem. Although they belong to the same forest ecosystem, topsoil and soil have very different but complementary functions. They cannot exist without each other until the limit of their reserves is exhausted.
Initial soils on acidic (Left
) and calcareous (Right
) ledges sampled by two student mountaineers 
. The centimeter scale bar shown in the center applies to both images. These profiles were obtained from vertical rock faces at an altitude of about 2000 m by driving a tube into the ground supported with ledges 50–150 cm wide. In the photo on the left, researchers can see the organic part matted with roots (OH), which is detached from the slightly lighter organo-mineral part (A) and resting on the rock; on the right, detachment occurs between a light brown organic part made of decomposing moss (OF) and a darker organic part in contact with the rock (OH). These micro-soils rested on a fragmented rock horizon that was not picked up with the probe (C horizon). These profiles illustrate the early stages of soil formation in an untouched, high-altitude environment. The diagnostic horizons are composed of arthropods and enchytraeids excrements, grains of fragmented rock and shredded and decomposed organic matter, owing to the action of bacteria and fungi.
The biological characteristics primarily concern biodiversity, which, in the soil, is clearly different from that out of the soil due to the lower presence of autotrophs and the dominance of a more complex network of heterotrophs. In terms of the number and variety of species and individuals, the soil biodiversity is richer than that outside of the soil 
Then, the storage function becomes important. Soil is the “living bank” of the forest system 
. It receives energy and matter from the outside and packages it in storage and release structures that are sensitive to biological stimuli. It looks like a “living sponge” placed on the rock. It can be several meters thick under particularly favorable conditions. At our latitudes, the thickness of the soil is less than 100 cm. The seeds of most forest plants lie dormant in the soil. When environmental conditions are appropriate, which, depending on scale, can correspond to a stand-replacing disturbance (e.g., wind storm avalanche, fire, etc.) as well as to a gap creation (e.g., fall of individual old trees), the activities of soil’s trophic network trigger the cessation of seed dormancy, and these seeds start the first dynamic phases of the forest ecosystem 
Finally, in soil genesis, Humipedon oversees the renewal and continuous growth of the forest ecosystem. In order to resist wear and tear and maintain an effective action over time, living organisms must undergo constant recycling. In a forest composed of complex and interconnected uni- and multicellular organisms, the natural death of these organisms involves the recycling of significant quantities of components that are transformed into the soil. This recycling takes place in two stages: decomposition of the original structures and composition of new structures ready for reuse inside and outside of the ground. The point is that both organic and mineral materials are concerned with the dichotomy between building and reuse fractions (also called inherited and newly formed). The inherited part of the soil is issued from dislocation, while the newly formed part of the soil generates fundamental materials such as clays and humic acids. The two opposing movements end at a point that is still not fully known today. This unknown is expressed in the word “humus”, which has a transcendental meaning that always stimulated scientists. A milestone in this scientific journey is certainly the study by Miller and Urey 
. They placed the molecules of a primitive soil soup in a test tube and, imitating the environment of a planet in its infancy, they attempted to create the first bricks of the living world. Using artificial electrical discharges mimicking distant storms, that primitive matrix produced many of the molecules that make up living cells.
It should be emphasized that soil is not a habitat. There is no “home for living organisms” called “soil” because soil microorganisms “are soil”. An example of a soil habitat could be that of the moon as it does not contain microorganisms 
. A “soil-habitat” can be made on our planet by sterilizing ordinary soil using autoclaving at 105 °C for 48 h. However, the problem is that once sterilized, it will no longer be soil. Soil made up of organo-mineral aggregates containing microorganisms looks more like a giant amoeba than an inert rock. It is certain that soil corresponds to an ecosystem 
, but to come to consider the soil as if it possessed the functional complexity of a supra-organism is still an open discussion 
The present article is based on a purely living soil definition. Some of the authors strongly disagree with a purely biological definition for the concept of soil, but researchers mutually agreed to keep it to provoke discussion and growth. Thus, researchers focus on a fundamental issue that cannot remain unresolved: the soil is either a biological matrix and, therefore, it must be treated as such (it must be protected as an inherited living structure) or it is a raw material inhabited by living beings and, therefore, researchers can destroy and modify it (for example, with the plow or by occupying it without thinking that researchers are destroying something irrecoverable) as if it were not an entity with a valuable historical path and heritage.
To conclude this introduction to the concept of soil, researchers now discuss earthworms, which are the main “builders” of soil on our planet, at least in all areas between −5 and +35 °C 
. Trying to summarize their action in a few lines is difficult. Therefore, researchers recommend downloading for free the fantastic book by Clive A. Edwards and Norman Q. Arancon 
, in which historical research on earthworms is collected, and reviewing the appropriate information. This book takes the reader very far and on the same path traced by Darwin in 1881 
, in search of the functional and ecological significance of these underground animals.
2. The Humipedon in Mountain and High-Mountain Forest Environments
The strength of the interactions within a couple can be tested at their limits. This is as true for the interactions between two human beings as it is for the interactions between vegetation and humus. Ponge et al. 
postulate that at the low-montane level, forest regeneration is limited most of the time by light alone because humus metabolism is always high; in contrast, at the high-mountain level, light becomes less and less of a limiting factor and humus becomes thicker. As a consequence, the role of burrowing animals such as earthworms in ecosystem functioning changes from important to critical as the transition from mountain to subalpine forest occurs. Earthworm burrowing activity alters humus on two levels. Macroscopically, the large anecic earthworms are able to modify their environment by spreading their organo-mineral feces along their main middens and secondary exploratory tunnels. Microscopically, earthworms ingest both organic and mineral material, mixing the two thoroughly. The result of their activity is thus the formation of the organo-mineral complex 
. As a result of the increasing rate of organo-mineral complexes near the surface, tree seeds can easily find a favorable microsite to grow. André et al. 
show that a key component in forest regeneration is the occurrence of mineral substance near the surface (Figure 2
). In an experimental plot in Savoy, almost all very young spruce trees were found to be growing on mineral deposits created with the burrowing activity of small mammals compared to the undisturbed soil 
. This regeneration micro-niche could be experimentally replicated by stripping the holorganic part of the humus.
Figure 2. Examples showing subalpine forest regeneration sites on a scree slope. (Left): on weakly stabilized skeletal soil. (Center): the few-centimeters-thin A horizon of a Mull system on skeletal soil on and between blocks of rock in Grand Follié forest—1550 m, Saint Foy Tarentaise, France. (Right): on a stump, i.e., an example of a 50 cm thick Ligno humus form (Somadida forest—1300 m, Auronzo, Italy).
In the subalpine belt, there are many reasons why minerals come close to the surface. The most effective is indeed the burrowing of earthworms (Figure 3
and Figure 4
, but it can also be the consequence of other biological activities as diverse as wild boars 
, tree uprooting 
, millipede activity 
, small mammals 
and human digging of forest roads 
. It can also have a physicochemical origin, such as an alteration to the bedrock of a small overhanging cliff and the subsequent debris on steep slopes. All these are drivers of humus form change, leading to a mineral enrichment near the surface that supports forest regeneration. However, such mineral input in a holorganic humus form is often discontinuous in space and time, or, if bioturbation is generalized, it is present at low levels in subalpine forests 
. In contrast to lowland forest bioturbation, which produces the Mull humus form, low-level and/or episodic bioturbation in mountain environments produces a range of Amphi humus forms.
The four Amphi humus forms (Leptoamphi, Eumacroamphi, Eumesoamphi, Pachyamphi), identified in the Amphi humus system using the thickness of the OH horizon and the structure of the A horizon 
, can be grouped into two main functional categories. The Bipolar Amphi forms evolve continuously between the Mull and Moder or Tangel humus forms, sometimes in the direction of Mull and sometimes in the direction of Moder or Tangel. They are characterized by the perturbation of large earthworms, which release their biomacrostructured
organo-mineral excrements into the organic layers, orienting the evolution in two opposite directions: toward Mull (Leptoamphi) when the excrements increase and toward Moder when instead they decrease (Eumacroamphi, with thicker OH). The Stable Amphi forms are stable intermediates between Moder or Tangel and Mull, reflecting an incomplete but homogeneous organo-mineral incorporation. They generate a holorganic horizon above (OH) and a biomesostructured
or a mixed biomeso- and biomicrostructured
organo-mineral horizon (meA) below (Eumesoamphi, or Pachyamphi with thicker OH).
In general, with an increase in altitude, the Humipedon passes from Mull to Moder and finally to Mor (rare in a forest environment on the Italian side of the Alps) on an acidic substrate, while it passes from Mull to Amphi and then to Tangel on a calcareous or dolomitic substrate. The difference between a very thick Dysmoder and a Mor or a Tangel is a specialist topic and requires knowledge related to soil fauna. To simplify presenting the dynamics of the Amphi in the following paragraphs, researchers prefer to use the name Raw humus for Moder, Mor and Tangel, in which anecic earthworm bioturbation is absent. researchers can say in a simplified way that researchers progressively pass from Mull to Raw humus with a rise in altitude due to a decrease in the biological activity of the soil. This dynamic is important for understanding how forest soil works. Below researchers propose a practical simplification that could facilitate an understanding of Humipedon in a mountain forest environment even for beginners.
3. Humipedon and Forest Management
An imaginary horizontal surface located below the forester’s feet divides the forest into two distinct yet integrated and highly interacting components of the same ecosystem. In general, the aboveground component is better known because it is more accessible for surveys and measurements. This component transforms solar radiation into a more useful form of energy thanks to the photosynthetic process, from which the forester extracts a part of the biomass for the needs of the human economy. Under the forester’s feet, a second “biological forge” develops the Humipedon. This also corresponds to a living, functional and efficient ecosystem component that supports and cooperates with the one above, from which it in turn receives the energy and matter needed for its distinctive processes, in the form of necromass (leaves, needles, bark, wood, the transformed remains of animals).
Therefore, Humipedon has its own ecological pyramid at different levels. Among the most typical animals in this ecosystem component, earthworms play a particularly important role as they feed on necromass along with the mineral soil. In their guts, which are almost as long as their bodies, they digest leaves and extract energy and minerals required for their metabolism. During this digestion process, part of the resulting molecules ends up in circulation in the earthworm’s body, which provides the energy and matter required to complete its life cycle. The waste pulp expelled from the body of earthworms is what Darwin (Figure 5
) discovered back in 1881 and called “soil” 
. It helps us to imagine an elastic and living tube, which moves in the soil by ingesting leaves and earth and expelling “living-soil”. This phenomenon became even more interesting when Marcel Bouché discovered that earthworms also eat their own excrement 
. It is not ingested fresh but only after the excrement has matured in the soil for a certain period of time. Earthworms can extract matter to live on from their own excrement, and during the process, they move the soil unceasingly, making it homogeneous. Undisturbed in the forest floor, year after year, earthworms increase the amount of carbon in the Humipedon and distribute it uniformly in the profile. Furthermore, their movement creates tunnels in which air and water can circulate, allowing the degradation of organic matter and the reproduction of organisms present in the depths. Thus, the Humipedon can be thought of as an “auto-recharging living battery” of the forest. The recycling of organic matter produced by the autotrophs on the surface depends on earthworms, which continuously recharge the Humipedon “organic battery” of the forest. On a small scale, the presence of innumerable microorganisms can explain the functioning of the entire ecosystem, which evolves almost autonomously, because it should contain all connected living things, including humans, whether they are forest operators or not (Figure 5
. researchers now return to the concept of soil
(Chapter 1): soil
(and even more, its most living part, the Humipedon) should not be considered a “habitat”. Only the “new soil” that is “made by the living”, “the organo-mineral sorption complex, the living things that make it and maintain it, the connection network between livings that keeps this network alive” is soil
. The rest can be considered habitat or substrate. Either the soil is alive, or it does not exist 
Earthworms characterize Mull Humipedons, which develop in temperate, mild, warm and humid climatic environments, such as those found in the meadows described by Darwin, and in all the deciduous lowland forests in Europe. They do not have the OH (humiferous) horizon typical of colder or drier environments with conifers or xerophilous species. Other animals (enchytraeids and/or especially arthropods) are responsible for recharging the battery of the soil in these colder or even drier environments, such as the Mediterranean 
. More generally, also considering wetlands, living organisms and the mechanisms for recharging and maintaining energy in the soil have evolved for each ecosystem environment (Figure 6
Synthetic keys for the classification of Humipedons in reference humus systems for terrestrial environments (non-asphyxiated soils) or for semiterrestrial environments (more or less submerged soils, peats). Modified from Zanella et al. 
with permission. These keys are based on the recognition of diagnostic horizons. The superimposed set of these horizons corresponds to a function of the Humipedon, in harmony with the rest of the soil, with the vegetation and climate of those environments. Summary legend: Horizons of Humipedon: Terrestrial
(aerated) organic horizons: OL, OF, OH + organo-mineral horizons: A; Semiterrestrial
(more or less submerged) organic horizons: HF, HM, HS + organo-mineral horizons: A. Meaning of lowercase letters applied as prefixes to diagnostic horizon codes: zo = zoogenic, noz = non-zoogenic, szo = weakly zoogenic, an = of Anmoor, ma = biomacrostructured, me = biomesostructured, mi = biomesostructured, ms = massive, sg = single-grain. Meaning of the initial part of the name of semiterrestrial systems: Fibri = generally submerged and very fibrous, Sapri = completely transformed into organic pulp, Mesi = transition between Fibri and Sapri, Moor = peats. About the relative thicknesses of the horizons and the accuracy of the transition between them: A ≥ 2 OH: thickness of the A horizon ≥ 2 times that of the OH; A < 2 OH: thickness of the horizon A < 2 times that of the OH; OH < gradual A: gradual transition between OH and A, inaccuracy > 5 mm; OH/net/A: sudden transition between OH and A, inaccuracy < 3 mm.
From what has been said, researchers must conclude that it is necessary to plan and practice specific management for forest soil. When you walk on the forest floor, your feet are resting on a very useful living sponge, which exists not only as a support for the static anchoring of trees, which play a dominant role as the driving force of the ecosystem, but also as a source of energy for all the whole forest life.
It is necessary to manage the soil, as already performed for the overlying tree part of the forest ecosystem, and to foresee a real plan for the conservation and restoration of the soil. This includes (i) an estimation of the average amount of organic carbon present in the soil in each management unit (above soil provision) and the structure and consistency of the eco-units (structural types in the sense of 
); (ii) identifying the systems and forms of humus dominant in each eco-unit; and (iii) creating a list of functional groups of soil animals with the number of individuals in relation to seasonality 
It would be also of great interest to obtain a list of Humipedon living organisms, using sequencing of the soil metagenome (“Shotgun Metagenomic Sequencing”), to understand which functional groups of microorganisms are present in the soil and their relationships with topsoil plant species and with the other living organisms in the ecosystem 
It has long been known that vegetation structural types have their own Humipedon related to the age of the trees and the forest cycle 
. A coordinated balance between the above and below ground is necessary for continuity in the forest cycle with a phase for consumption of soil resources in the young forest phase and replenishment and recapitalization of resources during the mature and final forest phases 
. Knowing how to distinguish earthworm horizons from the others in Humipedon (Figure 2
) allows us to check whether the evolution of Humipedon follows harmoniously that of the topsoil and to diagnose problems related to regeneration 
shows, as an example, the results of a doctoral thesis carried out in Trentino spruce forests 
. It shows how the stages of forest development (herbaceous opening, natural regeneration, intermediate age, adult/mature forest) correspond to Humipedons classified in different humus systems. Such systems are shown schematically in Figure 7
: Mull and Amphi (mainly Bipolar) humus are characterized by an A horizon formed by earthworms, which is not present in Moder or Mor humus. Basically, the forest floor evolves with the age of the trees it supports in balance with their needs. researchers consider that at the end of their life, trees prepare the soil for the next generation, feeding it and attracting the earthworms that prepare the organo-mineral aggregates that guarantee a good supply of water and nutrients to the new trees.
Figure 7. Humus systems in the development stages of an uneven spruce forest in acidic environments, facing south (AS) or north (AN), or in a basic environment facing south (BS) or north (BN). Notice how the humus system changes cyclically. In the Amphi and Mull systems, there are earthworms (with A horizons similar to those of the home garden); in the other systems, the earthworms are replaced with enchytraeids and arthropods, which form much more organic horizons, similar to those of shredded tobacco. The variance explained by the model presented on the plane of two axes (whose principal components are listed with increasing or decreasing importance along the axis) is about 68%.
The four stages of a forest can be clearly seen in the yellow–green–orange–blue sequence in Figure 6. In the upper right corner of the PCA plot, researchers have the northern (BN) and southern (BS) basic spruce forests, in contrast to the AN and AS acidic ones in the lower left corner. The basic ones would have higher pH, % of exchangeable bases, thickness of A horizons and average temperature in May (y-axis on the left). On the other hand, the acidic north environments show decidedly higher values of CO, thicknesses of the organic horizons and miA horizons, and absolute minimum temperatures in January and in the spring and fall (y-axis on the right). On the left of the x-axis, high values for the average temperature in the first 10 cm of soil are observed in all seasons, corresponding to both acidic and basic southern exposures. Interestingly, and probably true, is the pH compensation with AS versus AN exposure, with a soil cycling becoming similar to that encountered in BS. When this does not happen and the climatic conditions are harsh (north exposure in both cases), the two phytocoenoses separate on the basis of the parent rock, and the soil cycle that takes place in the BN is very different from that in the AN (absence of Mull and presence of Mor).
An important consequence of this diagnosis, i.e., the fact that the forest floor has a life cycle linked to that of the aerial part of the forest, is that it must be taken into account in the management and felling plans. The soil cycle must also be respected. If the ground is a bank, and if for 100–200 years, the aerial part has been borrowing energy and minerals from the ground, then it is imperative that this loan is repaid with interest to the soil before spending continues. A first management guideline could concern not only respect for some monumental trees 
and habitat trees 
but also a decisive lengthening of the life of forest trees using a doubling the duration of the cycles currently practiced. It is very probable that old age trees mainly nourish the soil, preparing it for renewal