Humusica 2, article 13: Para humus systems and forms

doi:10.1016/j.apsoil.2017.09.043

Applied Soil Ecology

Volume 122, Part 2, January 2018, Pages 181-199

https://doi.org/10.1016/j.apsoil.2017.09.043 Get rights and content

Highlights

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First attempt of classification of atypical humipedons called Para humus systems.
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Diagnostic features and horizons are presented with the help of photographs.
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Examples of classification of Para systems included in larger ecosystems.
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Cosmopedon, Aeropedon, Humi-Co-and Lithopedon, Hydropedon, Geopedon and Symbiopedon.

Abstract

Planet Earth is covered by very common Terrestrial (not submersed), Histic (peats) and Aqueous (tidal) humipedons. Beside these typical topsoils there are other more discrete humipedons, generated by the interaction of mineral matter with microorganisms, fungi and small plants (algae, lichens and mosses). In some cases roots and their symbionts can be a driving force of litter biotransformation, in other cases a large amount of decaying wood accommodates particular organisms which interfere and change the normal process of litter decomposition. Particular microorganisms inhabit submerged sediments or extreme environments and can generate specialised humipedons with grey-black or even astonishingly flashing colours. We describe all these common but still unknown humipedons, defining diagnostic horizons and proposing a first morpho-functional classification, which still has to be improved. At the end of the article, the hypothesis of evolving and interconnected Cosmo, Aero, Hydro, Humi, Co, Litho and Geopedons (related to the microbiota) is formulated as a speculative curiosity.

Section snippets

Introduction to Para humus systems

Beside Terrestrial and Histic humus systems and intergrades, it is possible to notice other humus systems in natural or artificial environments. Their ecological determinants are different from those of the main systems and are strongly related to specific habitats and/or plant covers. Labelled “Para” (from the ancient Greek παρά, aside), these novel humus systems and their main characters are reported in Table 1. We know that we are not in the mainstream of soil science, by extending the

Field assessment of Para humus systems

Specific diagnostic horizons allow the identification of each Para humus system. As listed below, Para systems are presented together with their associated prefixes. The adopted classification principle is to use the name alone (Crusto, Bryo, Rhizo, Ligno, Anaero, Archaeo) when the absence of more evolved horizons does not allow assigning them to the main classical Terrestrial or Histic humus systems or forms. Unless arrested in their development by erosion or climatic constraints, they

Crusto humus systems

Crusto humus systems are strongly controlled by the presence of bacteria, cyanobacteria, algae, lichens, bryophytes, and microfungi living in extremophile, aerated or periodically watered habitats (Evans and Johansen, 1999; Elbert et al., 2012). These organisms may form monospecific (microbial communities with a visible dominant member and many invisible associated members) or plurispecific (microbial communities with numerous invisible microscopic members) covers, from micrometric biofilms to

Bryo humus systems

Bryo are humus systems strongly influenced by dominant mosses or arbuscular lichens (mosses and lichens are often associated) or small stonecrop plants (Fig. 4, Fig. 5, Fig. 6, Fig. 7, Fig. 8, Fig. 9). These organisms may form mono- or multi-specific covers. Aeration is a mandatory prerequisite even if they prefer wet environments where they may form peatland (sphagnum bogs). These plants may grow on remains of a previous stem and when looking through a Bryo humipedon profile, old and new

Rhizo humus systems

Rhizo humus systems occur when roots are the driving factor of humus system development (Fig. 10, Fig. 11, Fig. 12, Fig. 13, Fig. 14, Fig. 15, Fig. 16). This humus system is not easy to detect because root-built aggregates are similar to other soil aggregates and, in addition, roots may penetrate and modify earthworm aggregates (Pouvelle et al., 2008). The humipedon is systematically assigned to a Rhizo system where more than 70% of the volume of the cumulated humus horizons is made of roots or

Ligno humus systems

Ligno are humus systems made of dominant wood under transformation by wood-feeding animals and/or wood-rotting fungi.

Anaero humus systems

Here the concept of humus system is extended to environments where water is the matrix. This should be considered as an innovation, which is proposed to the scientific community as a possible extension of names and concepts coined for terrestrial environments to aquatic environments. Although undergone by specialists of different disciplines, using different methodologies and different vocabularies, soil and sediment studies show that both environments ensure similar functions, such as for

Archaeo humus systems

Archaeo humus systems are under the influence of archaea or anaerobic bacteria living in extremophile submerged and emerged habitats without plants: highly saline, acidic or alkaline waters, hot springs among many others. These micro-ecosystems are formed by phototroph, lithotroph and/or organotroph microorganisms living in extremophile habitats (Fig. 28).

Can a column of plankton have humus inside? Is there humus even in the intergalactic space?

If we admit that the main components of a humus form are its specialized colonies of biodegrading and organic matter transforming microorganisms, we can say that there is humus in a column of plankton and even in the air. Soil microorganisms disperse out of the soil. Like pollen they are transported everywhere through the air in a kind of “microscopic dusty-misty-soil” represented by mineral and organic and/or organic-mineral particles, water, air and microorganisms.

A new more realistic concept

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As for other articles of Humusica 2, the reader must be informed that some knowledge of Humusica 1 (especially articles 1 and 7) is necessary for knowing what we mean by “humus form”, “humus system”, “humipedon”, “diagnostic horizon”, etc., and the same for the use of qualifiers, mosaic references, and most basic principles of our classification.

We must admit that Section 9 is largely speculative, and needs more developments in a paper specifically devoted to this topic. However, we think that

Authors’ contributions

Zanella A. and Ponge J.-F.: coordination of authors, structure of the article, redaction of the first draft of text, tables and figures, re-elaboration, final presentation; Zanella A.: non-cited author of photographs.

Others Authors: collaboration to the redaction of the article in specific domains and punctual corrections in other parts. Introduction and Field assessment: Matteodo M., Fritz I., Pietrasiak N.; Crusto: Pietrasiak N., Fritz I.; Bryo: Matteodo M.; Rhizo: Juilleret J., Matteodo M.;

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The humus forms are most comprehensively represented for forests on automorphic soils (Zanella et al., 2018g; Zanella et al., 2018h). In the latest version of the Classification, considerable attention is paid to temporary wet soils (Zanella et al., 2018i), permanently waterlogged soils (Zanella et al., 2018c; Zanella et al., 2018d), and Para systems (Zanella et al., 2018f). The Classification presents man-induced humus forms in much less detail and includes only artificially constructed substrates, urban soils, and agricultural soils (Zanella et al., 2018b; Zanella et al., 2018k; Zanella et al., 2018l).
Chemically contaminated soils are not included in the European morpho-functional classification of humus systems and forms. However, they differ from the uncontaminated analogs in biological, chemical, and physical features. We investigated the topsoils contaminated by long-term sulfur dioxide and metal emissions from a copper smelter (southern taiga; conifer and deciduous forests; the Middle Urals, Russia). The smelter has been in operation for more than 80 years, but ten years ago, emissions almost ceased which initiated the recovery of adjacent ecosystems. We performed 1,155 humus profile descriptions on 231 sampling plots in highly contaminated, moderately contaminated, and background sites. In the contaminated sites, we found 21 non-typical humus forms that do not fit the European classification. Earthworms’ extinction caused by soil toxicity resulted in a Mull-to-Mor transformation of humus profiles. Recent recolonization of poorly decomposed litter by earthworms and other soil macrodetritivores after emission cessation triggered a Mor-to-Mull shift. Because different topsoil layers react to environmental changes with unequal rates, signs of several stages of humus evolution can be imprinted in one profile. More rapid degradation of organic horizons than organic-mineral ones results in non-typical Mor forms when the thick non-zoogenic OF combines with zoogenic A horizon. Zoogenic O horizons’ heterogeneity due to non-zoogenic inclusions, or inversion of the litter layers sequence, or lagging of A horizon recovery behind the O horizons, represents the discrepancies between non-typical regraded and typical humus forms. Mor-to-Mull recovery can occur with and without passing the Moder forms. In the first path, all intermediate stages at least fitted to one of the known humus systems. In the second path, intermediate humus forms are represented by non-zoogenic OF conjoining highly zoogenic OF and lacking OH horizon (we named such forms Mormull). In both paths, the recovery starts from the upper litter layer and spreads downward, i.e., the signs of the zoogenic activity change in the opposite direction than within the natural Mor profile. We consider the non-typical humus forms as non-equilibrium topsoil states that are ephemeral on the forest succession scale. We described non-typical humus forms in detail, proposed their evolutionary diagram, nomenclature, and preliminary typology, and extended the Humus Index with non-typical forms.
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Forms such as “Para” and “Anthropogenic” have recently been added (Zanella et al., 2017c). Para-humus systems are driven by specific ecological determinants such as bryophytes, roots or even biological crusts (Zanella et al., 2018a), while Anthropogenic, or “techno” humus system result from human activities (Zanella et al., 2018b). One way to characterize the humus form developing on the soil surface is the use of thin-section micromorphology.
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