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Use of micromorphology for humus characterization and classification in some mediterranean calcareous soils
Applied Soil Ecology xxx (xxxx) xxx–xxx
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Applied field research article
Use of micromorphology for humus characterization and classification in some mediterranean calcareous soils ⁎
Olena Zaiets , Rosa M. Poch Departament de Medi Ambient i Ciències del Sòl − Universitat de Lleida, Catalonia, Spain
A R T I C L E I N F O
A B S T R A C T
Keywords: Soil organic matter Humus form Micromorphology Amphi Mull
A micromorphological approach is potentially useful in the classification of humus systems. However, the systematic description of organic material found in thin sections of humus profiles is poorly developed in comparison to already existing guides for mineral soils. This study presents the use of thin section microscopical analyses combined with digital images in order to set the highly detailed method of qualitative description and classification of some Mediterranean humus forms. Three forests and one meadow were selected for topsoil sampling in the Catalan Pre-Pyrenean mountain region. Fourteen thin sections were prepared, scanned and analyzed using the petrographic microscope. This resulted in the description of seven micromorphological fabric units based on their amount, shape, color, size, distribution and degree of decomposition. Digital images from thin section scans helped to classify three Amphi (Pachy-, Eumacro-, Eumesoamphi), and two Mull (Oligo-, Eumull) humus forms.
1. Introduction The development of any humus form is controlled by soil-forming factors such as climate, topography, soil parent material, organisms (autotrophs and heterotrophs) and human activity (Barratt, 1969). The successive transformation stages of litter under the different levels of faunal and microbial performance lead to the formation of definite horizons of a particular humus form. The morphological differences between diverse humus forms reveal information not only about the efficiency of soil organic matter (SOM) transformation but also about the reaction of forest ecosystem to management practices and growth cycle (Bernier and Ponge, 1994; Ponge, 2013). In this sense, they are thought to be the fastest and the cheapest way of indicating the level of nutrient cycling in ecosystems (Ponge, 2003; Andreetta et al., 2011). Humus forms rich in biological activity may well correspond to wide range of pH and performance of enzymes (Andreetta et al., 2013). As it is not quite clear why different humus forms have the same C/ N ratios and SOM contents (Brethes et al., 1995) and yet different rates of biological activity, it is important to apply micromorphology to humus profiles. The first investigations on humus micromorphology were related to soil zoology (Kubiëna, 1955; Zachariae, 1964, 1965; Babel, 1968; Bal, 1970; Pawluk, 1987). At the same time, the decomposition of different plant rests and tissues in the horizons of humus forms and the formation of microfabrics were described in studies of Barratt (1964, 1967).
⁎
Scanned thin sections and micromorphology were also used by a few researchers to characterize humus types and faunal activity in relation to soil structure (Ciarkowska and Niemyska-Łukaszuk, 2002; Davidson et al., 2002, 2016). Finally, the most common humus components and fabric types of several humus forms found in Central Europe were summarized in “Monographs in Soil Science” (Babel, 1975). The description of organic materials has not been taken into account at the same level as that of mineral soil materials. In fact, many of the basic concepts established by Bullock et al. (1985) and later by Stoops (2003) such as the coarse/fine parameters, b-fabric, or even pedofeatures, cannot be applied or are irrelevant when describing thin sections of organic horizons. Even if we have information about the types of organic components and the changes they undergo in soils (Babel, 1968; Stoops, 2003; Stolt et al., 2010; Kooistra, 2016) we are still missing a systematics of description of organic materials, as we already have for mineral ones. In classical works on humus morphology (Müller, 1878) the three main humus forms Mull, Moder and Mor are described. However, the European reference base of humus forms (Zanella et al., 2011) also mentions Amphi, which seems to be typical for Mediterranean environment (Andreetta et al., 2011, 2016; De Nicola et al., 2014). In contrast, some authors emphasize that Amphi is a Xeromoder which overlies a macrostructured A horizon of Mull (Bottner et al., 2000) and is also found in southern part of Alps (Graefe, 2007) and in Belgian forests (Ponge, 1999).
Corresponding author. E-mail address: [email protected] (O. Zaiets).
http://dx.doi.org/10.1016/j.apsoil.2017.09.016 Received 24 November 2016; Received in revised form 5 September 2017; Accepted 11 September 2017 0929-1393/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: ZAIETS, O., Applied Soil Ecology (2017), http://dx.doi.org/10.1016/j.apsoil.2017.09.016
Applied Soil Ecology xxx (xxxx) xxx–xxx
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2.2.1. Structure and porosity The terms to be applied are the same as in mineral horizons for the pedal material, although this is not the most frequent situation for organic materials. For the apedal material (soil particles not assembled in higher units), the structure should be described according to the dominant fabric or c/f related distribution: for instance, massive (sapric densely packed material), layered (flat components with the longer axis parallel to the soil surface) or single grain (monic c/f related distribution, no fine material, loosely packed organic components).
A previous work by the authors explored the differences between Amphi and Mull humus forms in Mediterranean mountain soils based on micromorphological, physical and chemical observations, and found that it is possible to significantly assign a given set of micromorphological features to the studied humus types (Zaiets and Poch, 2016). The aim of the present study is to show with higher detail these micromorphological tools in order to set a method of qualitative description of humus profiles that could be useful for the study of soil organic matter dynamics.
2.2.2. C/f limit, c/f ratio, c/f related distribution Although these parameters are very important in mineral materials, they do not have a very high relevance in most of the organic materials. The definition of c/f limit (the lowest size of a mineral particle we can identify by optical mineralogy) cannot be applied to organic components, since the lowest size of any organic particle could be that of a phytolith, a cell or an amorphous humus particle, which have very different origins. Nevertheless, it could be relevant in some cases (i.e. sapric materials with some recognizable organic tissues).
2. Materials and methods 2.1. Study sites The study area is situated in the Pre-Pyrenean mountain region in the north-east of the Iberian Peninsula. The relief is tabular sometimes with steep slopes over 20%. The altitude is 800–1100 m asl. The area is covered mainly by pine and oak forests developed on stony calcareous soils. The climate is typical Mediterranean with transition to a subalpine type of climate on higher altitudes. Four habitats within the area were selected for topsoil sampling: holm oak forest (T: Torra), pine brook forest (CAN: Canalda), mixed pine forest (CO: Cogulers shaded; CS: Cogulers sunny) and high meadow (P: Prat). At each site organic, organo-mineral and mineral horizons were simultaneously sampled and humus forms (Table 1) were classified according to the European Humus Forms Reference Base (Zanella et al., 2011).
2.2.3. Coarse (organic) components This is the most important and significant part of the description, since the identification of these components determines to a large extent the nature and behaviour of the organic material. The description (Table 2) from organs to amorphous organic material follows the four degrees of organic matter decomposition that can be recognized. Indeed, the different transformation paths of organic components (Kooistra, 2016) are closely related to the ecological conditions of the humus forms.
2.2. Micromorphology
2.2.4. Pedofeatures Pedofeatures, often used as indicators of pedogenesis in mineral horizons, are not as meaningful in organic materials. Nevertheless, it is important to record any possible pedofeature in the mineral components, as well as evidences of redoximorphic or textural features (cappings, coatings of mineral material).
Fourteen thin sections (5 × 13 cm) of topsoil from the study sites were prepared following the methods of Benyarku and Stoops (2005). All thin sections depicted several organic horizons at once except of those prepared from mineral (Bw) horizons. All thin sections were analyzed through an Olympus petrographic microscope (BX51) using 2 x magnification in order to identify and describe fabric units. The thin sectionś true color scans were made with a high-resolution Epson scanner in order to explore simpler method of topsoil investigation. Each image had around 2 MB. The guidelines of Stoops (2003) were followed for general fabric description (as shape, distribution, orientation, abundance among others). Observed plant residues were grouped in classes according to their morphological changes during their decomposition (Tian and Takeda, 1997; Blazejewski et al., 2005). Descriptions of the different evolution patterns of organic materials followed Kooistra (2016). A special feature class “organic fine matter” refers to amorphous organic material and organic material that does not have recognizable origin and organic pigment in the inorganic micromass (Stoops, 2003). The proposed qualitative description of the humus profiles is based on both microscope and scan images and is enlarged from the one proposed by Stoops (2003):
3. Results 3.1. Field description of humus forms According to humus forms classification in the field, one Amphi and two Mulls were observed. The litter horizon (OL) of the Pachyamphi (CAN) consisted of partly bleached pine needles, twigs, pieces of wood, grass residues and moss. The OF horizon had lightly decomposed wood and other plant rests with numerous fungal hyphae. The underlying OH horizon was twice as thin than the previous one and of darker color with thin roots. The organo-mineral A horizon had a biomesostructure. The litter horizon of the Eumesoamphi (CS) was much poorer and consisted of fresh and old pine needles, twigs, bark, wood and grass rests. An OF horizon was present, whereas the OH was discontinuous. The A horizon had a biomesostructure.
Table 1 Study sites characteristics (from Zaiets and Poch 2016). Site code
Canalda (C)
Cogulers O (shaded aspect) (CO)
Cogulers S (sunny aspect) (CS)
Torra (T)
Prat (P)
Number of samples and horizons Vegetation
4 OL, OF + OH, A, Bw Natural riparian Pinus nigra forest 800 Typic Ustifluvent/ Orthofluvic Fluvisol Pachyamphi
4 OL, OF + OH, A, Bw Natural not managed Pinus nigra and Pinus sylvestris mixed forest 800 Typic Ustorthent/Calcaric Regosol
4 OL, OF + OH, A, Bw Natural not managed Pinus nigra and Pinus sylvestris mixed forest 800 Typic Calciustept/Leptic Calcisol
Eumacroamphi
Eumesoamphi
4 OLv, OF, A, Bw Natural not managed Quercus ilex forest 900 Typic Calciustept/Leptic Calcisol Oligomull
2 A, Bw Thymus vulgaris high meadow 1100 Typic Haplustept/Leptic Calcaric Cambisol Eumull
Altitude, m asl Soil type (SSS, 2014/ IUSS 2014) Humus form (ERB, 2011)
2
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Table 2 Description of components of organic materials. Organs
Tissues Cells Amorphous organic material Charcoal Faunal excrements
Mineral components other than components of excrements Boundary
Leaves, needles and shoots
Relative amounts, degree of decomposition, type of decomposition, basic and referred distribution and orientation Roots Relative amounts, degree of decomposition, type of decomposition, basic and referred distribution and orientation Fungi Relative amounts, type of organs (sclerotia, hyphae), degree of decomposition, type of decomposition, basic and referred distribution and orientation, related distribution to other components (organs, tissues, excrements) e.g. cork, wood, moss fragments, cuticle, epidermis, parenchyma: relative amounts, degree of decomposition, type of decomposition, basic and referred distribution and orientation Relative amounts, distribution and orientation (basic, referred, related). Relative amounts, size and arrangement basic and referred distribution and orientation; if possible, use the c/f related distributions. It should be described separately, given the importance and environmental significance of this component. Describe relative amounts, size, shape, orientation and distribution, degree of combustion, original plant component. Big faunal droppings (> 150–200 μm), e.g. Shape, size, internal fabric, relative amounts, type of organisms, degree of diptera, earthworms decomposition, type of decomposition, basic and referred distribution and orientation, related distribution to other components (organs, tissues, mineral material, etc.), b-fabric. Small faunal droppings Shape, size, internal fabric, relative amounts, type of organisms, degree of (< 150–200 μm), e.g. enchytraeids, mites. decomposition, type of decomposition, basic and referred distribution and orientation, related distribution to other components (organs, tissues, mineral material, etc.), b-fabric. Should be subdivided into coarse components and micromass, and described according to Stoops (2003) We propose to use the same field terms, but with different distances: abrupt < 5 mm; clear 5–10 mm; gradual > 10 mm.
Fig. 1. Three thin sections’ scans of Amphi humus forms: approximate horizons boundaries marked with red; meso- and macrofaunal channels marked with blue. The traces of endogeic and anecic earthworms in A horizon of Pachyamphi are represented by horizontal and vertical channels and their casting. Channels found in organic horizons may belong to anecic earthworms as well. Each scan width: 18 mm.
3.2. Micromorphological analysis of thin sections
In contrast, the Eumacroamphi (CO) had all organic horizons (OL, OF, OH) and a biomacrostructured A horizon with noticeable soil fauna activity. The Oligomull (T) had no freshly formed litter horizon (OL), instead, bleached old oak leaves formed a thin OLv horizon. The OF horizon was discontinuous and consisted of crumbled plant rests and granulated aggregates of organic matter. The OH horizon was absent. The A horizon had a biomacrostructure.
The use of thin section scans has a great potential in the morphological classification of humus forms. Scans depict the vertical structure of topsoil, showing organic and organo-mineral horizons regarding their fabric types and biological traces of organic matter decomposition. Due to the shallowness of most of the litter layers, (large) thin sections are useful for determining their thickness and the boundaries between them. Nevertheless, microscope observations are essential to observe 3
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Fig. 2. Two thin sections’ scans of Mull humus forms: approximate horizons boundaries marked with red; meso- and macrofaunal channels marked with blue. Each scan width: 18 mm.
explained by the ability of soil fauna to migrate deeper into the soil under unfavorable drought conditions (Zanella et al., 2011). The first step in the disintegration of plant litter in organic horizons is its consumption by macro- (other than earthworms) and mesofauna. Near organic rests or in them are numerous droppings of diptera larvae, isopods and mites. They are mostly present in OL and OF horizons. Their faeces serve a good environment for bacterial and fungi expansion. The penetration of fungal hyphae into macro- and mesofauna droppings was commonly observed in all thin sections. Study on micromorphology of SOM (Babel, 1971) has indicated that the percentage of volume of organic fine matter (OFM) increases with the depth in Moders. The same was discovered in the present study in Amphi and Mull: under a myriad of biotic and abiotic factors, plant rests, which are abundant and recognizable in litter layer, lose their coherent structure and more tissue fragments are found in OF and OH horizons. This depletion of plant material forms a structureless organic fine matter. The interaction of organic fine matter with mineral particles leads to the formation of aggregates as was discussed by Babel (1975). This was often observed in the OF and OH horizons of all Amphi samples where organic fine matter occupied 22–50% of the total volume (Zaiets and Poch, 2016). Furthermore, the mineral micromass of the A horizon was stained by organic pigments, which justifies the positive relationship found between mineral particles and OFM in the A horizon (Zaiets and Poch, 2016). Another important group of micromorphological features are small granular organic aggregates (41–80 μm) found mainly in the organic OF and OH and following the organo-mineral A horizon. In Pachyamphi and Eumesoamphi they formed together with the mineral micromass a
some features such as fungi (sclerotia, hyphae) that cannot be distinguished in scans (Figs. 1 and 2), and to avoid overestimation of some components as organic micromass (Zaiets and Poch, 2016). The descriptions of the 5 humus profiles are shown in Tables 3–7. 4. Discussion The difference between Mull and Amphi humus forms can be detected macroscopically due to the presence or absence of particular horizons and their sequence. However, the use of thin sections and their analyses could give important information on humus forms development and their biology. The assemblages of micromorphological features found in each horizon of Mulls and Amphis were quantified in the study by Zaiets and Poch (2016). The obtained results were statistically meaningful and corresponded to classification of humus forms made on the field according to ERB of humus forms (Zanella et al., 2011). Two types of humus forms − Amphi and Mull − which were found within the study area reflect different systems of humus formation. All Amphis had a well-developed litter horizon which consisted of pine needles of different stages of decomposition and other aerial plant residues. Under the microscope, the OL horizon showed a laminated fabric with numerous pores. The formation of this type of fabric is due to the properties of parent material and its resistance to the fast decomposition. Under temperate pine forests the build-up of coniferous needles leads to the formation of Moders which was described from the micromorphological point of view by Babel (1971, 1975). However, Mediterranean conditions contribute to the formation of Amphis under the same type of forest cover (Andreetta et al., 2016), which is 4
Thickness, continuity and boundary
Continuous, 0.5 to 1 cm thick. Abrupt boundary.
Continuous, 0.5 to 1 cm thick. Clear boundary.
Continuous horizon, 1 to 3 cm thick. Clear boundary.
Continuous
Horizon
OL
OF
OH
A
Few quartz and calcite sandstone grains.
15%: calcite, calcareous sandstone and few quartz grains, coarse and medium sand, random.
c/f ratio about 3/1, coarse components: calcite, calcareous sandstone and few quartz grains, coarse and medium sand; micromass with a calcitic crystallitic b-fabric. Pedofeatures: pseudomorphs of vivianite inside cells in a lignified root section.
Few diptera excrements, about 10% of small mite droppings inside plant rests and loose small droppings between fragmented plant tissues, random. Some excrements of diptera and earthworms (Fig. 6), and frequent small droppings: 15%, mite droppings and loose small droppings between fragmented plant tissues, random or in clusters together with comminuted organic rests. Total droppings: 10%: some mesofaunal (large) excrements, mainly diptera and earthworms; and small mite and loose enchytraeid droppings between fragmented plant tissues, random or in clusters together with comminuted organic rests.
Plant rests 15% (10% pine needles), stems, wood and plant tissues, moderately well preserved (brown to red), very few of them blackened. 15%, made of partly decayed crumbled pine needles and leaves, plant tissues and hardly recognizable separate plant cells, some of them phlobaphenized, with transition to organic fine matter. 5%, consisting of plant tissues and recognizable separate plant cells, many of them phlobaphenized, with transition to organic fine matter.
Loose, 70% compound packing pores between excrements and plant remains, random fabric (Fig. 3a).
Highly separated crumb and granular; peds smaller than 250 μm. 50% complex packing pores between plant remains, excrements and mineral grains, random; few channels and chambers.
Single grain to highly separated, crumbs up to 0.3 mm 50% compound packing pores, few channels and chambers.
–
Few (2%), small mite droppings inside plant rests and loose small (50–200 μm) droppings, probably enchytraeids, between fragmented plant tissues, clustered.
28% plant rests mainly as pine needles (1 st − 4th stage of decomposition, 50% of all plant rests); stems, plant tissues and woody rests, mostly well preserved (brown to red), few of them blackened, colonized by fungi. Amorphous organic matter almost absent.
Loose, 70% compound packing pores between plant remains, random to laminated fabric
Mineral material
Excrements
Organic components
Microstructure and porosity
Table 3 Micromorphological description of humus forms: Pachyamphi (CAN).
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5
Discontinuous, max 0.2 cm. Abrupt boundary.
Continuous horizon, up to 0.5 cm. Clear boundary.
Continuous up to 1 cm. Clear boundary.
Continuous.
OL
OF
OH
A
Well separated crumb, peds up to 0.2 cm 50% complex packing pores between peds (small excrements) and mineral grains, random; some channels and chambers.
Well separated crumb, peds up to 0.2 cm 50% complex packing pores between peds (small excrements) and mineral grains, some channels and chambers.
Loose, 70% compound packing pores between excrements and plant remains, random.
Loose, 80% packing pores between plant remains and channels. Random to laminated fabric
Microstructure and porosity
6
Thickness, continuity and lower boundary
Discontinuous, up to 0.5 cm. Abrupt boundary.
Discontinuous, up to 0.5 cm. Clear boundary.
Discontinuous, up to 0.5 cm. Clear boundary
Continuous
Horizon
OL
OF
OH
A
Few large faunal excrements, mainly of earthworms, and about 10% small faunal droppings clustered and in plant remains. Few earthworm casts, many (20%) small droppings (enchytraeids, springtails, Fig. 7c), clustered and in plant remains.
20% of the section, made of moderately decomposed needles, shoots, cones, sclerotia, bark fragments.
About 10%, moderately decomposed roots, sclerotia, bark fragments.
c/f ratio 1/1, medium sand to gravels of sandstone, schists, quartz and limestone. Brown calcitic crystallitic b-fabric.
c/f ratio 1/1, medium sand to gravels of sandstone, schists, quartz and limestone. Brown undifferentiated to crystallitic bfabric.
Few gravels of sandstone, schists, quartz and limestone.
–
Mineral material
Few gravels of sandstone, schists, quartz and limestone. c/f ratio 1/1, medium sand to gravels of sandstone, schists, quartz and limestone. Brown undifferentiated to crystallitic b-fabric. Pedofeatures: plant tissue biocalcification. c/f ratio 1/1, medium sand to gravels of sandstone, schists, quartz and limestone. Brown crystallitic bfabric. Pedofeatures: some plant tissue biocalcification.
Excrements of diptera 2%, small fauna droppings < 1%.
Large excrements of diptera and earthworms. Small droppings: 5%, clustered Large excrements (diptera and earthworms). 10%. Small droppings: 5%, clustered and in plant remains. Frequent large droppings (mostly earthworms). Small droppings: 5% clustered and in plant remains.
18%. Pine needles of 1 st − 4th stage of decomposition, very common (40% of all plant rests). Browning and munching by soil animals and colonization by fungi. Common sclerotia. Amorphous organic matter: absent or in excrements. Barely decomposed needles, shoots, cones, sclerotia, root fragments (Fig. 4a), 20%. Amorphous organic matter: absent/in excrements. Moderately decomposed needles, shoots, cones, sclerotia, bark fragments. 20%. Amorphous organic matter in excrements. Moderately decomposed needles, shoots (Fig. 5b) and roots. Sclerotia and hyphae associated to plant rests, bark fragments. 20%.
Loose, 80% packing pores between plant remains, random to laminated fabric.
Highly separated crumb, crumbs up to 0.5 cm 40% compound packing pores, channels and chambers. Transpedal channels filled with oval-shaped dark coloured droppings of 40–100 μm.
Moderately separated crumb, peds up to 0.5 cm 50% complex packing pores between large excrements and mineral grains, frequent channels and chambers.
Loose, 70% compound packing pores between excrements and plant remains, random fabric
–
Excrements
Organic components
Mineral material
Small droppings: about 10% (Fig. 7b).
Small faunal droppings, about 2%, random.
About 18%, Pine needles of 1 st − 4th stage of decomposition, around 30% of all aerial plant rests. Together with shoots and cones, have undergone browning, munching by soil animals and colonization by fungi. Common sclerotia. About 20%, barely decomposed cones, seeds, needles, shoots, sclerotia, bark fragments.
Excrements
Organic components
Microstructure and porosity
Table 5 Micromorphological description of humus forms: Eumacroamphi (CO).
Thickness, continuity and lower boundary
Horizon
Table 4 Micromorphological description of humus forms: Eumesoamphi (CS).
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Continuous horizon, 0.5–1 cm. Abrupt boundary.
Continuous
OF
A
Common quartz and quartzite grains.
c/f ratio 1/1, quartz, quartzite, mica flakes and calcareous sandstone fragments; stipple speckled to undifferentiated b-fabric.
Large excrements: 10%, diptera and earthworm droppings. Small droppings: 10%, loose enchytraeid droppings, randomly distributed and oriented; and clustered mite droppings, the latter inside some plant fragments, 5%. Many earthworm droppings. Frequent, enchytraeid (random) and mite (clustered, associated to tissues) droppings (Fig. 7d).
Needles, leaf and shoot fragments (Fig. 5a), moderately decomposed, reddish brown, subjected to bleaching and often penetrated by fungi (Fig. 3b). Roots (Fig. 4b) and tissue fragments, some blackened, some reddish. Amorphous organic matter mixed with the micromass.
Loose, 60% compound packing pores between excrements and plant remains, random.
Weakly separated crumb structure (Fig. 2), crumbs up to 1 cm 30% compound packing pores, channels and few fissures.
Mineral material
Excrements
Organic components
Microstructure and porosity
7
Thickness, continuity and lower boundary
Continuous, about 1 cm. Abrupt boundary.
Continuous
Horizon
OF
A
5%, quartz and sandstone fragments.
c/f ratio 1/2, Quartz, quartzite, limestone and calcareous sandstone sand and gravels; brownish micromass, undifferentiated b-fabric.
Large excrements: 5%, mainly diptera, few earthworm. Small excrements: 30%, mainly enchytraeids, loose, randomly distributed and oriented. Large excrements: diptera and earthworm (Fig. 7a), 10%. Small excrements 50%.
Leaf and stem fragments, moderately decomposed, sclerotia. Amorphous organic matter in excrements. Some tissue and root sections, moderately decomposed. Amorphous organic matter mixed with the micromass.
Loose, 60% compound packing pores between small excrements and plant remains, random to somewhat laminated
Highly separated crumb microstructure (Fig. 2), crumbs of fine sand size (small droppings), 35% complex packing pores and fissures
Mineral material
Excrements
Organic components
Microstructure and porosity
Table 7 Micromorphological description of humus forms: Eumull (P).
Thickness, continuity and lower boundary
Horizon
Table 6 Micromorphological description of humus forms: Oligomull (T).
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Fig. 3. OF horizons of (a) Pachyamphi and (b) Oligomull PPL. Bl2–broad leaf 2d stage of decomposition: vascular bundles are intact as the outer sheath, mesophyll is moderately decomposed. Bl3–broad leaf 3d stage of decomposition: vascular bundles are partly destroyed; cuticle and epidermis are partly separated from mesophyll. C − separated cuticle. D − Diptera larva or Isopod/Arthropod dropping with recognized tissues of pine needles inside. E − epidermis of pine needle. ED − enchytraeids droppings. F − fungal hyphae. Pn1–pine needle 1st stage of decomposition: the outer sheath is complete; mesophyll and vascular bundle are intact. Pn3–pine needle 3d stage of decomposition: outer sheath is broken; mesophyll is moderately damaged.
Fig. 4. (a) Roots in OF horizon of Eumacroamphi PPL and (b) A horizon of Oligomull XPL. F − fungal hyphae. OM − organic fine material. R − intact root, note the anisotropy due to fresh cellulose. R2–root 2d stage of decomposition. R3–root 3d stage of decomposition, colonized by fungi and with phlobaphene-containing reddish cells in the outer sheath.
Fig. 5. (a) Young Q. ilex shoot, 3d level of decomposition, moderately disturbed by soil fauna and bark undergoing blackening in OF horizon of Oligomull and (b) pine wood in A horizon of Eumacroamphi, partly colonized by fungi and munching by soil fauna.
Fig. 6. (a) Isopod/Arthropod dropping type, containing light yellow, limpid, structureless material or with recognizable cellular plant material of pine needles or leaves other are darker and contain melanized rest of woody tissues. Sometimes they also contain small mineral grains. (b) Earthworm cast in OF and OH horizon of Pachyamphi. They are of irregular shape. Internal fabric is crumb and porous, it may include red organic punctuations (probably of phlobaphene). Micropores and rows in earthworḿs casts could be the traces of smaller soil fauna that feed on them. In OH and A horizons earthworms’ casts form entire spongy fabric. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(1964) they comminute organic fine material and partly mineral particles. Our thin section analyses of Amphi humus forms showed that enchytraeids do not contribute to intimate mixing between the mineral matrix and organic matter (Figs. 6 and 7), but instead the major contribution to those processes belongs to earthworms. Nevertheless, the
loose fabric and correspond to enchytraeidś faeces (Figs. 3). These Oligochaeta worms feed on the slime films of microorganisms and also droppings of other soil fauna in organic horizons. However, their faeces were noticed in cracks in between earthworms’ casts or organo-mineral aggregates in the A and Bw horizons. In deep horizons enchytraeids are only present when they have a source of food. According to Zachariae
8
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Fig. 7. Soil fauna droppings found in organic and organo-mineral horizons of Mull and Amphi PPL. (a) Earthworm’s casts in A horizon of Eumull. Note the mineral grains found in earthworḿs droppings that demonstrate the role of earthworms in intermixing of organic with mineral material and building of aggregates (b) Numerous oval droppings (50–100 μm in size) of oribatid mites in OF horizon of Eumesoamphi. Note their clustered distribution and the red fungal hyphae near them. They are usually brown to dark brown (altered droppings). When found in plant organs they are distributed in clusters with random orientation. (c) Enchytraeids’ droppings inside the plant rest in A horizon of Eumesoamphi. They are cylindricshaped droppings with about 40–110 μm and distributed in clusters. Latter weathering of enchytraeidal droppings leads to the loss of their shape and formation of porous microaggregates. (d) Enchytraeids’ droppings in A horizon of Oligomull. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
mesostructure of the A horizons of all Amphis − except Eumacroamphi − was formed due to the performance of enchytraeids. The dominant activity of earthworms in organic matter transformation results in the formation of organo-mineral aggregates which build Mull (Chen et al., 1998). Our micromorphological analyses demonstrate that with a well separated (large) crumb microstructure in both A horizons of Mulls (Oligomull and Eumull) and in Eumacroamphi. This spongy fabric influences the physical conditions in the topsoil. Plentiful channels and chambers (Figs. 6 and 7) formed by earthworms improve better aeration and infiltration. This is supported by the principle component analysis (PCA) results of the study conducted on the same plots and samples (Zaiets and Poch 2016). On the other hand, earthworms casting promotes faster mineralization of organic matter by creating supporting environment for microbes (Van Vliet et al., 2012). Thus, the accumulation of SOM in Mull is less than in Amphi, which explains the small amount of SOM observed in the Oligomull and Eumull. In Andreetta et al., 2013 this process is explained through the favourable faunal influence on the microbial community, which leads to degradation and mineralization of organic matter. This organic substrate mineralization is reduced as the result of SOM stabilization. Nevertheless, De Nicola et al. (2014) suggested that Mulls can store more organic matter in the whole profile compared to other humus forms. Our macromorphological and micromorphological analyses support the fact that recalcitrant litter input under Q. ilex cover fostered the formation of an OLv layer, which prompted the development of Oligomull (not sampled for micromorphology). De Nicola et al. (2014) also obtained the same result in Q. ilex and V. tinus evergreen sclerophyllous forest in Mediterranean conditions. The sequence of organic and organo-mineral horizons (OLv-OF-A in Oligomull versus discontinuous OF-A in Eumull) in the two studied Mulls strengthen the
conclusion that organic parent material is one of the key factors in the formation of a particular humus form. However, the activity of soil fauna is responsible for the shift from one humus form to another. For instance, the transition from Mull to Amphi happens when the consumption of litter by earthworms decreases, which promotes the formation of continuous organic horizons (Brethes et al., 1995). Contrariwise, Amphi can convert to Mull when the recalcitrant litter input is lower and there are suitable conditions for earthworms. The local humus formation factors such as solar radiation, soil texture, moisture and aeration also play an important role, and is the reason that Eumesoamphi under mixed P. nigra forest resembles Mulls in terms of sand percentage, infiltration rates and porosity and Amphis in higher litter input. 5. Conclusions The classification of humus forms in the frame of macromorphological observations corresponds to different assemblages of micromorphological features along a sequence of organic and organomineral horizons, in particular dropping type, structure, porosity and organic remains. The studied thin sections of Amphis and Mulls, according to the proposed scheme, reveal that the major factors affecting SOM transformation and the formation of a particular humus form are the quality and quantity of parent material (easy degradable or recalcitrant litter), soil fauna activity (especially of earthworms). Litter prone to fast degradation under the meadow herbs (P) does not have the ability to accumulate in organic horizons as it is quickly removed by soil fauna. This together with the presence of earthworms leads to the formation of Mulls with a porous spongy fabric in the A horizon. Meanwhile Amphi humus forms have developed organic horizons where the main agent of 9
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0038-0717(94)90161-9. Blazejewski, G. a., Stolt, M.H., Gold, A.J., Groffman, P.M., 2005. Macro- and micromorphology of subsurface carbon in riparian zone soils. Soil Sci. Soc. Am. J. 69, 1320. http://dx.doi.org/10.2136/sssaj2004.0145. Bottner, P., CoÛteaux, M.M., Anderson, J.M., Berg, B., Billès, G., Bolger, T., Casabianca, H., Romanyà, J., Rovira, P., 2000. Decomposition of 13C-labelled plant material in a European 65–40° latitudinal transect of coniferous soils: simulation of climate change by translocation of soils. Soil Biol. Biochem. 32, 527–543. Brethes, A., Brun, J.J., Jabiol, B., Ponge, J.F., Toutain, F., 1995. Classification of forest humus forms: a French proposal. Ann. For. Sci. 52, 535–546. http://dx.doi.org/10. 1016/0003-4312(96)89721-9. Chen, Z., Pawluk, S., Juma, N.G., 1998. Impact of variations in granular structures on carbon sequestration two Alberta Mollisols. In: Lal, R., Kimble, J.M., Follett, R.F., Sewart, B.A. (Eds.), Soil Processes and the Carbon Cycle. CRC Press, Boca Raton, pp. 225–245. Ciarkowska, K., Niemyska-Łukaszuk, J., 2002. Microstructure of humus horizons of gypsic soils from the Niecka Nidziańska area (South Poland). Geoderma 106 (3), 319–329. Davidson, D.A., Grieve, I.C., Young, I.M., 2002. Impacts of fauna on an upland grassland soil as determined by micromorphological analysis. Appl. Soil Ecol. 20 (2), 133–143. De Nicola, C., Zanella, A., Testi, A., Fanelli, G., Pignatti, S., 2014. Humus forms in a Mediterranean area (Castelporziano Reserve, Rome, Italy): classification, functioning and organic carbon storage. Geoderma 235-236, 90–99. http://dx.doi.org/10.1016/j. geoderma.2014.06.033. Graefe, U., 2007. Gibt Es in Deutschland Die Humusform Amphi? 110. Mitteilungen der Dtsch. Bodenkundlichen Gesellschaft, pp. 459–460. Kooistra, M.J., 2016. Descripción de los componentes orgánicos del suelo. In: Loaiza, J.C., Stoops, G., Poch, R.M., Casamitjana, M. (Eds.), Manual De Micromorfología De Suelos Y técnicas Complementarias. Fondo Editorial Pascual Bravo, Medellín, Colombia, pp. 261–291. Kubiëna, W.L., 1955. Animal activity in soils as a decisive factor in establishment of humus forms. In: Kevan, D.K.M. (Ed.), Soil Zoology. Butterworths, London United Kingdom, pp. 73–82. Müller, P.E., 1878. Studien über die natürlichen humusformen (in german). Berlin. Pawluk, S., 1987. Faunal micromorphological features in Moder humus of some western Canadian soils. Geoderma 40, 3–16. http://dx.doi.org/10.1016/0016-7061(87) 90010-3. Ponge, J.-F., 1999. Horizons and humus forms in beech forests of the Belgian Ardennes. Am. Journal Soil Sci Soc. Am. 63, 1888–1901. Ponge, J.-F., 2003. Humus forms in terrestrial ecosystems: a framework to biodiversity. Soil Biol. Biochem. 35, 935–945. http://dx.doi.org/10.1016/S0038-0717(03) 00149-4. Ponge, J.-F., 2013. Plant–soil feedbacks mediated by humus forms: a review. Soil Biol. Biochem. 57, 1048–1060. http://dx.doi.org/10.1016/j.soilbio.2012.07.019. Stolt, M.H., Lindbo, DL, 2010. Soil organic matter. In: Stoops, G., Marcelino, V.M., Mees, F. (Eds.), Interpretation of Micromorphological Features of Soils and Regoliths. Elsevier, Amsterdam, pp. 369–396. Stoops, G., 2003. Guidelines for Analysis and Description of Soil and Regolith Thin Sections. Soil Science Society of America Inc., Madison, WI. Tian, X., Takeda, H., 1997. Application of a rapid thin section method for observations on decomposing litter in mor humus form in a subalpine coniferous forest. Ecol. Res. 289–300. http://dx.doi.org/10.1007/BF02529458. Van Vliet, P.C.J., Hendrix, P.F., Callaham, M.A.J., 2012. Soil fauna: earthworms. In: Huang, P.M., Li, Y., Sumner, M.P. (Eds.), Handbook of Soil Sciences: Properties and ProcesseS. CRC Press, Boca Raton, pp. 35–44. Zachariae, G., 1964. Welche bedeutung haben enchytraeen im waldboden? (in german, with english summary). In: Jongerius, A. (Ed.), Soil Micromorphology. Elsevier, Amsterdam, The Netherlands, pp. 57–68. Zachariae, G., 1965. Spuren tierischer Tätigkeit im Boden des Buchenwaldes. Verlag Paul Parey Hamburg 20. pp. 1–68. Zaiets, O., Poch, R.M., 2016. Micromorphology of organic matter and humus in Mediterranean mountain soils. Geoderma 272, 83–92. http://dx.doi.org/10.1016/j. geoderma.2016.03.006. Zanella, A., Jabiol, B., Ponge, J.F., Sartori, G., De Waal, R., Van Delft, B., Graefe, U., Cools, N., Katzensteiner, K., Hager, H., Englisch, M., 2011. A European morphofunctional classification of humus forms. Geoderma 164, 138–145. http://dx.doi.org/ 10.1016/j.geoderma.2011.05.016.
SOM transformation is mesofauna. Hence, laminated and dropping fabric types are common in the organic horizons of Amphis in coniferous forests. The latter is characterized by higher amounts of plant residues, roots and SOM content. The specific microenvironment conditions of each study site influences the performance of soil fauna in SOM degradation. Thus, Eumesoamphi (CS) shares common features with both Mull and Amphi humus forms, while Eumacroamphi (CO) develops a macrostructured A horizon under more humid conditions which promote the abundance of macrofauna. While our study had limitations in the number of samples of humus forms, more detailed information on transformation of SOM in other humus forms under Mediterranean conditions is required. Nevertheless, it is apparent from the present study that the micromorphological approach in SOM examination is extremely useful and should be recommended for soil survey and management in rangelands. Acknowledgements The research was conducted under the Erasmus Mundus Master program MEDfOR (Mediterranean Forestry and Natural Resources Management). We thank Montserrat Antúnez (UdL) for consultancy and technical assistance and Prof. Dr. Augusto Zanella for including this article in Humusica 3. References Andreetta, A., Ciampalini, R., Moretti, P., Vingiani, S., Poggio, G., Matteucci, G., Tescari, F., Carnicelli, S., 2011. Forest humus forms as potential indicators of soil carbon storagein Mediterranean environments. Biol. Fertil. Soils 47 (1), 31–40. Andreetta, A., Macci, C., Giansoldati, V., Masciandaro, G., Carnicelli, S., 2013. Microbial activity and organic matter composition in Mediterranean humus forms. Geoderma 209, 198–208. http://dx.doi.org/10.1016/j.geoderma.2013.06.010. Andreetta, A., Cecchini, G., Bonifacio, E., Comolli, R., Vingiani, S., Carnicelli, S., 2016. Tree or soil? Factors influencing humus form differentiation in Italian forests. Geoderma 264, 195–204. Babel, U., 1968. Enchytraeen-Losungsgefuge in Löss (in German, with English summary) [Dropping fabrics from enchytraeidae in loess]. Geoderma 2, 57–63. http://dx.doi. org/10.1016/0016-7061(68)90006-2. Babel, U., 1971. Gliederung und beschreibung des humusprofils in mitteleuropäischen wäldern. Geoderma 5, 297–324. http://dx.doi.org/10.1016/0016-7061(71)90041-3. Babel, U., 1975. Micromorphology of soil organic matter. In: Gieseking, J.E. (Ed.), Soil Components. Springer, Berlin Heidelberg, pp. 369–473. Bal, L., 1970. Morphological investigation in two Moder-humus profiles and role of soil fauna in their genesis. Geoderma 4, 5–36. http://dx.doi.org/10.1016/0016-7061(70) 90030-3. Barratt, B.C., 1964. A classification of humus forms and micro-fabrics of temperate grasslands. J. Soil Sci. 15, 342–356. http://dx.doi.org/10.1111/j1365-2389. 1964. tb02231.x. Barratt, B.C., 1967. Differences in humus forms and their microfabrics induced by longterm topdressings in hayfields. Geoderma 1, 209–227. http://dx.doi.org/10.1016/ 0016-7061(67)90028-6. Barratt, B.C., 1969. A revised classification and nomenclature of microscopic soil materials with particular reference to organic components. Geoderma 2, 257–271. Benyarku, C.A., Stoops, G., 2005. Guidelines for Preparation of Rock and Soil Thin Sections and Polished Sections. Quaderns DMACS, 33. Universitat de Lleida, Lleida. Bernier, N., Ponge, J.F., 1994. Humus form dynamics during the sylvogenetic cycle in a mountain spruce forest. Soil Biol. Biochem. 26, 183–220. http://dx.doi.org/10.1016/
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Applied Soil Ecology journal homepage: www.elsevier.com/locate/apsoil
Applied field research article
Use of micromorphology for humus characterization and classification in some mediterranean calcareous soils ⁎
Olena Zaiets , Rosa M. Poch Departament de Medi Ambient i Ciències del Sòl − Universitat de Lleida, Catalonia, Spain
A R T I C L E I N F O
A B S T R A C T
Keywords: Soil organic matter Humus form Micromorphology Amphi Mull
A micromorphological approach is potentially useful in the classification of humus systems. However, the systematic description of organic material found in thin sections of humus profiles is poorly developed in comparison to already existing guides for mineral soils. This study presents the use of thin section microscopical analyses combined with digital images in order to set the highly detailed method of qualitative description and classification of some Mediterranean humus forms. Three forests and one meadow were selected for topsoil sampling in the Catalan Pre-Pyrenean mountain region. Fourteen thin sections were prepared, scanned and analyzed using the petrographic microscope. This resulted in the description of seven micromorphological fabric units based on their amount, shape, color, size, distribution and degree of decomposition. Digital images from thin section scans helped to classify three Amphi (Pachy-, Eumacro-, Eumesoamphi), and two Mull (Oligo-, Eumull) humus forms.
1. Introduction The development of any humus form is controlled by soil-forming factors such as climate, topography, soil parent material, organisms (autotrophs and heterotrophs) and human activity (Barratt, 1969). The successive transformation stages of litter under the different levels of faunal and microbial performance lead to the formation of definite horizons of a particular humus form. The morphological differences between diverse humus forms reveal information not only about the efficiency of soil organic matter (SOM) transformation but also about the reaction of forest ecosystem to management practices and growth cycle (Bernier and Ponge, 1994; Ponge, 2013). In this sense, they are thought to be the fastest and the cheapest way of indicating the level of nutrient cycling in ecosystems (Ponge, 2003; Andreetta et al., 2011). Humus forms rich in biological activity may well correspond to wide range of pH and performance of enzymes (Andreetta et al., 2013). As it is not quite clear why different humus forms have the same C/ N ratios and SOM contents (Brethes et al., 1995) and yet different rates of biological activity, it is important to apply micromorphology to humus profiles. The first investigations on humus micromorphology were related to soil zoology (Kubiëna, 1955; Zachariae, 1964, 1965; Babel, 1968; Bal, 1970; Pawluk, 1987). At the same time, the decomposition of different plant rests and tissues in the horizons of humus forms and the formation of microfabrics were described in studies of Barratt (1964, 1967).
⁎
Scanned thin sections and micromorphology were also used by a few researchers to characterize humus types and faunal activity in relation to soil structure (Ciarkowska and Niemyska-Łukaszuk, 2002; Davidson et al., 2002, 2016). Finally, the most common humus components and fabric types of several humus forms found in Central Europe were summarized in “Monographs in Soil Science” (Babel, 1975). The description of organic materials has not been taken into account at the same level as that of mineral soil materials. In fact, many of the basic concepts established by Bullock et al. (1985) and later by Stoops (2003) such as the coarse/fine parameters, b-fabric, or even pedofeatures, cannot be applied or are irrelevant when describing thin sections of organic horizons. Even if we have information about the types of organic components and the changes they undergo in soils (Babel, 1968; Stoops, 2003; Stolt et al., 2010; Kooistra, 2016) we are still missing a systematics of description of organic materials, as we already have for mineral ones. In classical works on humus morphology (Müller, 1878) the three main humus forms Mull, Moder and Mor are described. However, the European reference base of humus forms (Zanella et al., 2011) also mentions Amphi, which seems to be typical for Mediterranean environment (Andreetta et al., 2011, 2016; De Nicola et al., 2014). In contrast, some authors emphasize that Amphi is a Xeromoder which overlies a macrostructured A horizon of Mull (Bottner et al., 2000) and is also found in southern part of Alps (Graefe, 2007) and in Belgian forests (Ponge, 1999).
Corresponding author. E-mail address: [email protected] (O. Zaiets).
http://dx.doi.org/10.1016/j.apsoil.2017.09.016 Received 24 November 2016; Received in revised form 5 September 2017; Accepted 11 September 2017 0929-1393/ © 2017 Elsevier B.V. All rights reserved.
Please cite this article as: ZAIETS, O., Applied Soil Ecology (2017), http://dx.doi.org/10.1016/j.apsoil.2017.09.016
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2.2.1. Structure and porosity The terms to be applied are the same as in mineral horizons for the pedal material, although this is not the most frequent situation for organic materials. For the apedal material (soil particles not assembled in higher units), the structure should be described according to the dominant fabric or c/f related distribution: for instance, massive (sapric densely packed material), layered (flat components with the longer axis parallel to the soil surface) or single grain (monic c/f related distribution, no fine material, loosely packed organic components).
A previous work by the authors explored the differences between Amphi and Mull humus forms in Mediterranean mountain soils based on micromorphological, physical and chemical observations, and found that it is possible to significantly assign a given set of micromorphological features to the studied humus types (Zaiets and Poch, 2016). The aim of the present study is to show with higher detail these micromorphological tools in order to set a method of qualitative description of humus profiles that could be useful for the study of soil organic matter dynamics.
2.2.2. C/f limit, c/f ratio, c/f related distribution Although these parameters are very important in mineral materials, they do not have a very high relevance in most of the organic materials. The definition of c/f limit (the lowest size of a mineral particle we can identify by optical mineralogy) cannot be applied to organic components, since the lowest size of any organic particle could be that of a phytolith, a cell or an amorphous humus particle, which have very different origins. Nevertheless, it could be relevant in some cases (i.e. sapric materials with some recognizable organic tissues).
2. Materials and methods 2.1. Study sites The study area is situated in the Pre-Pyrenean mountain region in the north-east of the Iberian Peninsula. The relief is tabular sometimes with steep slopes over 20%. The altitude is 800–1100 m asl. The area is covered mainly by pine and oak forests developed on stony calcareous soils. The climate is typical Mediterranean with transition to a subalpine type of climate on higher altitudes. Four habitats within the area were selected for topsoil sampling: holm oak forest (T: Torra), pine brook forest (CAN: Canalda), mixed pine forest (CO: Cogulers shaded; CS: Cogulers sunny) and high meadow (P: Prat). At each site organic, organo-mineral and mineral horizons were simultaneously sampled and humus forms (Table 1) were classified according to the European Humus Forms Reference Base (Zanella et al., 2011).
2.2.3. Coarse (organic) components This is the most important and significant part of the description, since the identification of these components determines to a large extent the nature and behaviour of the organic material. The description (Table 2) from organs to amorphous organic material follows the four degrees of organic matter decomposition that can be recognized. Indeed, the different transformation paths of organic components (Kooistra, 2016) are closely related to the ecological conditions of the humus forms.
2.2. Micromorphology
2.2.4. Pedofeatures Pedofeatures, often used as indicators of pedogenesis in mineral horizons, are not as meaningful in organic materials. Nevertheless, it is important to record any possible pedofeature in the mineral components, as well as evidences of redoximorphic or textural features (cappings, coatings of mineral material).
Fourteen thin sections (5 × 13 cm) of topsoil from the study sites were prepared following the methods of Benyarku and Stoops (2005). All thin sections depicted several organic horizons at once except of those prepared from mineral (Bw) horizons. All thin sections were analyzed through an Olympus petrographic microscope (BX51) using 2 x magnification in order to identify and describe fabric units. The thin sectionś true color scans were made with a high-resolution Epson scanner in order to explore simpler method of topsoil investigation. Each image had around 2 MB. The guidelines of Stoops (2003) were followed for general fabric description (as shape, distribution, orientation, abundance among others). Observed plant residues were grouped in classes according to their morphological changes during their decomposition (Tian and Takeda, 1997; Blazejewski et al., 2005). Descriptions of the different evolution patterns of organic materials followed Kooistra (2016). A special feature class “organic fine matter” refers to amorphous organic material and organic material that does not have recognizable origin and organic pigment in the inorganic micromass (Stoops, 2003). The proposed qualitative description of the humus profiles is based on both microscope and scan images and is enlarged from the one proposed by Stoops (2003):
3. Results 3.1. Field description of humus forms According to humus forms classification in the field, one Amphi and two Mulls were observed. The litter horizon (OL) of the Pachyamphi (CAN) consisted of partly bleached pine needles, twigs, pieces of wood, grass residues and moss. The OF horizon had lightly decomposed wood and other plant rests with numerous fungal hyphae. The underlying OH horizon was twice as thin than the previous one and of darker color with thin roots. The organo-mineral A horizon had a biomesostructure. The litter horizon of the Eumesoamphi (CS) was much poorer and consisted of fresh and old pine needles, twigs, bark, wood and grass rests. An OF horizon was present, whereas the OH was discontinuous. The A horizon had a biomesostructure.
Table 1 Study sites characteristics (from Zaiets and Poch 2016). Site code
Canalda (C)
Cogulers O (shaded aspect) (CO)
Cogulers S (sunny aspect) (CS)
Torra (T)
Prat (P)
Number of samples and horizons Vegetation
4 OL, OF + OH, A, Bw Natural riparian Pinus nigra forest 800 Typic Ustifluvent/ Orthofluvic Fluvisol Pachyamphi
4 OL, OF + OH, A, Bw Natural not managed Pinus nigra and Pinus sylvestris mixed forest 800 Typic Ustorthent/Calcaric Regosol
4 OL, OF + OH, A, Bw Natural not managed Pinus nigra and Pinus sylvestris mixed forest 800 Typic Calciustept/Leptic Calcisol
Eumacroamphi
Eumesoamphi
4 OLv, OF, A, Bw Natural not managed Quercus ilex forest 900 Typic Calciustept/Leptic Calcisol Oligomull
2 A, Bw Thymus vulgaris high meadow 1100 Typic Haplustept/Leptic Calcaric Cambisol Eumull
Altitude, m asl Soil type (SSS, 2014/ IUSS 2014) Humus form (ERB, 2011)
2
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Table 2 Description of components of organic materials. Organs
Tissues Cells Amorphous organic material Charcoal Faunal excrements
Mineral components other than components of excrements Boundary
Leaves, needles and shoots
Relative amounts, degree of decomposition, type of decomposition, basic and referred distribution and orientation Roots Relative amounts, degree of decomposition, type of decomposition, basic and referred distribution and orientation Fungi Relative amounts, type of organs (sclerotia, hyphae), degree of decomposition, type of decomposition, basic and referred distribution and orientation, related distribution to other components (organs, tissues, excrements) e.g. cork, wood, moss fragments, cuticle, epidermis, parenchyma: relative amounts, degree of decomposition, type of decomposition, basic and referred distribution and orientation Relative amounts, distribution and orientation (basic, referred, related). Relative amounts, size and arrangement basic and referred distribution and orientation; if possible, use the c/f related distributions. It should be described separately, given the importance and environmental significance of this component. Describe relative amounts, size, shape, orientation and distribution, degree of combustion, original plant component. Big faunal droppings (> 150–200 μm), e.g. Shape, size, internal fabric, relative amounts, type of organisms, degree of diptera, earthworms decomposition, type of decomposition, basic and referred distribution and orientation, related distribution to other components (organs, tissues, mineral material, etc.), b-fabric. Small faunal droppings Shape, size, internal fabric, relative amounts, type of organisms, degree of (< 150–200 μm), e.g. enchytraeids, mites. decomposition, type of decomposition, basic and referred distribution and orientation, related distribution to other components (organs, tissues, mineral material, etc.), b-fabric. Should be subdivided into coarse components and micromass, and described according to Stoops (2003) We propose to use the same field terms, but with different distances: abrupt < 5 mm; clear 5–10 mm; gradual > 10 mm.
Fig. 1. Three thin sections’ scans of Amphi humus forms: approximate horizons boundaries marked with red; meso- and macrofaunal channels marked with blue. The traces of endogeic and anecic earthworms in A horizon of Pachyamphi are represented by horizontal and vertical channels and their casting. Channels found in organic horizons may belong to anecic earthworms as well. Each scan width: 18 mm.
3.2. Micromorphological analysis of thin sections
In contrast, the Eumacroamphi (CO) had all organic horizons (OL, OF, OH) and a biomacrostructured A horizon with noticeable soil fauna activity. The Oligomull (T) had no freshly formed litter horizon (OL), instead, bleached old oak leaves formed a thin OLv horizon. The OF horizon was discontinuous and consisted of crumbled plant rests and granulated aggregates of organic matter. The OH horizon was absent. The A horizon had a biomacrostructure.
The use of thin section scans has a great potential in the morphological classification of humus forms. Scans depict the vertical structure of topsoil, showing organic and organo-mineral horizons regarding their fabric types and biological traces of organic matter decomposition. Due to the shallowness of most of the litter layers, (large) thin sections are useful for determining their thickness and the boundaries between them. Nevertheless, microscope observations are essential to observe 3
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Fig. 2. Two thin sections’ scans of Mull humus forms: approximate horizons boundaries marked with red; meso- and macrofaunal channels marked with blue. Each scan width: 18 mm.
explained by the ability of soil fauna to migrate deeper into the soil under unfavorable drought conditions (Zanella et al., 2011). The first step in the disintegration of plant litter in organic horizons is its consumption by macro- (other than earthworms) and mesofauna. Near organic rests or in them are numerous droppings of diptera larvae, isopods and mites. They are mostly present in OL and OF horizons. Their faeces serve a good environment for bacterial and fungi expansion. The penetration of fungal hyphae into macro- and mesofauna droppings was commonly observed in all thin sections. Study on micromorphology of SOM (Babel, 1971) has indicated that the percentage of volume of organic fine matter (OFM) increases with the depth in Moders. The same was discovered in the present study in Amphi and Mull: under a myriad of biotic and abiotic factors, plant rests, which are abundant and recognizable in litter layer, lose their coherent structure and more tissue fragments are found in OF and OH horizons. This depletion of plant material forms a structureless organic fine matter. The interaction of organic fine matter with mineral particles leads to the formation of aggregates as was discussed by Babel (1975). This was often observed in the OF and OH horizons of all Amphi samples where organic fine matter occupied 22–50% of the total volume (Zaiets and Poch, 2016). Furthermore, the mineral micromass of the A horizon was stained by organic pigments, which justifies the positive relationship found between mineral particles and OFM in the A horizon (Zaiets and Poch, 2016). Another important group of micromorphological features are small granular organic aggregates (41–80 μm) found mainly in the organic OF and OH and following the organo-mineral A horizon. In Pachyamphi and Eumesoamphi they formed together with the mineral micromass a
some features such as fungi (sclerotia, hyphae) that cannot be distinguished in scans (Figs. 1 and 2), and to avoid overestimation of some components as organic micromass (Zaiets and Poch, 2016). The descriptions of the 5 humus profiles are shown in Tables 3–7. 4. Discussion The difference between Mull and Amphi humus forms can be detected macroscopically due to the presence or absence of particular horizons and their sequence. However, the use of thin sections and their analyses could give important information on humus forms development and their biology. The assemblages of micromorphological features found in each horizon of Mulls and Amphis were quantified in the study by Zaiets and Poch (2016). The obtained results were statistically meaningful and corresponded to classification of humus forms made on the field according to ERB of humus forms (Zanella et al., 2011). Two types of humus forms − Amphi and Mull − which were found within the study area reflect different systems of humus formation. All Amphis had a well-developed litter horizon which consisted of pine needles of different stages of decomposition and other aerial plant residues. Under the microscope, the OL horizon showed a laminated fabric with numerous pores. The formation of this type of fabric is due to the properties of parent material and its resistance to the fast decomposition. Under temperate pine forests the build-up of coniferous needles leads to the formation of Moders which was described from the micromorphological point of view by Babel (1971, 1975). However, Mediterranean conditions contribute to the formation of Amphis under the same type of forest cover (Andreetta et al., 2016), which is 4
Thickness, continuity and boundary
Continuous, 0.5 to 1 cm thick. Abrupt boundary.
Continuous, 0.5 to 1 cm thick. Clear boundary.
Continuous horizon, 1 to 3 cm thick. Clear boundary.
Continuous
Horizon
OL
OF
OH
A
Few quartz and calcite sandstone grains.
15%: calcite, calcareous sandstone and few quartz grains, coarse and medium sand, random.
c/f ratio about 3/1, coarse components: calcite, calcareous sandstone and few quartz grains, coarse and medium sand; micromass with a calcitic crystallitic b-fabric. Pedofeatures: pseudomorphs of vivianite inside cells in a lignified root section.
Few diptera excrements, about 10% of small mite droppings inside plant rests and loose small droppings between fragmented plant tissues, random. Some excrements of diptera and earthworms (Fig. 6), and frequent small droppings: 15%, mite droppings and loose small droppings between fragmented plant tissues, random or in clusters together with comminuted organic rests. Total droppings: 10%: some mesofaunal (large) excrements, mainly diptera and earthworms; and small mite and loose enchytraeid droppings between fragmented plant tissues, random or in clusters together with comminuted organic rests.
Plant rests 15% (10% pine needles), stems, wood and plant tissues, moderately well preserved (brown to red), very few of them blackened. 15%, made of partly decayed crumbled pine needles and leaves, plant tissues and hardly recognizable separate plant cells, some of them phlobaphenized, with transition to organic fine matter. 5%, consisting of plant tissues and recognizable separate plant cells, many of them phlobaphenized, with transition to organic fine matter.
Loose, 70% compound packing pores between excrements and plant remains, random fabric (Fig. 3a).
Highly separated crumb and granular; peds smaller than 250 μm. 50% complex packing pores between plant remains, excrements and mineral grains, random; few channels and chambers.
Single grain to highly separated, crumbs up to 0.3 mm 50% compound packing pores, few channels and chambers.
–
Few (2%), small mite droppings inside plant rests and loose small (50–200 μm) droppings, probably enchytraeids, between fragmented plant tissues, clustered.
28% plant rests mainly as pine needles (1 st − 4th stage of decomposition, 50% of all plant rests); stems, plant tissues and woody rests, mostly well preserved (brown to red), few of them blackened, colonized by fungi. Amorphous organic matter almost absent.
Loose, 70% compound packing pores between plant remains, random to laminated fabric
Mineral material
Excrements
Organic components
Microstructure and porosity
Table 3 Micromorphological description of humus forms: Pachyamphi (CAN).
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Discontinuous, max 0.2 cm. Abrupt boundary.
Continuous horizon, up to 0.5 cm. Clear boundary.
Continuous up to 1 cm. Clear boundary.
Continuous.
OL
OF
OH
A
Well separated crumb, peds up to 0.2 cm 50% complex packing pores between peds (small excrements) and mineral grains, random; some channels and chambers.
Well separated crumb, peds up to 0.2 cm 50% complex packing pores between peds (small excrements) and mineral grains, some channels and chambers.
Loose, 70% compound packing pores between excrements and plant remains, random.
Loose, 80% packing pores between plant remains and channels. Random to laminated fabric
Microstructure and porosity
6
Thickness, continuity and lower boundary
Discontinuous, up to 0.5 cm. Abrupt boundary.
Discontinuous, up to 0.5 cm. Clear boundary.
Discontinuous, up to 0.5 cm. Clear boundary
Continuous
Horizon
OL
OF
OH
A
Few large faunal excrements, mainly of earthworms, and about 10% small faunal droppings clustered and in plant remains. Few earthworm casts, many (20%) small droppings (enchytraeids, springtails, Fig. 7c), clustered and in plant remains.
20% of the section, made of moderately decomposed needles, shoots, cones, sclerotia, bark fragments.
About 10%, moderately decomposed roots, sclerotia, bark fragments.
c/f ratio 1/1, medium sand to gravels of sandstone, schists, quartz and limestone. Brown calcitic crystallitic b-fabric.
c/f ratio 1/1, medium sand to gravels of sandstone, schists, quartz and limestone. Brown undifferentiated to crystallitic bfabric.
Few gravels of sandstone, schists, quartz and limestone.
–
Mineral material
Few gravels of sandstone, schists, quartz and limestone. c/f ratio 1/1, medium sand to gravels of sandstone, schists, quartz and limestone. Brown undifferentiated to crystallitic b-fabric. Pedofeatures: plant tissue biocalcification. c/f ratio 1/1, medium sand to gravels of sandstone, schists, quartz and limestone. Brown crystallitic bfabric. Pedofeatures: some plant tissue biocalcification.
Excrements of diptera 2%, small fauna droppings < 1%.
Large excrements of diptera and earthworms. Small droppings: 5%, clustered Large excrements (diptera and earthworms). 10%. Small droppings: 5%, clustered and in plant remains. Frequent large droppings (mostly earthworms). Small droppings: 5% clustered and in plant remains.
18%. Pine needles of 1 st − 4th stage of decomposition, very common (40% of all plant rests). Browning and munching by soil animals and colonization by fungi. Common sclerotia. Amorphous organic matter: absent or in excrements. Barely decomposed needles, shoots, cones, sclerotia, root fragments (Fig. 4a), 20%. Amorphous organic matter: absent/in excrements. Moderately decomposed needles, shoots, cones, sclerotia, bark fragments. 20%. Amorphous organic matter in excrements. Moderately decomposed needles, shoots (Fig. 5b) and roots. Sclerotia and hyphae associated to plant rests, bark fragments. 20%.
Loose, 80% packing pores between plant remains, random to laminated fabric.
Highly separated crumb, crumbs up to 0.5 cm 40% compound packing pores, channels and chambers. Transpedal channels filled with oval-shaped dark coloured droppings of 40–100 μm.
Moderately separated crumb, peds up to 0.5 cm 50% complex packing pores between large excrements and mineral grains, frequent channels and chambers.
Loose, 70% compound packing pores between excrements and plant remains, random fabric
–
Excrements
Organic components
Mineral material
Small droppings: about 10% (Fig. 7b).
Small faunal droppings, about 2%, random.
About 18%, Pine needles of 1 st − 4th stage of decomposition, around 30% of all aerial plant rests. Together with shoots and cones, have undergone browning, munching by soil animals and colonization by fungi. Common sclerotia. About 20%, barely decomposed cones, seeds, needles, shoots, sclerotia, bark fragments.
Excrements
Organic components
Microstructure and porosity
Table 5 Micromorphological description of humus forms: Eumacroamphi (CO).
Thickness, continuity and lower boundary
Horizon
Table 4 Micromorphological description of humus forms: Eumesoamphi (CS).
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Continuous horizon, 0.5–1 cm. Abrupt boundary.
Continuous
OF
A
Common quartz and quartzite grains.
c/f ratio 1/1, quartz, quartzite, mica flakes and calcareous sandstone fragments; stipple speckled to undifferentiated b-fabric.
Large excrements: 10%, diptera and earthworm droppings. Small droppings: 10%, loose enchytraeid droppings, randomly distributed and oriented; and clustered mite droppings, the latter inside some plant fragments, 5%. Many earthworm droppings. Frequent, enchytraeid (random) and mite (clustered, associated to tissues) droppings (Fig. 7d).
Needles, leaf and shoot fragments (Fig. 5a), moderately decomposed, reddish brown, subjected to bleaching and often penetrated by fungi (Fig. 3b). Roots (Fig. 4b) and tissue fragments, some blackened, some reddish. Amorphous organic matter mixed with the micromass.
Loose, 60% compound packing pores between excrements and plant remains, random.
Weakly separated crumb structure (Fig. 2), crumbs up to 1 cm 30% compound packing pores, channels and few fissures.
Mineral material
Excrements
Organic components
Microstructure and porosity
7
Thickness, continuity and lower boundary
Continuous, about 1 cm. Abrupt boundary.
Continuous
Horizon
OF
A
5%, quartz and sandstone fragments.
c/f ratio 1/2, Quartz, quartzite, limestone and calcareous sandstone sand and gravels; brownish micromass, undifferentiated b-fabric.
Large excrements: 5%, mainly diptera, few earthworm. Small excrements: 30%, mainly enchytraeids, loose, randomly distributed and oriented. Large excrements: diptera and earthworm (Fig. 7a), 10%. Small excrements 50%.
Leaf and stem fragments, moderately decomposed, sclerotia. Amorphous organic matter in excrements. Some tissue and root sections, moderately decomposed. Amorphous organic matter mixed with the micromass.
Loose, 60% compound packing pores between small excrements and plant remains, random to somewhat laminated
Highly separated crumb microstructure (Fig. 2), crumbs of fine sand size (small droppings), 35% complex packing pores and fissures
Mineral material
Excrements
Organic components
Microstructure and porosity
Table 7 Micromorphological description of humus forms: Eumull (P).
Thickness, continuity and lower boundary
Horizon
Table 6 Micromorphological description of humus forms: Oligomull (T).
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Fig. 3. OF horizons of (a) Pachyamphi and (b) Oligomull PPL. Bl2–broad leaf 2d stage of decomposition: vascular bundles are intact as the outer sheath, mesophyll is moderately decomposed. Bl3–broad leaf 3d stage of decomposition: vascular bundles are partly destroyed; cuticle and epidermis are partly separated from mesophyll. C − separated cuticle. D − Diptera larva or Isopod/Arthropod dropping with recognized tissues of pine needles inside. E − epidermis of pine needle. ED − enchytraeids droppings. F − fungal hyphae. Pn1–pine needle 1st stage of decomposition: the outer sheath is complete; mesophyll and vascular bundle are intact. Pn3–pine needle 3d stage of decomposition: outer sheath is broken; mesophyll is moderately damaged.
Fig. 4. (a) Roots in OF horizon of Eumacroamphi PPL and (b) A horizon of Oligomull XPL. F − fungal hyphae. OM − organic fine material. R − intact root, note the anisotropy due to fresh cellulose. R2–root 2d stage of decomposition. R3–root 3d stage of decomposition, colonized by fungi and with phlobaphene-containing reddish cells in the outer sheath.
Fig. 5. (a) Young Q. ilex shoot, 3d level of decomposition, moderately disturbed by soil fauna and bark undergoing blackening in OF horizon of Oligomull and (b) pine wood in A horizon of Eumacroamphi, partly colonized by fungi and munching by soil fauna.
Fig. 6. (a) Isopod/Arthropod dropping type, containing light yellow, limpid, structureless material or with recognizable cellular plant material of pine needles or leaves other are darker and contain melanized rest of woody tissues. Sometimes they also contain small mineral grains. (b) Earthworm cast in OF and OH horizon of Pachyamphi. They are of irregular shape. Internal fabric is crumb and porous, it may include red organic punctuations (probably of phlobaphene). Micropores and rows in earthworḿs casts could be the traces of smaller soil fauna that feed on them. In OH and A horizons earthworms’ casts form entire spongy fabric. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
(1964) they comminute organic fine material and partly mineral particles. Our thin section analyses of Amphi humus forms showed that enchytraeids do not contribute to intimate mixing between the mineral matrix and organic matter (Figs. 6 and 7), but instead the major contribution to those processes belongs to earthworms. Nevertheless, the
loose fabric and correspond to enchytraeidś faeces (Figs. 3). These Oligochaeta worms feed on the slime films of microorganisms and also droppings of other soil fauna in organic horizons. However, their faeces were noticed in cracks in between earthworms’ casts or organo-mineral aggregates in the A and Bw horizons. In deep horizons enchytraeids are only present when they have a source of food. According to Zachariae
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Fig. 7. Soil fauna droppings found in organic and organo-mineral horizons of Mull and Amphi PPL. (a) Earthworm’s casts in A horizon of Eumull. Note the mineral grains found in earthworḿs droppings that demonstrate the role of earthworms in intermixing of organic with mineral material and building of aggregates (b) Numerous oval droppings (50–100 μm in size) of oribatid mites in OF horizon of Eumesoamphi. Note their clustered distribution and the red fungal hyphae near them. They are usually brown to dark brown (altered droppings). When found in plant organs they are distributed in clusters with random orientation. (c) Enchytraeids’ droppings inside the plant rest in A horizon of Eumesoamphi. They are cylindricshaped droppings with about 40–110 μm and distributed in clusters. Latter weathering of enchytraeidal droppings leads to the loss of their shape and formation of porous microaggregates. (d) Enchytraeids’ droppings in A horizon of Oligomull. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
mesostructure of the A horizons of all Amphis − except Eumacroamphi − was formed due to the performance of enchytraeids. The dominant activity of earthworms in organic matter transformation results in the formation of organo-mineral aggregates which build Mull (Chen et al., 1998). Our micromorphological analyses demonstrate that with a well separated (large) crumb microstructure in both A horizons of Mulls (Oligomull and Eumull) and in Eumacroamphi. This spongy fabric influences the physical conditions in the topsoil. Plentiful channels and chambers (Figs. 6 and 7) formed by earthworms improve better aeration and infiltration. This is supported by the principle component analysis (PCA) results of the study conducted on the same plots and samples (Zaiets and Poch 2016). On the other hand, earthworms casting promotes faster mineralization of organic matter by creating supporting environment for microbes (Van Vliet et al., 2012). Thus, the accumulation of SOM in Mull is less than in Amphi, which explains the small amount of SOM observed in the Oligomull and Eumull. In Andreetta et al., 2013 this process is explained through the favourable faunal influence on the microbial community, which leads to degradation and mineralization of organic matter. This organic substrate mineralization is reduced as the result of SOM stabilization. Nevertheless, De Nicola et al. (2014) suggested that Mulls can store more organic matter in the whole profile compared to other humus forms. Our macromorphological and micromorphological analyses support the fact that recalcitrant litter input under Q. ilex cover fostered the formation of an OLv layer, which prompted the development of Oligomull (not sampled for micromorphology). De Nicola et al. (2014) also obtained the same result in Q. ilex and V. tinus evergreen sclerophyllous forest in Mediterranean conditions. The sequence of organic and organo-mineral horizons (OLv-OF-A in Oligomull versus discontinuous OF-A in Eumull) in the two studied Mulls strengthen the
conclusion that organic parent material is one of the key factors in the formation of a particular humus form. However, the activity of soil fauna is responsible for the shift from one humus form to another. For instance, the transition from Mull to Amphi happens when the consumption of litter by earthworms decreases, which promotes the formation of continuous organic horizons (Brethes et al., 1995). Contrariwise, Amphi can convert to Mull when the recalcitrant litter input is lower and there are suitable conditions for earthworms. The local humus formation factors such as solar radiation, soil texture, moisture and aeration also play an important role, and is the reason that Eumesoamphi under mixed P. nigra forest resembles Mulls in terms of sand percentage, infiltration rates and porosity and Amphis in higher litter input. 5. Conclusions The classification of humus forms in the frame of macromorphological observations corresponds to different assemblages of micromorphological features along a sequence of organic and organomineral horizons, in particular dropping type, structure, porosity and organic remains. The studied thin sections of Amphis and Mulls, according to the proposed scheme, reveal that the major factors affecting SOM transformation and the formation of a particular humus form are the quality and quantity of parent material (easy degradable or recalcitrant litter), soil fauna activity (especially of earthworms). Litter prone to fast degradation under the meadow herbs (P) does not have the ability to accumulate in organic horizons as it is quickly removed by soil fauna. This together with the presence of earthworms leads to the formation of Mulls with a porous spongy fabric in the A horizon. Meanwhile Amphi humus forms have developed organic horizons where the main agent of 9
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0038-0717(94)90161-9. Blazejewski, G. a., Stolt, M.H., Gold, A.J., Groffman, P.M., 2005. Macro- and micromorphology of subsurface carbon in riparian zone soils. Soil Sci. Soc. Am. J. 69, 1320. http://dx.doi.org/10.2136/sssaj2004.0145. Bottner, P., CoÛteaux, M.M., Anderson, J.M., Berg, B., Billès, G., Bolger, T., Casabianca, H., Romanyà, J., Rovira, P., 2000. Decomposition of 13C-labelled plant material in a European 65–40° latitudinal transect of coniferous soils: simulation of climate change by translocation of soils. Soil Biol. Biochem. 32, 527–543. Brethes, A., Brun, J.J., Jabiol, B., Ponge, J.F., Toutain, F., 1995. Classification of forest humus forms: a French proposal. Ann. For. Sci. 52, 535–546. http://dx.doi.org/10. 1016/0003-4312(96)89721-9. Chen, Z., Pawluk, S., Juma, N.G., 1998. Impact of variations in granular structures on carbon sequestration two Alberta Mollisols. In: Lal, R., Kimble, J.M., Follett, R.F., Sewart, B.A. (Eds.), Soil Processes and the Carbon Cycle. CRC Press, Boca Raton, pp. 225–245. Ciarkowska, K., Niemyska-Łukaszuk, J., 2002. Microstructure of humus horizons of gypsic soils from the Niecka Nidziańska area (South Poland). Geoderma 106 (3), 319–329. Davidson, D.A., Grieve, I.C., Young, I.M., 2002. Impacts of fauna on an upland grassland soil as determined by micromorphological analysis. Appl. Soil Ecol. 20 (2), 133–143. De Nicola, C., Zanella, A., Testi, A., Fanelli, G., Pignatti, S., 2014. Humus forms in a Mediterranean area (Castelporziano Reserve, Rome, Italy): classification, functioning and organic carbon storage. Geoderma 235-236, 90–99. http://dx.doi.org/10.1016/j. geoderma.2014.06.033. Graefe, U., 2007. Gibt Es in Deutschland Die Humusform Amphi? 110. Mitteilungen der Dtsch. Bodenkundlichen Gesellschaft, pp. 459–460. Kooistra, M.J., 2016. Descripción de los componentes orgánicos del suelo. In: Loaiza, J.C., Stoops, G., Poch, R.M., Casamitjana, M. (Eds.), Manual De Micromorfología De Suelos Y técnicas Complementarias. Fondo Editorial Pascual Bravo, Medellín, Colombia, pp. 261–291. Kubiëna, W.L., 1955. Animal activity in soils as a decisive factor in establishment of humus forms. In: Kevan, D.K.M. (Ed.), Soil Zoology. Butterworths, London United Kingdom, pp. 73–82. Müller, P.E., 1878. Studien über die natürlichen humusformen (in german). Berlin. Pawluk, S., 1987. Faunal micromorphological features in Moder humus of some western Canadian soils. Geoderma 40, 3–16. http://dx.doi.org/10.1016/0016-7061(87) 90010-3. Ponge, J.-F., 1999. Horizons and humus forms in beech forests of the Belgian Ardennes. Am. Journal Soil Sci Soc. Am. 63, 1888–1901. Ponge, J.-F., 2003. Humus forms in terrestrial ecosystems: a framework to biodiversity. Soil Biol. Biochem. 35, 935–945. http://dx.doi.org/10.1016/S0038-0717(03) 00149-4. Ponge, J.-F., 2013. Plant–soil feedbacks mediated by humus forms: a review. Soil Biol. Biochem. 57, 1048–1060. http://dx.doi.org/10.1016/j.soilbio.2012.07.019. Stolt, M.H., Lindbo, DL, 2010. Soil organic matter. In: Stoops, G., Marcelino, V.M., Mees, F. (Eds.), Interpretation of Micromorphological Features of Soils and Regoliths. Elsevier, Amsterdam, pp. 369–396. Stoops, G., 2003. Guidelines for Analysis and Description of Soil and Regolith Thin Sections. Soil Science Society of America Inc., Madison, WI. Tian, X., Takeda, H., 1997. Application of a rapid thin section method for observations on decomposing litter in mor humus form in a subalpine coniferous forest. Ecol. Res. 289–300. http://dx.doi.org/10.1007/BF02529458. Van Vliet, P.C.J., Hendrix, P.F., Callaham, M.A.J., 2012. Soil fauna: earthworms. In: Huang, P.M., Li, Y., Sumner, M.P. (Eds.), Handbook of Soil Sciences: Properties and ProcesseS. CRC Press, Boca Raton, pp. 35–44. Zachariae, G., 1964. Welche bedeutung haben enchytraeen im waldboden? (in german, with english summary). In: Jongerius, A. (Ed.), Soil Micromorphology. Elsevier, Amsterdam, The Netherlands, pp. 57–68. Zachariae, G., 1965. Spuren tierischer Tätigkeit im Boden des Buchenwaldes. Verlag Paul Parey Hamburg 20. pp. 1–68. Zaiets, O., Poch, R.M., 2016. Micromorphology of organic matter and humus in Mediterranean mountain soils. Geoderma 272, 83–92. http://dx.doi.org/10.1016/j. geoderma.2016.03.006. Zanella, A., Jabiol, B., Ponge, J.F., Sartori, G., De Waal, R., Van Delft, B., Graefe, U., Cools, N., Katzensteiner, K., Hager, H., Englisch, M., 2011. A European morphofunctional classification of humus forms. Geoderma 164, 138–145. http://dx.doi.org/ 10.1016/j.geoderma.2011.05.016.
SOM transformation is mesofauna. Hence, laminated and dropping fabric types are common in the organic horizons of Amphis in coniferous forests. The latter is characterized by higher amounts of plant residues, roots and SOM content. The specific microenvironment conditions of each study site influences the performance of soil fauna in SOM degradation. Thus, Eumesoamphi (CS) shares common features with both Mull and Amphi humus forms, while Eumacroamphi (CO) develops a macrostructured A horizon under more humid conditions which promote the abundance of macrofauna. While our study had limitations in the number of samples of humus forms, more detailed information on transformation of SOM in other humus forms under Mediterranean conditions is required. Nevertheless, it is apparent from the present study that the micromorphological approach in SOM examination is extremely useful and should be recommended for soil survey and management in rangelands. Acknowledgements The research was conducted under the Erasmus Mundus Master program MEDfOR (Mediterranean Forestry and Natural Resources Management). We thank Montserrat Antúnez (UdL) for consultancy and technical assistance and Prof. Dr. Augusto Zanella for including this article in Humusica 3. References Andreetta, A., Ciampalini, R., Moretti, P., Vingiani, S., Poggio, G., Matteucci, G., Tescari, F., Carnicelli, S., 2011. Forest humus forms as potential indicators of soil carbon storagein Mediterranean environments. Biol. Fertil. Soils 47 (1), 31–40. Andreetta, A., Macci, C., Giansoldati, V., Masciandaro, G., Carnicelli, S., 2013. Microbial activity and organic matter composition in Mediterranean humus forms. Geoderma 209, 198–208. http://dx.doi.org/10.1016/j.geoderma.2013.06.010. Andreetta, A., Cecchini, G., Bonifacio, E., Comolli, R., Vingiani, S., Carnicelli, S., 2016. Tree or soil? Factors influencing humus form differentiation in Italian forests. Geoderma 264, 195–204. Babel, U., 1968. Enchytraeen-Losungsgefuge in Löss (in German, with English summary) [Dropping fabrics from enchytraeidae in loess]. Geoderma 2, 57–63. http://dx.doi. org/10.1016/0016-7061(68)90006-2. Babel, U., 1971. Gliederung und beschreibung des humusprofils in mitteleuropäischen wäldern. Geoderma 5, 297–324. http://dx.doi.org/10.1016/0016-7061(71)90041-3. Babel, U., 1975. Micromorphology of soil organic matter. In: Gieseking, J.E. (Ed.), Soil Components. Springer, Berlin Heidelberg, pp. 369–473. Bal, L., 1970. Morphological investigation in two Moder-humus profiles and role of soil fauna in their genesis. Geoderma 4, 5–36. http://dx.doi.org/10.1016/0016-7061(70) 90030-3. Barratt, B.C., 1964. A classification of humus forms and micro-fabrics of temperate grasslands. J. Soil Sci. 15, 342–356. http://dx.doi.org/10.1111/j1365-2389. 1964. tb02231.x. Barratt, B.C., 1967. Differences in humus forms and their microfabrics induced by longterm topdressings in hayfields. Geoderma 1, 209–227. http://dx.doi.org/10.1016/ 0016-7061(67)90028-6. Barratt, B.C., 1969. A revised classification and nomenclature of microscopic soil materials with particular reference to organic components. Geoderma 2, 257–271. Benyarku, C.A., Stoops, G., 2005. Guidelines for Preparation of Rock and Soil Thin Sections and Polished Sections. Quaderns DMACS, 33. Universitat de Lleida, Lleida. Bernier, N., Ponge, J.F., 1994. Humus form dynamics during the sylvogenetic cycle in a mountain spruce forest. Soil Biol. Biochem. 26, 183–220. http://dx.doi.org/10.1016/
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