# The impact of parent material, climate, soil type and vegetation on Venetian forest humus forms: a direct gradient approach

Jean-François Ponge, Giacomo Sartori, Adriano Garlato, Fabrizio Ungaro, Augusto Zanella, Bernard Jabiol, Silvia Obber

► To cite this version:

Jean-François Ponge, Giacomo Sartori, Adriano Garlato, Fabrizio Ungaro, Augusto Zanella, et al. The impact of parent material, climate, soil type and vegetation on Venetian forest humus forms: a direct gradient approach. Geoderma, 2014, 226-227, pp.290-299. 10.1016/j.geoderma.2014.02.022. hal-00980797

HAL Id: hal-00980797

https://hal.science/hal-00980797v1

Submitted on 18 Apr 2014

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons CC0 1.0 - Universal - International License
# The impact of parent material, climate, soil type and vegetation on Venetian forest humus forms: a direct gradient approach

Jean-François Ponge<sup>a,*</sup>, Giacomo Sartori<sup>b</sup>, Adriano Garlato<sup>c</sup>, Fabrizio Ungaro<sup>d</sup>, Augusto Zanella<sup>e</sup>,
Bernard Jabiol<sup>f</sup>, Silvia Obber<sup>c</sup>

<sup>a</sup> Muséum National d'Histoire Naturelle, CNRS UMR 7179, 4 avenue du Petit-Château, 91800 Brunoy, France

<sup>b</sup> Museo delle Scienze, Corso del Lavoro e della Scienza 3, 38123 Trento, Italy

<sup>c</sup> ARPAV, Servizio Osservatorio Suolo e Bonifiche, Via Santa Barbara 5/A, 31100 Treviso, Italy

<sup>d</sup> CNR-IBIMET, Via Madonna del Piano 10, 50019 Sesto Fiorentino, Italy

<sup>e</sup> University of Padua, Department of Land, Environment, Agriculture and Forestry, Viale dell'Università 16, 35020 Legnaro, Italy

<sup>f</sup> AgroParisTech, INRA UMR 1092, Laboratoire d'Etude des Ressources Foret Bois (LERFoB), ENGREF, 14 rue Girardet, 54042 Nancy Cedex, France

## ABSTRACT

The impact of geology, climate, soil type and vegetation on forest humus forms was studied in the Veneto Region (northern Italy). A total of 352 study sites were compared by Redundance Analysis (RDA). Humus forms were described by the structure (micro-, meso-, or macro-aggregated) of the organo-mineral A horizon, by the thickness of litter horizons and by their nomenclature, which followed the morpho-functional classification recently proposed for inclusion in the WRB-FAO. The size of aggregates within the A horizon was distributed along a common gradient embracing geology,

<footnote>* Corresponding author. Muséum National d'Histoire Naturelle, CNRS UMR 7179, 4 avenue du Petit-Château, 91800 Brunoy, France. Tel. +33 6 78930133; fax: +33 1 60465719. E-mail address: ponge@mnhn.fr (J.F. Ponge).</footnote>
climate, soils and vegetation. Macro-aggregation (as opposed to micro-aggregation, meso-aggregation being intermediate) was favoured by carbonated (as opposed to silicated) parent rocks, warmer climate associated to lower elevation, lower soil acidity, deciduous (as opposed to coniferous) forest vegetation and relatively high clay content. The amphi group of humus forms, associated with carbonated substrates in Esalpic and Mesalpic climate districts, was distributed according to thickness of litter horizons along a gradient of soil stoniness. Biological reasons for the observed environmental influences on the size of soil aggregates, a criterion of humus form classification, were discussed to the light of knowledge on annelid (earthworm and enchytraeid) ecology. Humus forms can be easily identified and classified on the field, using a table included in the article. Our results can be used for mapping the distribution of forest humus forms in the Veneto Region, implying a better understanding of carbon cycling processes in the frame of present-day global warming.

Keywords: Veneto, humus forms, mull, moder, amphi, aggregate size

# 1. Introduction

Following the United Nations Framework Convention on Climate Change (UNFCCC, 1992), there is an urgent need to characterize organic carbon pools both quantitatively and qualitatively worldwide, in order to (i) optimize models of global climate warming (Bottner et al., 1995; Marland et al., 2003; Thum et al., 2011), and (ii) discern trends in C turnover/sequestering and nutrient availability/retention at ecosystem level (De Deyn et al., 2008; Zhang et al., 2008; Egli et al., 2010). Some countries, such as the Netherlands, implemented soil organic carbon (SOC) inventories, based on forest standing crops (Nabuurs and Mohren, 1993) and agricultural soil maps (Kuikman et al., 2003). However, in forests soils carbon stocks vary to a great extent according to the development of organic layers (Schulp et al., 2008; Andreetta et al., 2011; Bonifacio et al., 2011), although more stable C stocks in the mineral horizons should not be neglected (Garlato et al., 2009; Rumpel and Kögel-Knabner, 2011). Humus forms, patterning the way organic matter is distributed and
transformed within the forest soil profile (Bal, 1970; Klinka et al., 1990; Kindel and Garay, 2002), and known as key components of plant-soil relationships (Ponge, 2003, 2013), are easy to identify directly on the field without expensive laboratory analyses. They also could be mapped with the aid of digital mapping techniques (Aberegg et al., 2009), taking into account their local variability by standardized protocols (Ponge et al., 2002; Lalanne et al., 2008). The systematic census of humus forms could allow in a near future a clearer assessment of the amount and distribution of fast-recycling (organic horizons) and stable (organo-mineral horizons) carbon at scales varying from local to regional then to global levels (Thornley and Cannell, 2001; Hedde et al., 2008; Andreetta et al., 2011). In parallel, searching proxies for humus forms in geology, climate, soils and vegetation, mapped for a long time or easily accessible by remote sensing, and known as main determinants of forest soil conditions (Egli et al., 2009, 2010; Li et al., 2010), would help to achieve this goal even more rapidly.

A recent study by Ponge et al. (2011) showed that at national level (France) forest humus forms were mainly ‘explained’ (in statistical sense) by geology and climate. Tree canopies (coniferous vs deciduous) exhibit only a minor additive influence. The present study was undertaken in the mountain and hilly part of the Veneto Region (northern Italy), embracing a wide array of climate and geologic conditions on a relatively small area (6,000 km²) owing to its particular position on the south-eastern side of the Alps arch and on the northern border of the Adriatic Sea (Del Favero and Lasen, 1993). The aim was to assess whether the influence of (i) components of climate (temperature, precipitation), elevation, aspect, soil type and substrate (carbonated versus silicated parent materials), known to affect forest humus forms in other North Italian contexts (Carletti et al., 2009; Bonifacio et al., 2011; Ascher et al., 2012), and (ii) vegetation, whose effects have been well-established at a very local level (Schulp et al., 2008; Trap et al., 2011, 2013), could be retrieved in the regional context of Veneto. For that purpose we used for the first time a new classification of humus forms, based on morpho-functional principles previously defined at European level by Zanella et al. (2011), then refined by Jabiol et al. (2013) in the frame of the World Reference Base for Soil Resources (IUSS Working Group WRB, 2007.
## 2. Materials and methods

### 2.1. Climate, geology and distribution of forest and soil types in Veneto

Veneto is an Alpine Italian Region, displaying an altitudinal gradient from the Mediterranean Sea (Gulf of Venice) up to the summits of the eastern Alps (Carnic Alps, Austrian boundary) (Fig. 1). Four climatic districts have been described by Del Favero and Lasen (1993).

The Avanalpic climatic district (numbered 1 in Fig. 1) concerns the lowland area between the Mediterranean Sea and the Alpine reliefs. The area is comprised of the wide Veneto plain and a hilly zone at the piedmont of the Alps, with a climatic gradient between them. On the whole, the Mediterranean district is characterized by a mean annual precipitation of 900-950 mm with a maximum in spring, and a mean annual temperature of 12-13°C with July as the warmest month but without any arid summer season. Ericaceous species and sweet chestnut (*Castanea sativa* Mill.) are common on silicated substrates or on acid soils as opposed to oak (often downy oak, *Quercus pubescens* Willd.), hornbeam (often hop hornbeam, *Ostrya carpinifolia* Scop.) and ash (often Manna ash, *Fraxinus ornus* L.), which grow preferably on carbonated substrates. Remains of the natural oak-hornbeam forest which covered the Po plain between 6000 BC and a few centuries ago (Susmel, 1994), are nowadays managed as hedgerows in a mechanized agricultural landscape or as small private woods.

The Esalpic district is characterized by Mediterranean-type temperatures but mean annual precipitation is higher (ca. 1500 mm) owing to wet-warm air coming from the sea and raising on pre-alpine reliefs (Fig. 1). Carbonated substrates (calcareous and dolomitic), largely dominant, are favourable to the development of pure or mixed forests of hop hornbeam, with Manna ash, downy oak, field maple (*Acer campestre* L.), which occupy about one third of the forest area in Veneto (Del Favero, 2010). Above 800 m altitude, hop hornbeam is replaced by common beech (*Fagus sylvatica* L.) in thermophile beech forests which constitute the second most important element of forest vegetation.
The more internal Mesalpic district is characterized by a high mean annual precipitation (ca. 1400 mm, snowfall included), which is mostly distributed from April to November. Temperatures are much lower than in the Esalpic district, averaging 7-8 °C. The whole district is covered with mountain beech forests, pure or mixed with Norway spruce [Picea abies (L.) Karst.] and silver fir (Abies alba Mill.), and by mixed spruce-fir forests. The composition of these forests varies according to substrate (95 % carbonated), slope and exposure.

The Endalpic climatic district corresponds to the highest part of the region. The mean annual precipitation is lower than in Mesalpic and Esalpic districts (ca. 1000 mm) and is distributed according to a continental regime with a single peak in July. Mean annual temperature is 5 °C, being too cold for many broadleaved tree species. Above 1500 m altitude, Swiss pine (Pinus cembra L.) is common, while Norway spruce and European larch (Larix decidua Mill.) grow at the same altitude but also lower, often favoured by human activities (pasture or forest management). Spruce forests cover silicated substrates in the high north-eastern part of the district.

In short, there is a gradient of decreasing temperature from the Mediterranean Sea (Avanalpic district) to the higher Carnic Alps (Endalpic district), while precipitation is distributed in a humped back fashion (Fig. 1), maximum precipitation occurring at mid elevation (Esalpic and Mesalpic districts).

Geologically, the study area can be divided, from North to South, into Alpine, Prealpine and Hilly areas. The Alpine sector displays a remarkable lithological variability, from metamorphic acid rocks of the crystalline basement of the Dolomites to the overlying stratigraphic succession (mainly calcareous, dolomitic and terrigenous rocks). In the Prealpine area limestone and marly limestone are common, but in the eastern part basalts of Tertiary volcanism are widespread, while the area of Recoaro and the Piccole Dolomiti (Small Dolomites), due to particular tectonic conditions, show a Permo-Triassic stratigraphic succession, similar to Dolomite reliefs. The Hilly Area is characterized by extremely heterogeneous geological substrates, ranging from limestone of the Berici to volcanic rocks of the Euganean hills with a prevalence of carbonated substrates (Antonelli et al., 1990).
Lithological variability results in prominent soil heterogeneity (Antonelli et al., 1990) in mountain areas. On carbonated rocks, shallow soils with a high content in skeleton and a low differentiation of profiles, classified by the World Reference Base (IUSS Working Group WRB, 2007) as Rendzic Leptosols, prevail besides deeper and moderately developed soils with cambic (Bw) horizons (Endoleptic Cambisols [Calcaric, Skeletic]). On silicated rocks with cold climate and high altitude the podzolization process is dominant (Podzols). At lower altitude this process is less intense with translocation of iron and aluminium sesquioxides (Bs horizons) but not of organic matter (Haplic Cambisols [Dystric]). On marly limestone, especially in the Prealpine area, well-differentiated and deep soils free of carbonates dominate even on very steep slopes, with a horizon of clay accumulation (Cutanic Luvisols). In the Hilly Area sandy and marly lithotypes are associated with moderately deep soils, partially decarbonated (Haplic Cambisols [Eutric] and [Calcaric]). In Euganean hills acid soils are prevalent (Haplic Cambisols [Dystric]).

## 2.2. *Selection, description and classification of humus profiles*

Study sites (N = 348) were selected in order to embrace the widest possible variety of geological, climate, soil and vegetation conditions found in forests of the Veneto region, in the framework of the Soil Map of Veneto Region at 1:250,000 scale (ARPAV, 2005). Humus profiles were described according to the thickness of OL (not retained for data analysis due to strong seasonal variations), OF and OH organic horizons (litter s. l.) and to the structure of the A horizon: organo-mineral aggregates were classified in macro- (> 4 mm), meso- (2–4 mm), and micro- (< 2 mm) aggregates (Zanella et al., 2011). Humus profiles were then assigned to humus forms according to Zanella et al. (2011). Table 1 summarizes main features useful for the identification of humus forms found during the survey. Given the scarcity of acid silicated substrates in the study region (Fig. 1) only a restricted array of acidic humus profiles were found, all of them classified as dysmoder, while mull and amphi were represented by all described humus forms. Amphi is a recently acknowledged group of humus forms (mostly carbonated), displaying a combination of moder (in organic horizons) and
mull features (in organo-mineral horizons), which were first described under the collective name
amphimull (Jabiol et al., 1994; Brêthes et al., 1995; Jabiol et al., 1995, 2007), then separated from
mull under the name amphi (Graefe, 2007; Galvan et al., 2008) or amphimus (AFES, 2008; Gobat et
al., 2010) and more recently shared in several humus classification systems (Zanella et al., 2009, 2011;
Jabiol et al., 2013). Considered (explained) variables were then the thicknesses of OL, OF, OH and A
horizons and the presence of macro-, meso- and micro-aggregates.

2.3. *Explanatory data (Table 2)*

Among distal factors (not affected by humus forms), parent rocks were classified in silicated
(most of them acid) and carbonated rocks (limestone, dolomite and marls). At each site, the elevation
(a.s.l.) was recorded, as well as the aspect according to the nearest cardinal direction. The climate
variables considered were: mean annual precipitation (precipitation, in mm), mean annual temperature
(temperature in °C), annual and summer (June-July-August) potential evapotranspiration (in mm)
calculated according to Thornthwaite and Mather (1957), Gams continentality index
(precipitation/elevation), simplified Lang aridity index (precipitation/temperature), and the site's
occurrence in one of the four climate districts.

Among proximal or site factors (in mutual relationship with humus forms, Ponge, 2003, 2013)
forest types were classified in 8 groups: beech, chestnut, other deciduous trees, spruce-beech, spruce-
pine, spruce-larch, silver-fir and larch forests. A vegetation index, varying along a scale from 1 to 3
expressed the dominance of coniferous versus deciduous canopies within forests. Herbaceous cover
was measured as a dummy variable (presence > 50% cover; absence < 50% cover). Soils were
classified in six groups based on the World Reference Base (IUSS Working Group WRB, 2007): rzLP
= Rendzic Leptosols, CMca = calcareous Cambisols, CMeu = non calcareous Cambisols with high
base saturation, LVhe = Luvisols with high base saturation, LVdy = very acid Luvisols, CMdy/PZ =
aggregate group embracing Podzols and very acid Cambisols. Several soil analyses were done in the A
horizon of humus profiles: pH<sub>water</sub> (glass electrode method in 1:5 soil:water suspension, ISO
10390:2005), presence of carbonates (effervescence to cold dilute hydrochloric acid, scale 0–4 according to USDA Soil Survey Manual), clay, sand and rock fragments in mass percentages (sieving and sedimentation method, ISO 11277:2009), organic C and N in mass percentages (dry combustion method, ISO 10694:1995 and 10878:2009, respectively), and C/N ratio. The pH<sub>water</sub> of the topsoil was also measured with the same method in a bulk sample from 0 to 40 cm, including the A horizon but not the O horizon.

## 2.4. Data analysis

Data were analyzed by Redundancy Analysis (RDA), a direct gradient multivariate method (Van den Wollenberg, 1977; Kenkel, 2006), allowing discerning trends in a set of explained variables (humus form categories and humus profile features) which could be explained by a set of explanatory variables (geology, climate, soil, vegetation). For the sake of clarity, we projected independent (explanatory) and dependent (explained) variables in separate bi-plots, and dependent variables were arbitrarily subdivided (for easier visual interpretation) in climate and geology in one bi-plot, and vegetation and soil in another bi-plot. All three bi-plots belong to the same analysis.

Major trends detected by visualizing RDA bi-plots were checked for statistical significance by chi-square test. Quantitative variables to be analyzed by chi-square test were classified in two or three categories on the basis of arbitrary class limits, taking into account the need for having balanced class sizes among categories, to the exception of pH, which was divided in two classes with a limit at pH 5.2, separating soils with (pH < 5.2) or without (pH ≥ 5.2) soluble aluminum (Walker et al., 1990), a threshold known to affect the composition of annelid communities (Graefe and Beylich, 2003). All statistical calculations were performed under XLSTAT® version 2013.4.05 (Addinsoft, Paris, France).

# 3. Results
The main features of the 9 recorded humus forms are presented in Table 2. Most represented humus forms were Oligomull (84 sites) and Eumacroamphi (61 sites), and the least represented ones were Mesomull (8 sites) and Pachyamphi (15 sites). The four mull humus forms (Eumull – Mesomull – Oligomull – Dysmull) were more often recorded on carbonated substrates, with a dominance of macro- and meso-structures in the A horizon. Dysmoder was found exclusively on silicated (mostly acid) substrates with very acid and podzolic soils (CMdy/PZ) and at high altitude. Within the amphi group, which was mostly represented (but not exclusively) on carbonated substrates, there was a trend of increasing dominance of carbonated substrates, from 60 to more than 90%, along the sequence Leptoamphi → Eumesoamphi → Eumacroamphi → Pachyamphi, which was associated with increasing thickness of OF horizon from 1.1 to 2.5 cm and OH horizon from 0.5 to 6.4 cm (Fig. 2). This trend was concomitant with an increase in the representation of shallow or very shallow calcareous soils (rzLP and CMca), from 30 to 80% (Table 2). Amphi humus forms were found near exclusively in Esalpic and Mesalpic climate districts and were absent from Endalpic and Avanalpic districts. Mor and Tangel have not been observed in forest soils of Veneto.

The first canonical component F1 extracted 37% of the explained variance. Mull and moder were projected on opposite sides of this component. Humus forms were distributed along F1 mainly according to the structure of the A horizon, with a gradient from macro- to meso- then to micro- aggregates (Fig. 3a). The thickness of fragmented and humified O horizons (OF and OH, respectively) varied in a direction opposite to that of OL and A horizons. The ends of the gradient were characterized by Oligomull and Dysmoder, respectively, other humus forms being projected in an intermediary position.

The projection of environmental variables showed that the first canonical component could be interpreted as a composite factor embracing both geological, climate and soil gradients. Among distal factors (Fig. 3b) the positive side of F1 was characterized by silicated rocks (as opposed to carbonated rocks), high elevation, low temperature, low summer and to a lesser extent annual potential evapotranspiration, high Lang index (low aridity), and Mesalpic district (as opposed to Esalpic and Mediterranean districts on left side). Among site factors (Fig. 3c), the positive side of F1 was
characterized by most acidic soils (CMdy/PZ: Podzols and dystric Cambisols), low pH (whether A horizon or first top 40 cm), high C/N and organic C, high sand content (as opposed to clay content) and high vegetation index (dominance of coniferous trees), exemplified by spruce-pine forests (right side) opposed to other deciduous forests (left side) with opposite characters (soil types CMeu, LVhe, CMca).

Mean annual precipitation, organic N, exposure (four cardinal directions), Endalpic district, rock fragments content and soil effervescence were projected not far from the origin along F1, thus did not seem to contribute to a great extent to the gradient depicted by F1. Thus temperature and potential evapotranspiration (in particular during summer months) for climate, and pH, organic C and C/N for soil, are given more importance in this gradient than mean annual precipitation and organic N, respectively.

We tested the significance of the gradient from macro- to meso- then to micro-aggregated structure of the A horizon according to some possible proximal (soil, vegetation) and distal (geology, climate) factors (Fig. 4). The impact of geology (silicated vs. carbonated rocks, Fig. 4a), elevation (three classes, < 800m, 800-1500m, > 1500m, Fig. 4b), temperature (three classes, < 6°C, 6–10°C, > 10°C, Fig. 4c), aridity (three classes of Lang index, < 150, 150–250, > 250, Fig. 4d) and climate district (four districts, Fig. 4e) was verified, with a very high significance level (P < 0.0001 for all tests, to the exception of P < 0.001 for Lang index). The impact of vegetation (three classes of vegetation index, Fig. 4f), soil pH (two classes, pH < 5.2, pH ≥ 5.2, Fig. 4g) and clay content (three classes, < 15%, 15–25%, > 25%, Fig. 4h) was verified with a significant level for clay content (P < 0.05) and a highly significant level for vegetation and pH (P < 0.0001). The variation along geological, climate, soil and vegetation gradients of the proportion of meso-aggregated A horizons was much less pronounced than that of micro- and macro-aggregated A horizons, which exhibited strongly contrasted trends, exemplifying the intermediate nature of meso-aggregates.

The second canonical component F2 extracted 21% of the explained variance. Along F2 amphi (negative side) was opposed to both mull and moder (positive side). Humus forms were
distributed along this factor mainly according to the thickness of OF and OH horizons, in particular
within the amphi group, Pachyamphi (with thickest OF and OH horizons, see Table 2) being projected
far from the origin on the negative side of F2, followed by Eumacroamphi then by Eumesoamphi and
at last Leptoamphi (Figs. 2 and 3a). Within the mull group, Dysmull was opposed to Oligomull along
F2, Mesomull and Eumull being in an intermediate position, thus not depicting a similar gradient of
thickness of OF (OH absent in mull) horizon (see Table 2). Among distal and site environmental
factors, F2 mainly depicted an opposition between silicated and carbonated rocks (Fig. 3b), and a
gradient in soil effervescence, clay and rock content (Fig. 3c): very shallow calcareous soils (rsLP)
with low clay and high rock fragment content, developed over carbonated substrates are on the
negative side of F2 (Fig. 3c).

The thickness of OH horizons, the % of carbonated substrates among recorded profiles (Fig. 2)
and the rock fragments content of A horizons (Fig. 5) increased along the gradient Leptoamphi –
Eumesoamphi – Eumacroamphi – Pachyamphi. The thickness of the OH horizon and the
effervescence level (varying from 0 to 4) were positively correlated with the rock fragment content of
the soil on carbonated substrates (r<sub>s</sub> = 0.26 and 0.37, respectively, P < 0.0001).

**4. Discussion and conclusions**

We showed that a large variety of humus forms, mainly associated with carbonated substrates
(hard limestone, dolomite and marl), were recorded in Veneto. We also showed that geology, climate,
soil and vegetation exert a prominent influence on the distribution of humus forms, especially when
considering the micro-, meso- or macro-aggregated structure of the A horizon. The coldest climatic
district in the Veneto region is situated in areas with the highest elevation, the most acidic substrate
(acid silicated rocks), the most recalcitrant litter (spruce and pine), and a finer structure in the A
horizon. This means that all factors (low temperature, high acidity, litter recalcitrance), known to
decrease litter decomposition and soil biological activity (reviewed in Ponge, 2003, 2013), may affect
the size of soil aggregates, whether these factors co-occur or not. In the study region, substrate acidity
and litter recalcitrance (spruce and pine) are correlated. Coniferous forest vegetation, especially spruce, is ubiquitous and man-induced for a large part, being mostly established on poorest soils as commonly observed in other regions (Willis et al., 1997; Wilson et al., 2001). On the contrary, elevation and soil acidity are not correlated in Veneto (Fig. 1), where the highest mountain summits and cliffs are of carbonated (dolomitic) nature (ARPAV, 2005; Viero et al., 2010), while in eastern and western parts of the Alps acidity increases with altitude, since carbonated rocks are eroded at a higher rate than core metamorphic and igneous silicated rocks during orogeny (Frisch et al., 2008), among many other causal factors (Körner, 2007).

The structure of the organo-mineral horizon is mainly governed by the activity of soil organisms, among which annelids, after roots, take a prominent position (Wolters, 1991, 2000; Zangerlé et al., 2011). Among annelids, large worms (generally earthworms) and small worms (generally enchytraeids) diverge in their tolerance to low temperatures (Holmstrup et al., 2007), acidity (Graefe and Beylich, 2003), and litter quality (Ponge et al., 1997). Annelids create different structures according to their size, macro- (> 4 mm) and meso-structures (2–4 mm) of mull and amphi-humus forms resulting from the burrowing and defecating activity of earthworms (Galvan et al., 2008), while microstructures (< 2 mm) of moder humus forms result mainly from the activity of enchytraeids (Didden, 1990; Dawod and FitzPatrick, 1993; Ponge, 1999). Enchytraeids, in particular the common, asexual *Cognettia sphagnetorum*, replace earthworms when conditions become more severe along latitudinal and altitudinal gradients (Bernier et al., 1993; Briones et al., 2007). Exclusion mechanisms between enchytraeids and earthworms, whether competitive or behavioral in advance of competition, have been demonstrated (Haukka, 1987; Ponge, 2003), in addition to abovementioned differences in tolerance, which might explain observed gradients of annelid composition and resulting changes in humus forms (Ponge, 2003).

As expected, main groups of humus forms present in the Veneto region, namely mull, moder and amphi, are distributed according to their main morphological features and their contrasted responses to environmental factors (Ponge, 2003; Graefe and Beylich, 2003; Graefe, 2007; Ponge, 2013). Our results show a strong contrast between (i) Dysmoder (as the unique representative of the
moder group) in environmental conditions favoring fungal and mesofaunal activity (aggregates < 2 mm, mainly done by enchytraeids), as in very acid podzolic soils, and (ii) mull and amphi, representatives of soils with intense bacterial and macrofaunal activity (aggregates > 2 mm, mainly done by earthworms). We hypothesize that the gradient depicted by the first canonical factor of RDA (embracing climate, geology, soil and vegetation) corresponded to the balance between fungal-based and bacterial-based soil trophic networks, a balance largely controlled by the environment (Hedlund et al., 2004). The impact of environmental gradient on humus forms is mainly mediated by the activity of annelids (enchytraeids and earthworms) which transform the litter into humus and incorporate it (or not) to mineral horizons, with strong variations according to their size and behavioral (burrowing) type. Low temperature, acidity and poor litter quality favor small-sized annelids (enchytraeids) to the detriment of big-sized annelids (anecic earthworms), medium-sized annelids such as epigeic earthworms being intermediary in their environmental requirements (Ponge, 2003, 2013). Awaiting a study embracing both animal communities and humus profiles in a wide range of environmental conditions, this may contribute to explain the observed gradient of aggregate size distribution.

The contrast between mull and amphi was mainly associated with physicochemical properties of parent materials, in particular texture (clay → sand → rock fragments) and calcium content (soil effervescence). Our results do not point to a great importance given to climate in the contrast between amphi and mull, although climate has been suspected to be a driving factor of amphi build-up (Graefe, 2007). Amphi, being characterized by thick organic horizons with a well-developed OH horizon (otherwise typical of moder and mor) overlying a macro- or meso-aggregated A horizon (otherwise typical of mull), can be explained by the combination of (i) large earthworm populations with (ii) some factors preventing them to incorporate litter at least during part of the year. The former is linked to calcium availability (here approximated by soil pH and presence of carbonates), a prominent factor of earthworm activity (Piearce, 1972), while the latter is linked to climate harshness (summer drought and/or winter frost) and hardness of the parent rock (here approximated by percent rock fragments), known to be stressful in shallow soils (Aussenac, 2002). According to present results, and given our still incipient knowledge on amphi humus forms (Andreetta et al., 2011), high calcium availability,
shallow soils and associated poor water storage capacity seem to be driving factors in the genesis of the most typical humus form of this group: Pachyamphi. In some sense it can be said that on carbonated substrates with summer and/or winter climate stress for earthworm communities the development of a thick OH horizon saturated in Ca may compensate soil shallowness.

Galvan et al. (2008) observed that the size of aggregates reflected the size of species which were dominant in the earthworm community, in particular the balance between epigeic (small earthworms) and anecic (big earthworms) species. According to their records in subalpine forests of the Trento Province (northern Italy), the group they called amphi 1 (meso-structured, embracing Eumesoamphi and Pachyamphi in the present classification) was restricted to acid crystalline substrates, while the group they called amphi 2 (macro-structured, embracing Leptoamphi and Eumacroamphi) was found on dolomitic and mixed substrates. This means that substrates poor in nutrients, in particular calcium, are unfavorable to more exacting litter-feeding earthworms, i.e. anecics, a result supported by studies on earthworm communities (Nordström and Rundgren, 1974; Satchell, 1980; Ponge et al., 1999). Here we found that the gradient Leptoamphi (macro) → Eumesoamphi (meso) → Eumacroamphi (macro) → Pachyamphi (meso) did not reflect the structure of the A horizon (macro- versus meso-aggregates) but rather the thickness of organic layers, and was positively correlated with the percentage of carbonated substrates among samples (from 60% for Leptoamphi to 90% for Pachyamphi) and the percentage of rock fragments in the A horizon (from 14% for Leptoamphi to 38% for Pachyamphi). This suggests that the richer is the topsoil in rock fragments (in calcareous context) the more difficult the incorporation of organic matter to mineral matter by burrowing earthworms (Callaham and Blair, 1999). In the absence of earthworm records we cannot go deeper in elucidating this causal relationship. The balance between unincorporated carbon and that incorporated by annelids in the mineral soil would not be entirely driven by climate, as suspected so far (Bottner et al., 1995; Graefe, 2007; Ascher et al., 2012), although other influences such as phosphorus availability have been suspected (Tagger et al., 2008), but also by the restricted soil volume available for burrowing due to the abundance of rock fragments. This ‘rock fragment effect’, typical of shallow calcareous soils (rzLP) on dolomite and hard limestone substrates might
have important consequences on carbon sequestration (Wironen and Moore, 2006; Andreetta et al., 2011; Bonifacio et al., 2011), beside climate effects (Marland et al., 2003; Egli et al., 2010; Thornley and Cannell, 2001) and their interactions with vegetation (Paré et al., 2006; De Deyn et al., 2008; Podwojewski et al., 2011) and mineral composition (Rasmussen et al., 2006).

## Acknowledgements

All data were obtained within the framework of the research project "Soil map of Veneto Region at 1:250,000 scale". This project was financially supported by the Environmental Protection Agency of the Veneto Region (ARPAV), which is greatly acknowledged. The authors also acknowledge two anonymous reviewers, for language editing and many improvements suggested to a previous draft of our paper.

## References

* Aberegg, I., Egli, M., Sartori, G., Purves, R., 2009. Modelling spatial distribution of soil types and characteristics in a high Alpine valley (Val di Sole, Italy). Studi Trentini di Scienze Naturali 85, 39-50.

* AFES, 2008. Référentiel Pédologique 2008. Quae, Paris.

* 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 storage in Mediterranean environments. Biology and Fertility of Soils 47, 31-40.

* Antonelli, R., Barbieri, G., Dal Piaz, G.V., Dal Pra, A., De Zanche, V., Grandesso, P., Mietto, P., Sedea, R., Zanferrari A., 1990. Carta geologica del Veneto, scala 1:250,000: una storia di cinquecento milioni di anni fa. Regione del Veneto, Segreteria Regionale per il Territorio, Venice.
ARPAV, 2005. Carta dei suoli del Veneto. Osservatorio Regionale Suolo, Castelfranco Veneto, Italy.

Ascher, J., Sartori, G., Graefe, U., Thornton, B., Ceccherini, M.T., Pietramellara, G., Egli, M., 2012.
Are humus forms, mesofauna and microflora in subalpine forest soils sensitive to thermal
conditions ? Biology and Fertility of Soils 48, 709-725.

Aussenac, G., 2002. Ecology and ecophysiology of circum-Mediterranean firs in the context of
climate change. Annals of Forest Science 59, 823-832.

Bal, L., 1970. Morphological investigation in two moder-humus profiles and the role of the soil fauna
in their genesis. Geoderma 4, 5-36.

Bernier, N., Ponge, J.F., André, J., 1993. Comparative study of soil organic mayers in two bilberry-
spruce forest stands (*Vaccinio-Piceetea*): relation to forest dynamics. Geoderma 59, 89-108.

Bonifacio, E., Falsone, G., Petrillo, M., 2011. Humus forms, organic matter stocks and carbon
fractions in forest soils of northwestern Italy. Biology and Fertility of Soils 47, 555-566.

Bottner, P., Coûteaux, M.M., Vallejo, V.R., 1995. Soil organic matter in Mediterranean-type
ecosystems and global climate changes: a case study, the soils of the Mediterranean Basin. In:
Moreno, J.M., Oechel, W.C. (Eds.), Global Change and Mediterranean-Type Ecosystems.
Springer, New York, pp. 306-325.

Brêthes, A., Brun, J.J., Jabiol, B., Ponge, J.F., Toutain, F., 1995. Classification of forest humus forms:
a French proposal. Annales des Sciences Forestières 52 535-546.

Briones, M.J.I., Ineson, P., Heinemeyer, A., 2007. Predicting potential impacts of climate change on
the geographical distribution of enchytraeids: a meta-analysis approach. Global Change
Biology 13, 2252-2269.

Callaham, M.A., Blair, J.M., 1999. Influence of differing land management on the invasion of North
American tallgrass prairie soils by European earthworms. Pedobiologia 43, 507-512.
Carletti, P., Vendramin, E., Pizzeghello, D., Concheri, G., Zanella, A., Nardi, S., Squartini, A., 2009.
Soil humic compounds and microbial communities in six spruce forests as function of parent
material, slope aspect and stand age. Plant and Soil 315, 47–65.

Dawod, V., FitzPatrick, E.A., 1993. Some population sizes and effects of the Enchytraeidae
(Oligochaeta) on soil structure in a selction of Scottish soils. Geoderma 56, 173–178.

De Deyn, G.B., Cornelissen, J.H.C., Bargett, R.D., 2008. Plant functional traits and soil carbon sequestration in contrasting biomes. Ecology Letters 11, 516–531.

Del Favero, R., 2010. I boschi delle regioni alpine italiane: tipologia, funzionamento, selvicoltura.
Cooperativa Libraria Editrice Università di Padova, Padova.

Del Favero, R., Lasen, C., 1993. La Vegetazione Forestale del Veneto, 2nd ed. Progetto, Padova.

Didden, W.A.M., 1990. Involvement of Enchytraeidae (Oligocheata) in soil structure evolution in agricultural fields. Biology and Fertility of Soils 9, 152–158.

Egli, M., Sartori, G., Mirabella, A., Favilli, F., Giaccai, D., Delbos, E., 2009. Effect of north and south exposure on organic matter in high Alpine soils. Geoderma 149, 124–136.

Egli, M., Sartori, G., Mirabella, A., Giaccai, D., 2010. The effects of exposure and climate on the weathering of Late Pleistocene and Holocene Alpine soils. Geomorphology 114, 466–482.

Frisch, W., Kuhlemann, J., Dunkl, I., Szekely, B., 2008. The geomorphologic evolution of the eastern
Alps. Mitteilungen der Österreichischen Geographischen Gesellschaft 150, 123–162.

Galvan, P., Ponge, J.F., Chersich, S., Zanella, A., 2008. Humus components and soil biogenic structures in Norway spruce ecosystems. Soil Science Society of America Journal 72, 548–557.

Garlato, A., Obber, S., Vinci, I., Sartori, G., Manni, G., 2009. Stock attuale di carbonico nei suoli di montagna del Veneto. Studi Trentini di Scienze Naturali 85, 69–81.
Gobat, J.M., Aragno, M., Matthey, W., 2010. Le Sol Vivant: Bases de Pédologie, Biologie des Sols. Presses Polytechniques Universitaires Romandes, Lausanne, Switzerland.

Graefe, U., 2007. Gibt es in Deutschland die Humusform Amphi? Mitteilungen der deutschen Bodenkundlichen Gesellschaft 110, 459–460.

Graefe, U., Beylich, A., 2003. Critical values of soil acidification for annelid species and the decomposer community. Newsletter on Enchytraeidae 8, 51–55.

Haukka, J.K., 1987. Growth and survival of *Eisenia fetida* (Sav.) (Oligochaeta: Lumbricidae) in relation to temperature, moisture and presence of *Enchytraeus albidus* (Henle) (*Enchytraeidae*). Biology and Fertility of Soils 3, 99–102.

Hedde, M., Aubert, M., Decaëns, T., Bureau, F., 2008. Dynamics of soil carbon in a beechwood chronosequences forest. Forest Ecology and Management 255, 193–202.

Hedlund, K., Griffiths, B., Christensen, S., Scheu, S., Setälä, H., Tscharntke, T., Verhoef, H., 2004. Trophic interactions in changing landscapes: responses of soil food webs. Basic and Applied Ecology 5, 495-503.

Holmstrup, M., Overgaard, J., Bayley, M., 2007. Small *Dendrobaena* earthworms survive freezing better than large worms. Cryobiology 54, 298–300.

IUSS Working Group WRB, 2007. World Reference Base for Soil Resources 2006, first update 2007. Food and Agriculture Organization of the United Nations, Rome.

Jabiol, B., Brêthes, A., Ponge, J.F., Toutain, F., Brun, J.J., 1995. L'humus sous Toutes ses Formes. École Nationale du Génie Rural des Eaux et Forêts, Nancy, France.

Jabiol, B., Brêthes, A., Brun, J.J., Ponge, J.F., Toutain, F., 1994. Une classification morphologique et fonctionnelle des formes d'humus: propositions du Référentiel Pédologique 1992. Revue Forestière Française 46, 152–166.
Jabiol, B., Brêthes, A., Ponge, J.F., Toutain, F., Brun, J.J., 2007. L'Humus sous Toutes ses Formes, 2ème édition. École Nationale du Génie Rural des Eaux et Forêts, Nancy, France.

Jabiol, B., Zanella, A., Ponge, J.F., Sartori, G., Englisch, M., Van Delft, B., De Waal, R., Le Bayon, R.C., 2013. A proposal for including humus forms in the World Reference Base for Soil Resources (WRB-FAO). Geoderma 192, 286–294.

Kenkel, N.C., 2006. On selecting an appropriate multivariate analysis. Canadian Journal of Plant Science 86, 663–676.

Kindel, A., Garay, I., 2002. Humus form in ecosystems of the Atlantic Forest, Brazil. Geoderma 108, 101–118.

Klinka, K., Wang, Q., Carter, R.E., 1990. Relationships among humus forms, forest floor nutrient properties, and understory vegetation. Forest Science 36, 564–581.

Körner, C., 2007. The use of ‘altitude’ in ecological research. Trends in Ecology and Evolution 22, 569–574.

Kuikman, P.J., De Groot, W.J.M., Hendriks, R.F.A., Verhagen, J., De Vries, F., 2003. Stocks of C in Soils and Emissions of CO₂ from Agricultural Soils in the Netherlands. Alterra Green World Research, Wageningen.

Lalanne, A., Bardat, J., Lalanne-Amara, F., Gautrot, T., Ponge, J.F., 2008. Opposite responses of vascular plant and moss communities to changes in humus form, as expressed by the Humus Index. Journal of Vegetation Science 19, 645–652.

Li, P.H., Wang, Q., Endo, T., Zhao, X., Kakubari, Y., 2010. Soil organic carbon stock is closely related to aboveground vegetation properties in cold-temperate mountainous forests. Geoderma 154, 407–415.

Marland, G., Pielke, R.A. Sr, Apps, M., Avissar, R., Betts, R.A., Davis, K.J., Frumhoff, P.C., Jackson, S.T., Joyce, L.A., Kauppi, P., Katzenberger, J., McDicken, K.G., Neilson, R.P., Niles, J.O.,
Niyogi, D.D.S., Norby, R.J., Pena, N., Sampson, N., Xue, Y.K., 2003. The climatic impacts of land surface change and carbon management, and the implications for climate-change mitigation policy. Climate Policy 3, 149–157.

Nabuurs, G.J., Mohren, G.M.J., 1993. Carbon in Dutch forest ecosystems. Netherlands Journal of Agricultural Science 41, 309–326.

Nordström, S., Rundgren, S., 1974. Environmental factors and lumbricid associations in southern Sweden. Pedobiologia 14, 1–27.

Paré, D., Boutin, R., Larocque, G.R., Raulier, F., 2006. Effect of temperature on soil organic matter decomposition in three forest biomes of eastern Canada. Canadian Journal of Soil Science 86, 247–256.

Piearce, T.G., 1972. The calcium relations of selected Lumbricidae. Journal of Animal Ecology 41, 167–188.

Podwojewski, P., Poulenard, J., Minh Luu Nguyen, De Rouw, A., Van Thiet Nguyen, Quang Ha Pham, Duc Toan Tran, 2011. Climate and vegetation determine soil organic matter status in an alpine inner-tropical soil catena in the Fan Si Pan Mountain, Vietnam. Catena 87, 226–239.

Ponge, J.F., 1999. Horizons and humus forms in beech forests of the Belgian Ardennes. Soil Science Society of America Journal 63, 1888–1901.

Ponge, J.F., 2003. Humus forms in terrestrial ecosystems: a framework to biodiversity. Soil Biology and Biochemistry 35, 935–945.

Ponge, J.F., 2013. Plant-soil feedbacks mediated by humus forms: a review. Soil Biology and Biochemistry 57, 1048–1060.

Ponge, J.F., Arpin, P., Sondag, F., Delecour, F., 1997. Soil fauna and site assessment in beech stands of the Belgian Ardennes. Canadian Journal of Forest Research 27, 2053–2064.
Ponge, J.F., Chevalier, R., Loussot, P., 2002. Humus Index: an integrated tool for the assessment of forest floor and topsoil properties. Soil Science Society of America Journal 66, 1996-2001.

Ponge, J.F., Jabiol, B., Gégout, J.C., 2011. Geology and climate conditions affect more humus forms than forest canopies at large scale in temperate forests. Geoderma 162, 187-195.

Ponge, J.F., Patzel, N., Delhaye, L., Devigne, E., Levieux, C., Beros, P., Wittebroodt, R., 1999.
Interactions between earthworms, litter and trees in an old-growth beech forest. Biology and
Fertility of Soils 29, 360-370.

Rasmussen, C., Southard, R.J., Horwath, W.R., 2006. Mineral control of organic carbon mineralization in a range of temperate conifer forests soils. Global Change Biology 12, 834-847.

Rumpel, C., Kögel-Knabner, I., 2011. Deep soil organic matter: a key but poorly understood component of terrestrial C cycle. Plant and Soil 338, 143-158.

Satchell, J.E., 1980. Potential of the Silpho Moor experimental birch plots as a habitat for *Lumbricus terrestris*. Soil Biology and Biochemistry 12, 317-323.

Schulp, C.J.E., Nabuurs, G.J., Verburg, P.H., De Waal, R., 2008. Effect of tree species on carbon stocks in forest floor and mineral soil and implications for soil carbon inventories. Forest Ecology and Management 256, 482-490.

Susmel, L., 1994. I rovereti di pianura della Szerenissima. Cooperativa Libraria Editrice Università di Padova, Padova.

Tagger, S., Périssol, C., Criquet, S., Aubert, G., Neville, P., Le Petit, J., Toutain, F., 2008.

Characterization of an amphimull under Mediterranean evergreen oak forest (*Quercus ilex*):
micromorphological and biodynamic descriptions. Canadian Journal of Forest Research 38,
268-277.
Thornley, J.H.M., Cannell, M.G.R., 2001. Soil carbon storage response to temperature: an hypothesis.
Annals of Botany 87, 591–598.

Thornthwaite C.W., Mather J.R., 1957. Instructions and tables for computing potential
evapotranspiration and the water balance. Publications in Climatology 10, 183–311.

Thum, T., Räisänen, P., Sevanto, S., Tuomi, M., Reick, C., Vesala, T., Raddatz, T., Aalto, T., Järvinen, H., Altimir, N., Pilegaard, K., Nagy, Z., Rambal, S., Liski, J., 2011. Soil carbon model alternatives for ECHAM5/JSBACH climate model: evaluation and impacts on global carbon cycle estimates. Journal of Geophysical Research 116, G02028,
doi:10.1029/2010JG001612.

Trap, J., Bureau, F., Akpa-Vinceslas, M., Decaëns, T., Aubert, M., 2011. Changes in humus forms and soil N pathways along a 130-year-old pure beech forest chronosequence. Annals of Forest Science 68, 595–606.

Trap, J., Bureau, F., Perez, G., Aubert, M., 2013. PLS-regressions highlight litter quality as the major
predictor of humus form shift along forest maturation. Soil Biology and Biochemistry 57,
969–971.

UNFCCC, 1992. United Nations Framework Convention on Climate Change. United Nations, New York, New York.

Van den Wollenberg, A.L., 1977. Redundancy analysis: an alternative for canonical correlation analysis. Psychometrika 42, 207–219.

Viero, A., Teza, G., Massironi, M., Jaboyedoff, M., Galgaro, A., 2010. Laser scanning-based
recognition of rotational movements on a deep seated gravitational instability: the Cinque
Torri case (north-eastern Italian Alps). Geomorphology 122, 191–204.

Walker, W.J., Cronan, C.S., Bloom, P.R., 1990. Aluminum solubility in organic soil horizons from northern and southern forested watersheds. Soil Science Society of America Journal 54, 369–374.
Willis, K.J., Braun, M., Sümegi, P., Tóth, A., 1997. Does soil change cause vegetation change or vice versa? A temporal perspective from Hungary. Ecology 78, 740–750.

Wilson, S.McG., Pyatt, D.G., Malcolm, D.C., Connolly, T., 2001. The use of ground vegetation and humus type as indicators of soil nutrient regime for an ecological site classification of British forests. Forest Ecology and Management 140, 101–116.

Wironen, M., Moore, T.R., 2006. Exotic earthworm invasion increases soil carbon and nitrogen in an old-growth forest in southern Quebec. Canadian Journal of Forest Research 36, 845–854.

Wolters, V., 1991. Soil invertebrates, effects on nutrient turnover and soil structure: a review. Zeitschrift für Pflanzenernährung und Bodenkunde 154, 389–402.

Wolters, V., 2000. Invertebrate control of soil organic matter stability. Biology and Fertility of Soils 31, 1–19.

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 morpho-functional classification of humus forms. Geoderma 164, 138–145.

Zanella, A., Jabiol, B., Ponge, J.F., Sartori, G., De Waal, R., Van Delft, B., Graefe, U., Cools, N., Katzentsteiner, K., Hager, H., Englisch, M., Brêthes, A., 2009. Toward a European humus forms reference base. Studi Trentini di Scienze Naturali 85, 145–151.

Zangerlé, A., Pando, A., Lavelle, P., 2011. Do earthworms and roots cooperate to build macroaggregates ? Geoderma 167/168, 303–309.

Zhang, D.Q., Hui, D.F., Luo, Y.Q., Zhou, G.Y., 2008. Rates of litter decomposition in terrestrial ecosystems: global patterns and controlling factors. Journal of Plant Ecology 1, 85–93.
**Figure captions**

**Figure 1.** The distribution of climate and geology over the Veneto region, with the distribution of climate districts according to Del Favero and Lasen (1993), modified for the Mediterranean district, which has been updated to Avanalpic.

**Figure 2.** Distribution of amphi humus forms and thickness of organic horizons according to geological substrate. Error bars for OH thickness indicate standard errors of the means.

**Figure 3.** Redundancy Analysis (RDA). Projection of explained variables (Fig. 1a) and explanatory variables (Fig. 1b) in the plane of the first two canonical factors (see list in Table 1).

**Figure 4.** Distribution of micro- (< 2 mm), meso- (2–4 mm) and macro-aggregates (> 4 mm) according to geology (Fig. 4a), climate (Figs. 4b to 4e), vegetation (Fig. 4f), pH (Fig. 4g) and clay content (Fig. 4h) of the A horizon.

**Figure 5.** Distribution of amphi humus forms and thickness of organic horizons according to soil stoniness. Error bars for OH thickness and % rock fragments indicate standard errors of the means.
<table><caption>**Table 1.** Criteria used for the classification of terrestrial humus forms from Veneto Region. Never = horizon never present; always = horizon always present; transition O-A < 3 mm = thickness of the junction zone between O (organic) and A (organo-mineral) horizons < 3 mm; bracketed horizon = discontinuous or in pockets; OH < 1cm = thickness of OH horizon < 1 cm; zo = zoogenic; noz = non zoogenic; mA = biomacrostructured A horizon; mA = biomesostructured A; mA = biomicrostructured A; mA ≥ OH/2 = thickness of mA ≥ half that of OH; * = not included in Veneto dataset.</caption><thead><tr><th>Main morpho-functional types</th><th>Humus Form<br>Reference Groups<br>(Jabiol et al. (2013))</th><th>Diagnostic horizons</th></tr></thead><tbody><tr><td rowspan="4"><b>MULL:</b> never: OH, OFnoz, Anoz, miA; always: mA or meA; transition O-A &lt; 3 mm</td><td>Eumull</td><td>(OLn), mA</td></tr><tr><td>Mesomull</td><td>OLn, (OLv), mA</td></tr><tr><td>Oligomull</td><td>OLn, OLv, (OFzo) mA or meA</td></tr><tr><td>Dysmull</td><td>OLn, OLv, OFzo, mA or meA</td></tr><tr><td rowspan="3"><b>MODER:</b> never: mA, OFnoz; always: Anoz or miA; transition O-A ≥ 5mm</td><td>Hemimoder*</td><td>OLn, OLv, OFzo, (OH), miA</td></tr><tr><td>Eumoder*</td><td>OLn, OLv, OFzo, OH≤1cm, (Anoz) or miA</td></tr><tr><td>Dysmoder</td><td>OLn, OLv, OFzo, OH&gt;1cm, (Anoz and OH?2A) or miA</td></tr><tr><td rowspan="3"><b>MOR*:</b> never: mA, meA; never: mA, meA; always: OFnoz; transition O-A &lt; 3 mm</td><td>Hemimor*</td><td>OLn, OLv, OFzo, (OFnoz), (OH), miA or Anoz or E</td></tr><tr><td>Humimor*</td><td>OLn, OLv, OFzo, OFnoz, (OH), miA or Anoz or E</td></tr><tr><td>Eumor*</td><td>OLn, OLv, OFzo, OFnoz, OH, Anoz or E</td></tr><tr><td rowspan="4"><b>AMPHI:</b> never: OFnoz, Anoz; always: OH, mA or meA; always: A ≥ OH/2</td><td>Leptoamphi</td><td>OLn, OLv, OFzo, OH &lt; 1cm, mA ≥ OH/2</td></tr><tr><td>Eumacroamphi</td><td>OLn, OLv, OFzo, OH ≥ 1cm, mA ≥ OH/2</td></tr><tr><td>Eumesoamphi</td><td>OLn, OLv, OFzo, OH &lt; 3cm, A ≥ OH/2, miA and/or meA or only meA</td></tr><tr><td>Pachyamphi</td><td>OLn, OLv, OFzo, OH ≥ 3cm, A ≥ OH/2, miA and/or meA or only meA</td></tr><tr><td rowspan="2"><b>TANGEL*:</b> never: OFnoz; always: OH &gt; 2A</td><td>Eutangel*</td><td>OLn, OLv, OFzo, OH &gt; 2A, Anoz or meA, transition OH-A &lt; 5 mm</td></tr><tr><td>Dystangel*</td><td>OLn, OLv, OFzo, OH &gt; 2A, Anoz, transition OH-A ≥ 5 mm</td></tr></tbody></table>
<table><caption>**Table 2.** Mean features of Veneto humus forms according to morphology of humus profiles, geology, topography, climate, vegetation and soil characteristics. When data are of analytical nature (measures), values are given as means with standard deviations. When data are of typological nature (occurrences), they are indicated as percent occurrence of the different categories.</caption><thead><tr><th></th><th>EUMULL<br>(n = 48)</th><th>MESOMULL<br>(n = 8)</th><th>OLIGOMULL<br>(n = 84)</th><th>DYSMULL<br>(n = 55)</th><th>DYSMODER<br>(n = 24)</th><th>LEPTOAMPHI<br>(n = 10)</th><th>EUMESOAMPHI<br>(n = 43)</th><th>EUMACROAMPHI<br>(n = 61)</th><th>PACHYAMPHI<br>(n = 15)</th></tr></thead><tbody><tr><td>OF (cm)</td><td>0±0</td><td>0±0</td><td>0.08±0.19</td><td>2.6±1.9</td><td>3.0±1.8</td><td>1.1±1.0</td><td>1.5±1.6</td><td>2.2±2.6</td><td>2.5±2.0</td></tr><tr><td>OH (cm)</td><td>0±0</td><td>0±0</td><td>0±0</td><td>0±0</td><td>3.7±3.2</td><td>0.5±0.0</td><td>1.3±0.5</td><td>3.5±4.3</td><td>6.4±3.7</td></tr><tr><td>A (cm)</td><td>10.3±5.7</td><td>17.6±9.9</td><td>14.0±10.0</td><td>9.9±7.7</td><td>7.2±4.7</td><td>7.6±3.9</td><td>14.6±9.8</td><td>14.0±10.9</td><td>12.7±7.5</td></tr><tr><td>Macro-aggregates</td><td>24%</td><td>38%</td><td>61%</td><td>52%</td><td>0%</td><td>100%</td><td>0%</td><td>100%</td><td>0%</td></tr><tr><td>Meso-aggregates</td><td>74%</td><td>50%</td><td>38%</td><td>48%</td><td>0%</td><td>0%</td><td>100%</td><td>0%</td><td>100%</td></tr><tr><td>Micro-aggregates</td><td>2%</td><td>13%</td><td>1%</td><td>0%</td><td>96%</td><td>0%</td><td>0%</td><td>0%</td><td>0%</td></tr><tr><td>Silicated rock</td><td>36%</td><td>25%</td><td>32%</td><td>28%</td><td>100%</td><td>40%</td><td>37%</td><td>21%</td><td>7%</td></tr><tr><td>Carbonated rock</td><td>64%</td><td>75%</td><td>68%</td><td>72%</td><td>0%</td><td>60%</td><td>63%</td><td>79%</td><td>93%</td></tr><tr><td>Elevation (m)</td><td>1325±443</td><td>931±712</td><td>640±396</td><td>1087±465</td><td>1494±329</td><td>817±387</td><td>1073±521</td><td>981±412</td><td>1186±311</td></tr><tr><td>South aspect</td><td>18%</td><td>25%</td><td>30%</td><td>31%</td><td>46%</td><td>30%</td><td>30%</td><td>26%</td><td>40%</td></tr><tr><td>West aspect</td><td>28%</td><td>0%</td><td>23%</td><td>14%</td><td>8%</td><td>20%</td><td>5%</td><td>13%</td><td>13%</td></tr><tr><td>East aspect</td><td>28%</td><td>25%</td><td>23%</td><td>28%</td><td>8%</td><td>40%</td><td>51%</td><td>45%</td><td>40%</td></tr><tr><td>North aspect</td><td>26%</td><td>50%</td><td>25%</td><td>26%</td><td>38%</td><td>10%</td><td>14%</td><td>16%</td><td>7%</td></tr><tr><td>Annual precipitation (mm)</td><td>1321±277</td><td>1225±259</td><td>1292±311</td><td>1304±242</td><td>1258±195</td><td>1540±348</td><td>1212±208</td><td>1349±262</td><td>1453±146</td></tr><tr><td>Mean annual temperature (°C)</td><td>6.1±2.4</td><td>8.4±4.0</td><td>10.0±2.3</td><td>7.4±2.6</td><td>5.2±1.6</td><td>8.8±2.2</td><td>7.4±2.9</td><td>7.9±2.3</td><td>6.9±1.6</td></tr><tr><td>Annual potential evapotranspiration (mm)</td><td>549±68</td><td>604±122</td><td>645±75</td><td>582±69</td><td>518±48</td><td>635±77</td><td>591±97</td><td>606±76</td><td>564±57</td></tr><tr><td>Summer potential evapotranspiration (mm)</td><td>116±15</td><td>129±26</td><td>140±15</td><td>124±15</td><td>108±10</td><td>132±13</td><td>123±19</td><td>127±15</td><td>120±11</td></tr><tr><td>Gams index</td><td>1.2±0.6</td><td>3.1±3.5</td><td>2.6±1.3</td><td>2.6±6.5</td><td>0.9±0.3</td><td>2.4±1.3</td><td>1.7±1.5</td><td>1.9±2.1</td><td>1.3±0.4</td></tr><tr><td>Lang index</td><td>241±79</td><td>193±122</td><td>142±59</td><td>200±77</td><td>261±80</td><td>182±51</td><td>189±74</td><td>185±57</td><td>226±74</td></tr><tr><td>Mediterranean climate district</td><td>2%</td><td>38%</td><td>27%</td><td>10%</td><td>0%</td><td>0%</td><td>12%</td><td>5%</td><td>0%</td></tr><tr><td>Esalpic climate district</td><td>36%</td><td>13%</td><td>62%</td><td>48%</td><td>8%</td><td>80%</td><td>35%</td><td>55%</td><td>60%</td></tr><tr><td>Mesalpic climate district</td><td>44%</td><td>50%</td><td>8%</td><td>33%</td><td>79%</td><td>20%</td><td>51%</td><td>35%</td><td>40%</td></tr><tr><td>Endalpic climate district</td><td>18%</td><td>0%</td><td>2%</td><td>9%</td><td>13%</td><td>0%</td><td>2%</td><td>5%</td><td>0%</td></tr><tr><td>Vegetation index</td><td>2.4±0.7</td><td>2.0±0.9</td><td>1.5±0.7</td><td>2.2±0.8</td><td>2.5±0.6</td><td>1.7±0.7</td><td>2.2±0.8</td><td>1.9±0.8</td><td>2.3±0.5</td></tr><tr><td>Spruce-pine forests</td><td>34%</td><td>25%</td><td>8%</td><td>38%</td><td>42%</td><td>10%</td><td>37%</td><td>19%</td><td>27%</td></tr><tr><td>Beech forests</td><td>6%</td><td>0%</td><td>8%</td><td>22%</td><td>13%</td><td>30%</td><td>14%</td><td>19%</td><td>40%</td></tr><tr><td>Chestnut forests</td><td>8%</td><td>25%</td><td>15%</td><td>7%</td><td>0%</td><td>10%</td><td>12%</td><td>3%</td><td>0%</td></tr><tr><td>Other deciduous forests</td><td>14%</td><td>38%</td><td>60%</td><td>21%</td><td>4%</td><td>40%</td><td>23%</td><td>32%</td><td>0%</td></tr><tr><td>Spruce-beech forests</td><td>8%</td><td>0%</td><td>5%</td><td>9%</td><td>13%</td><td>10%</td><td>9%</td><td>15%</td><td>33%</td></tr><tr><td>Subalpine spruce-larch forests</td><td>18%</td><td>13%</td><td>4%</td><td>2%</td><td>8%</td><td>0%</td><td>2%</td><td>5%</td><td>0%</td></tr><tr><td>Silver fir forests</td><td>6%</td><td>0%</td><td>0%</td><td>2%</td><td>17%</td><td>0%</td><td>2%</td><td>5%</td><td>0%</td></tr><tr><td>Larch forests</td><td>6%</td><td>0%</td><td>0%</td><td>0%</td><td>4%</td><td>0%</td><td>0%</td><td>2%</td><td>0%</td></tr><tr><td>Herbaceous cover</td><td>73%</td><td>80%</td><td>37%</td><td>35%</td><td>32%</td><td>40%</td><td>51%</td><td>38%</td><td>53%</td></tr><tr><td>rzLP (WRB classification)</td><td>17%</td><td>25%</td><td>14%</td><td>25%</td><td>0%</td><td>10%</td><td>33%</td><td>31%</td><td>47%</td></tr><tr><td>CMca (WRB classification)</td><td>27%</td><td>38%</td><td>29%</td><td>13%</td><td>0%</td><td>20%</td><td>14%</td><td>26%</td><td>33%</td></tr><tr><td>CMeu (WRB classification)</td><td>15%</td><td>13%</td><td>17%</td><td>9%</td><td>0%</td><td>10%</td><td>16%</td><td>3%</td><td>0%</td></tr><tr><td>LVhe (WRB classification)</td><td>15%</td><td>13%</td><td>23%</td><td>31%</td><td>0%</td><td>20%</td><td>12%</td><td>16%</td><td>13%</td></tr><tr><td>LVdy (WRB classification)</td><td>10%</td><td>13%</td><td>8%</td><td>9%</td><td>0%</td><td>20%</td><td>14%</td><td>5%</td><td>0%</td></tr><tr><td>CMdy/PZ (WRB classification)</td><td>17%</td><td>0%</td><td>10%</td><td>11%</td><td>100%</td><td>20%</td><td>12%</td><td>18%</td><td>7%</td></tr><tr><td>pH water (A horizon)</td><td>6.2±1.3</td><td>7.2±0.9</td><td>6.7±1.2</td><td>6.3±1.3</td><td>4.5±0.5</td><td>6.1±1.2</td><td>6.7±1.3</td><td>6.6±1.4</td><td>7.2±0.9</td></tr><tr><td>pH water (0-40 cm)</td><td>6.6±1.2</td><td>7.4±0.4</td><td>6.7±1.2</td><td>6.5±1.1</td><td>4.8±0.4</td><td>6.5±1.3</td><td>6.8±1.2</td><td>6.7±1.2</td><td>7.1±1.0</td></tr><tr><td>Effervescence</td><td>0.6±1.3</td><td>0.8±1.4</td><td>0.7±1.3</td><td>0.5±1.1</td><td>0±0</td><td>0.4±0.8</td><td>0.9±1.3</td><td>1.0±1.4</td><td>1.9±1.7</td></tr><tr><td>Clay%</td><td>19±9</td><td>22±11</td><td>26±12</td><td>21±11</td><td>16±8</td><td>21±9</td><td>20±11</td><td>20±12</td><td>13±7</td></tr><tr><td>Sand%</td><td>31±13</td><td>28±15</td><td>27±17</td><td>32±16</td><td>41±12</td><td>26±12</td><td>34±18</td><td>30±18</td><td>33±17</td></tr><tr><td>Rock fragments%</td><td>11±14</td><td>5±4</td><td>15±15</td><td>16±16</td><td>12±10</td><td>14±13</td><td>17±16</td><td>24±22</td><td>38±19</td></tr><tr><td>Organic C%</td><td>4.9±2.5</td><td>4.7±3.6</td><td>5.3±3.8</td><td>5.4±2.0</td><td>8.1±3.9</td><td>5.9±2.3</td><td>5.4±3.5</td><td>5.2±2.8</td><td>5.0±2.8</td></tr><tr><td>Organic N%</td><td>4.6±2.5</td><td>3.3±1.6</td><td>5.1±3.4</td><td>4.9±2.2</td><td>5.2±2.4</td><td>4.2±1.1</td><td>4.7±2.7</td><td>4.7±2.0</td><td>4.4±2.1</td></tr><tr><td>C/N</td><td>11.3±8.1</td><td>9.5±2.5</td><td>11.0±2.4</td><td>11.9±3.0</td><td>14.9±2.6</td><td>15.8±5.6</td><td>13.6±2.7</td><td>11.9±2.8</td><td>11.5±2.5</td></tr></tbody></table>
<figure><img src="image_2.png" /><figcaption>Fig. 1</figcaption></figure>
<figure><img src="image_3.png" /><figcaption>Fig. 2</figcaption></figure>
<figure><img src="image_4.png" /></figure>

(a)

<figure><img src="image_5.png" /></figure>

(b)

<figure><img src="image_6.png" /></figure>

(c)

<caption>Fig. 3</caption>
Geology

$$
\chi^2 = 55.9 \\
P < 0.0001
$$

(a)

<figure><img src="image_7.png" /></figure>

Elevation

$$
\chi^2 = 30.6 \\
P < 0.0001
$$

(b)

<figure><img src="image_8.png" /></figure>

Mean temperature

$$
\chi^2 = 32.1 \\
P < 0.0001
$$

(c)

<figure><img src="image_9.png" /></figure>

Lang index

$$
\chi^2 = 22.5 \\
P < 0.001
$$

(d)

<figure><img src="image_10.png" /></figure>

Climate districts

$$
\chi^2 = 33.4 \\
P < 0.0001
$$

(e)

<figure><img src="image_11.png" /></figure>

Vegetation

$$
\chi^2 = 31.8 \\
P < 0.0001
$$

(f)

<figure><img src="image_12.png" /></figure>

Soil pH

$$
\chi^2 = 42.7 \\
P < 0.0001
$$

(g)

<figure><img src="image_13.png" /></figure>

Clay content

$$
\chi^2 = 10.6 \\
P < 0.05
$$

(h)

<figure><img src="image_14.png" /><figcaption>Fig. 4</figcaption></figure>
<figure><img src="image_15.png" /><figcaption>Fig. 5</figcaption></figure>