Volcanic soils and landslides : the case study of the Ischia island ( southern Italy ) and relationship with other Campania events

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by the 20 %, (ii) k s values one order of magnitude lower, (iii) a porosity concentrated in the small size (15 to 30 µm modal class) pores characterized by very low percolation threshold (around 15-25 µm), (iv) occurrence of expandable clay minerals and (v) higher Na content in the exchange complex.Therefore, most of these properties indicated 2C as a lower permeability horizon than the above.Nevertheless, only the identification of a thin (6.5 mm) finely stratified ash layer on the top of 2C enabled to assume this interface as an impeding layer to vertical and horizontal water fluxes, as testified by the hydromorphic features (e.g.Fe/Mn concretions) within and on the top of the layer.Despite the Mt.Vezzi soil environment has many properties (high gradient northern facing slope, similar forestry, volcanic origin of the parent material) in common with those of many Campania debris-mud flows, the results of this study did not support the found relationship between Andosols and debris-mudflows, but emphasize the role of vertical discontinuities as landslide predisposing factor.

Introduction
The Italian territory is vulnerable from a hydraulic and geological standpoint, due to its distinguishing geological, geomorphological and climatic factors, along with anthropic pressures.Landslides are frequent and widespread geomorphological phenomena (Guzzetti et al., 1994;Guzzetti and Tonelli, 2004) and the caused damages are severe.
In the last 60 years , all the 20 Italian regions have suffered fatal events -ANSA (2013) census with data from CNR-IRPI: 541 floods in 388 municipalities caused 1760 casualties (762 killed, 67 missing, 931 injured) and 812 landslides in 536 towns produced 5368 casualties, for a total of 3413 deaths, including the tragic event occurred in Vajont on 1963Vajont on (1917Vajont on killed, 14 missing, 1941 wounded) wounded) (Ward and Day, 2011).The Campania is the third region in Italy for number of landslide events (363).In the ecosystems of this Region, the most dangerous type of landslide are debris and mudflows, i.e. very rapid to extremely rapid flows of detritic and plastic materials having high water content (Hungr et al., 2001).One of the most catastrophic historical event occurred at Sarno on 4 and 5 May 1998: debris and mudflows, detached from the slopes of Pizzo D'Alvano, extended 3-4 km into the surrounding lowlands, causing the lost of 161 human lives and severe destruction.Anyhow, other disastrous events struck the mountainous reliefs of the Region before and after the events of Sarno.Soil properties and relationships between debris-mudflows and soil type were investigated on twenty catastrophic landslides occurred in Campania (Terribile et al., 2000(Terribile et al., , 2007)).The results showed that the most catastrophic events are related to soils developing upon pyroclastic materials, covering the carbonatic relieves of the Campania Apennines, and belonging to the Andosols group, generally Molli-Vitric e Pachi-Vitric Andosols (WRB, 2014).In these soils the aspect of the slope affects soil susceptibility to landslide triggering and the landslide susceptibility is related to the hydrological properties, which induce higher water storage especially in northern facing soils.In another study on the soils related to the Sarno landslide it was found (Basile et al., 2003) an increase in soil water storage, due to anisotropy of vertical hydraulic conductivity and water retention Figures characteristics, connected to the occurrence of soil discontinuities (e. g. soil mantle cutting due to track ways) and it was demonstrated that landslide susceptibility is also strongly affected by human-induced disturbances to forestry ecosystems.Therefore, when five debris-mudflows were triggered on 30 April 2006 on the northern slope of Mt.Vezzi in the Ischia Island and caused 4 victims, an investigation aimed to assess the occurrence of andic properties also in the Ischia soils was carried out, due to the close similarity of the environments (volcanic), type of landslides (debrismudflows) and aspect of the slope (northern) with those of the other Campania landslides.
Hence, the principal aim of this work was to investigate the soil properties (chemical, physical, hydrological, mineralogical and micromorphological) at Mt. Vezzi, in order to (i) define whether soil properties may have a role in determining these landslides, (ii) verify if the relationship between Andosols and debris-mudflows, as occurred for most catastrophic Campanian landslides, is still valid for the Ischia landslides and (iii) carry out a detailed investigation at microscale of the sliding surface in order to understand causes of the occurred events.

Events and geomorphological data related to landslides area
Five landslides (namely f1 to f5, following Ascione et al., 2007) (Fig. 1b) occurred on 30 April 2006 along the northern slope of Mt.Vezzi, in two times.The eastern three landslides (f1 to f3) occurred firstly (around 7 a.m.), while the western ones (f4 and f5) about one hour later.The source area is located in the uppermost Costa Ruscello hillslope, in a narrow belt (between 315 and 345 m a.s.l.) close to Mt. Vezzi summit, characterised by high slope gradient (35-45 • ) and variable morphology: f1 initiated at the outer rim of an access track, f2 to f5 started very close to the Costa Ruscello summit free face (Ascione et al., 2007;De Vita et al., 2007).The landslides were classified (De Vita et al., 2007) complex debris slide -debris flows (Cruden and Varnes, 1996).Data recorded before the events, on 29 and 30 April, by the closest meteorological station (Forio d'Ischia, approximately 6 km far from Mt. Vezzi, at 150 m a.s.l., rain gauge Introduction

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Full records reported by Mazzarella and De Luise, 2009) evidenced that rainfall was barely 14 mm.Nevertheless, reliable witnesses reported that landslides were preceded by 12 h of intense rainfall.

Geological and environmental setting
The Ischia island represents the emerged part of a large active volcanic complex rising more than 1000 m a.s.l., located in the easternmost portion of the Naples city area (Fig. 1c); together with the Procida island and the Campi Flegrei caldera, it constitutes the Phlegrean volcanic district (De Vita et al., 2010).Mt.Vezzi (392 m a.s.l.) is located in the south-eastern area of the island and, together with Mt.Torrione (375 m a.s.l.), belongs to a group of relics of a major volcanic complex related to a phase of the island volcanism (Rittmann and Gottini, 1980;Vezzoli, 1988;Civetta et al., 1991).The top of Mt.Vezzi consists of a lava dome (Di Nocera et al., 2007) covered by the Piano Liguori Formation, pyroclastic products of the recent hydromagmatic explosive activity (about 5347-5807 years BP, 14 C calibrated) (Orsi et al., 1996), made up by trachytic ash and pumice layers.Different land use characterises the summit landscape and the north slope, with vineyards occurring on Piano Liguori and close surrounding slopes, and chestnut coppice woodland on high gradient northern slopes.The woodland appears poorly managed in the areas of the detachment crowns, because of clear signs of unplanned cuttings and evidences of anthropic disturbances, like dense trail network and fire traces on tree barks.

Materials and methods
The first survey on the detachment crowns of Ischia landslides was done in the period just after the events (on 12 May 2006) and first soil samples analysed (Vingiani and Terribile, 2007).Nevertheless the pedological study and the sampling was carried out only during the following summer, due to the wet state of the deep soil horizons limiting Introduction

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Full the digging.Two (f2 and f3) of the five detachment crowns were studied (Fig. 1a): P1 and P2 profiles were described and sampled in f2 and P3 in f3, following the guidelines of FAO (2006).Bulk and undisturbed soil samples were collected for chemical, physical (particle size distribution and hydraulic properties) and microtomographic analyses.
For soil classification, the WRB system was used (WRB, 2014).Two field sensitivity tests were used in order to estimate the soil andic properties: (1) one test enables to evaluate the thixotropy (smeary upon pressure) on moist undisturbed soil samples applying a simple pressure between fingers (Wells and Furkert, 1972), because if allophanes (therefore andic properties) occur in soils, the material become sensitive and suddenly shears under increasing pressure; (2) the other test evaluates the amount of non-crystalline (amorphous) material in soil: the action of the sodium fluoride (NaF) upon non-crystalline soil material induces release of hydroxide ions (OH − ) to the soil solution and increases the pH (Fields and Perrott, 1966).

Chemical analysis
Soil bulk samples were air dried and sieved at 2 mm.Percentages of coarse fragments (ϕ > 2 mm) and fine earth fraction (ϕ < 2 mm) were measured after sieving.Chemical analysis were carried out on the fine earth fraction, according to the Soil Survey Laboratory Methods Manual (USDA, 2004): the soil pH was determined potenziometrically on soil-H 2 O (1 : 2.5 ratio) and soil-solution (KCl 1M and NaF 1M, 1 : 2.5 and 1 : 5 ratios, respectively) suspensions; the organic carbon (OC) content following the Walkley and Black (1934) procedure; the cation exchange capacity (CEC) according to Mehlich (1938), with barium chloride-triethanolamine buffered at pH 8.2, and the exchangeable cations (Ca, Mg, Na, K) were measured by ICP-AES Varian Liberty model 150; the phosphate retention according to Blakemore et al. (1987); the electrical conductivity (EC) was measured on soil-H 2 O (1 : 5 ratio) suspensions.The sodium adsorption ratio (SAR) was calculated (Lesch and Suarez, 2009) as the square root of the ratio of the measured exchangeable sodium (Na) to calcium + magnesium (Ca + Mg), divided for Introduction

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Full two, while the exchangeable sodium percentage (ESP) was calculated as the percentage ratio of the Na to the CEC.Due to the volcanic origin of the parent material, specific analyses were also required in order to assess occurrence of andic properties.Selective extraction procedures were applied to soil samples and the different Al, Fe and Si forms were determined.
The acid ammonium-oxalate extractable forms (Al o , Fe o and Si o ) were obtained after Schwertmann (1964) and Blakemore et al. (1987), the Na-dithionite citrate bicarbonate (DCB) extractable components (Al d , Fe d and Si d ), also called total "free iron oxides", by the Mehra and Jackson (1960) procedure and the Na-pyrophosphate extractable complexes (Al p and Fe p ) according to Bascomb (1968).The concentration of elements was then measured by ICP-AES Varian Liberty model 150.Allophane and ferrihydrite contents were calculated using Parfitt (Parfitt and Wilson, 1985;Parfitt, 1990) and Childs (1985) formulas, respectively.

Hydrological analysis
Hydrological analyses were carried out on undisturbed soil samples (ϕ = 86 mm, h = 150 mm) collected in selected horizons of the P2 (Bw 1 , Bw 2 , Bw 4 , 2CB, 2C) and P3 (Bw 4 , 2C, 2Btb) profiles.The saturated hydraulic conductivity, k s , was measured by means of a constant head permeameter (Reynolds and Elrick, 2003).The soil water retention, θ(h), which is the relationship between water content θ and water pressure head h, and the hydraulic conductivity, k(θ), which is the relationship between hydraulic conductivity k and water content θ, were both determined applying the evaporation method (Arya, 2002).The undisturbed soil samples were slowly saturated from the bottom.Three tensiometers were installed at three different depths in the samples and placed on a load cell; starting from saturation and during a 1-dimensional transient upward flow, pressure head and weight of the sample were automatically recorded at these depths until air was observed in the circuit of the uppermost tensiometer.After that, the soil sample was dismantled and placed for 24 h in the oven at 105 • C to determine the water content from the weight data set.An iterative method was applied to 7 Introduction

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Full obtain the θ(h) relationship (Basile et al., 2012).The unsaturated hydraulic conductivity curve was obtained applying the method proposed by Watson (1966).Details on the calculus procedure can be found in Bonfante et al. (2010).

Physical and microtomographic analyses
The particle size distribution (PSD) was determined on soil samples after dispersion with Na-hexametaphosphate and the measurements were carried out by the laser diffraction technique, using a Malvern Mastersizer 2000 system (Glendon and Dani, 2002).X-ray micro-tomography was performed in order to reconstruct 3-D images of the inner structure of the soil samples collected from three subsurface contiguous horizons identified in the P2 profile.The SKYSCAN 1172 desktop system (www.skyscan.be),based on a microfocus cone beam X-ray source, was used and an optimised inverse Radon transform (Kak and Slaney, 1988) was applied on the X-ray attenuation coefficient dataset for image reconstruction.Then the "successive opening" image analysis algorithm (e.g.Serra, 1982;Horgan, 1998) was applied on the reconstructed 3-D images in order to obtain a Pore Size Distribution (PoSD) consisting in the pore space classification according to the wall spacing.Finally the procedure of Lantuejoul and Maisonneuve (1984) was applied to calculate the percolation curves in order to evaluate the connectivity of the pore space.
The inter-aggregate PoSD of each horizon was calculated scanning undisturbed soil samples collected with cylinders in PMMA 3.4 cm in height and diameter, then a 3.5 cm 3 cubic Region Of Interest (ROI) was reconstructed at 30 µm pixel size (image resolution).The intra-aggregate 3-D pore structure was reconstructed and analysed on a 3.5 mm 3 ROI at 1.5 µm image resolution scanning on two selected aggregates from the bulk samples, separately.
In addition, a pore size distribution was derived from the hydrological analysis.Specifically, the scale of the soil water pressure head on the axis of the soil water retention curve can be replaced by that of the diameter of the capillaries according to 8 Introduction

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Full the capillarity law |h| = 2σ cos γ/rρ w g ≈ 0.3/d for length units in cm.This procedure supplies the cumulative pore size distribution curve.The derivative of this curve is the curve of pore size density distribution.Such a schematization, which assumes the simple conceptual model of the soil porous system as a bundle of parallel capillaries to be correct, leads to the derivation of the "equivalent" pore size distribution.In this manner the equivalent diameters are those of the bundle of capillary tubes, which behave, in functional terms, as the soil sample.

Mineralogical and micromorphological analyses
Mineralogical analyses were carried out by means of X-ray diffractometry (XRD) on sand, silt and clay fractions.Samples were dispersed by the use of Nahexametaphosphate and separated in different sizes by means of sieving, for sand fraction, and centrifuge, for clay fractions.The clay fraction was saturated using CaCl 2 and washed with water and ethanol, till removal of chlorides.Mineralogical spectra were acquired with a Rigaku Geigerflex D/Max IIIC, CuKa radiation, Ni filtered, at 35 kW and 35 mA.Powder samples of sand (2 mm-50 µm), silt (50-2 µm) and clay (< 2 µm) fractions were analyzed randomly, in order to detect primary and secondary minerals.Undisturbed samples collected by means of Kubiena boxes for micromorphological studies were impregnated with crystic resin and large (15 × 7.5 cm) thin sections were produced using the FitzPatrick methodology (1984,1993).The micromorphological description referred to the terminology used by FitzPatrick (1993).

Soil morphology and main chemical properties
All the studied soils (Table 1) were very deep, characterised by thin (about 5 cm) brownish surface horizons (A), covering dark yellowish brown Bw horizons abruptly overlying Introduction

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Full greysh brown to light brownish grey 2C horizons, at a variable depth from 70/90 to 190 cm.The yellowish and greyish colours suggested a generally low degree of weathering of the soil and ashy materials, as a consequence of scarce iron oxides formation.
Both tests for field identification of andic properties resulted negative, showing absence of thixotropy and andic properties.All horizons did not reacted to 10 % hydrochloric acid (HCl) due to the absence of carbonates.The soil structure of all soils abruptly changed from sub-angular blocky of the Bw horizons to massive of the ashy layer of the 2C horizons.During the sampling, the soil horizons showed also very different moisture status: the A and all the Bw horizons were slightly moist, while the massive 2C was wet and limited the digging to 100 cm of depth, just after the landslide events.Moreover, the authors of the survey referred that "jumping on the 2C appeared similar to jump on a waterbed", a kind of "blob effect" which indicated a high plasticity of the wet 2C horizon.Therefore, in order to collect undisturbed samples of the 2C and underneath horizons, it was necessary to postpone the sampling in dryer soil conditions.Anyhow, the above described 2C horizon was found in all the studied soil profiles and identified as sliding surface of the occurred landslides (De Vita et al., 2007).In P1 and P3, a yellowish-red 2Btb horizon of a buried soil, having a moderate sub-angular blocky structure, was found below the 2C.Very few to few roots occurred in A and Bw horizons, but they were always absent in 2C and 2Btb.
The results of the chemical analyses showed large accumulation of organic carbon (46.1-71.3g kg −1 ) (Table 2) in all A horizons.As expected, the OC rapidly decreased with depth (Bw 1 horizons: 12.3-6.0g kg −1 ; 2C horizons: 0.8-1.2g kg −1 ).A similar trend was followed by the CEC, whose highest values -as expected -were found in correspondence of the A horizons.Soils had a generally neutral reaction (pH H 2 O ) and the pH was weakly increasing in correspondence of the 2C horizons (Table 2).The soil pH measured in KCl solution (pH KCl ) was lower of 1 to 2.5 units than pH H 2 O , which suggested a potential exchangeable acidity (due to H + and Al 3+ ions) particularly high for the 2C horizons (values between 2.0 and 2.5).Despite the exchangeable complex was dominated by the Ca, high Na + Mg contents (40-65 %) were measured in the Introduction

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Full deepest horizons of the three profiles, which showed also a calculated exchangeable sodium percentage (ESP) generally higher than 15 %.Nevertheless, they cannot be classified as salic or natric horizons (WRB, 2014), for lack of requirements in terms of electrical conductivity (EC < 15 dS m −1 ), soil structure and texture.Data from the selectively dissolved procedures showed for all the soils generally low values of % Al o + 0.5Fe o (AI = andicity index), ranging from 0.1 to 0.7, as well as low allophane (0.9-2.6 %) and ferrihydrite (0.1-0.6 %) contents (Table 4).The low AI, associated with the low phosphate retention -Pret -(2-35 %) (Table 4), evidenced that generally these soils did not satisfy the requirements for Andosols classification (WRB, 2014), except for the P1, which falls in the Andosols just at the limit with the Cambisols.Therefore they were classified: P1 -Vitric Andosol (loamic, novic, sodic), P2 -Sodic Cambisol (loamic, tephric) and P3 -Sodic Cambisol (loamic, colluvic, novic, tephric).Moreover, the Na-pyrophosphate extractable forms (Al and Fe-humus complexes) were always absent, according with the very low Al p /Al o ratios (< 0.4), even in the most rich of OC (71.3 g kg −1 ) horizon.The calculated Fe o /Fe d ratios showed values generally decreasing with depth (from 0.7 to 0.1), which enabled to suppose a higher degree of weathering of the deeper horizons compared with the upper.

Particle size distribution (PSD)
The rock fragment (ϕ > 2 mm) distribution showed the highest pumices content in A and B horizons (Table 3).The medium size (5-20 mm) pumices ranged from few (3-4 %) to many (18-22 %) in A and B horizons, while only few (around 1 % or less) and very fine (2-5 mm) pumices were found in all 2C horizons and in 2Btb of both P1 and P3 (data not shown).Concerning the fine earth fraction, the A and most of Bw horizons were sandy loam, even the former resulted coarser compared with the latter, while few Bw, the 2BC, 2CB and 2C were silty loam (Table 3), due to the higher silt content (more than 50 %).By the analyses of the particle size distribution curves, all Introduction

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Full the horizons showed a bimodal PSD.Generally, A and Bw horizons (Fig. 2a and c) exhibited a stronger bimodal distribution of soil particles, one population having a peak at a diameter around 30-50 µm and another of 600-900 µm.Very different revealed the PSD of the 2C horizons (Fig. 2b and d), having a much less pronounced or as a whole a unimodal distribution, in which the most abundant population had a diameter around 30 µm.The PSD of the Bw 4 showed intermediate characters between the Bw series and the underlying 2C horizons.Therefore, the above data are consistent with the discontinuity (found in field) in terms of morphological and chemical properties.

Hydraulic properties
All the investigated horizons showed high water retention capacity.Values of saturated soil water content ranged from 0.56 to 0.64 cm 3 cm −3 , with a slight decrease from the surface to the bottom of the soil profile.Despite the soils did not exhibit andic properties, these high values were indicative of a complex porous system, similar to that shown by Andosols, which are well known for their high water retention capacity (Basile et al., 2007).The water retention curves and the equivalent pore size distributions of selected horizons of P2 were reported in Fig. 3a and b, correspondingly.The water retention curves of the Bw horizons showed a smoothed shape underlying a rather platicurtic equivalent pore size distribution, indicating the presence of a wide range of pore diameters (Fig. 3b).Moreover, the Bw water retention curves exhibited a slight bimodality, with a secondary porous system having an equivalent pore dimension ranging between 1 and 10 mm.This behaviour is common in many natural soils in which the presence of aggregates frequently results in a water retention curve having at least two points of inflection (Coppola, 2000).Different was the shape of the 2C retention curve, which almost overlapped to the others till around 20 cm, but sharply decreased between 100 and 1000 cm.This trend is related to a leptokurtic distribution of pore size (Fig. 3b), with the prevailing class at 10 µm.An intermediate trend between the Bw horizons and the 2C was shown by the retention curve of the 2CB.

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Full The unsaturated hydraulic conductivity curves of selected horizons of P2 were shown in Fig. 3c.In the range of pressure head where the measured curves overlap (h between −190 and −330 cm), the 2C horizon showed higher unsaturated hydraulic conductivity with respect to the upper horizons.This is consistent with the higher slope showed by the 2C water retention curve in the same explored range.In fact, the relative hydraulic conductivity at a specific value of pressure head is function of the slope of the water retention curve in the same point, more than its absolute value (Basile et al., 2006;Kutilek and Nielsen, 1994).Moreover, the values of the saturated hydraulic conductivity decreased with depth (Table 3).Indeed, the highest values were found in correspondence of the upper Bw horizons (k > 100 cm h −1 ), while they decreased one order of magnitude for Bw 4 and 2CB in P2 (k = 80.7 and 23.3 cm h −1 ) and two order of magnitude for 2C in P2 (k = 5.5 cm h −1 ), while in P3 the decrease was of one order of magnitude between Bw 4 and 2C.The different rank along the depth of the saturated and unsaturated hydraulic conductivity is not surprising because they apply to different pore ranges.

Optical microscopy
A selection of results of the micromorphological analyses of P2 were reported in Fig. 4. The surface A horizon showed a very fine granular structure, presence of living roots and blackish granules of decomposed organic residues (Fig. 4a), typical characters of forestry soils.The following Bw horizon, characterised by a granular structure, exhibited very abundant (from 25 to 50 %) and generally no weathered pumice fragments, enveloped by silty -fine sandy brown material (Fig. 4b).A clear microstructure change was observed in correspondence of the deeper 2CB horizon, characterised by an incomplete sub-angular blocky structure less porous than the above.Chaotically immersed in the soil matrix, angular to sub-angular peds (Fig. 4c 1 and c 2 ) pseudostratified with finer brownish and coarser materials were found and interpreted as fragments of the underlying horizon (2C), probably originated by the pedofauna activity.The soil matrix and some aggregates of 2CB showed clay coatings occurrence in pores and 13 Introduction

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Full The following 2C horizon showed a thin stratification in the upper 6-7 mm, which was commonly incrusted by Fe-Mn segregations (Fig. 4e).Moreover, in a root pore localised just above the described stratification and having its same direction, anisotropic clay films and frequent Fe-Mn segregations (Fig. 4g 1 and g 2 ) coated the walls.The combination of these features are likely accountable of the massive silty ash layer described in field (Table 1) and identify a low drainage soil environment, favouring waterlogging and hydromorphic conditions (possibly temporary).In the lower part, the 2C horizon became coarser due to the occurrence of a layer having a mineral single grain structure, with pores filled by clay materials (Fig. 4f 1 and f 2 ).

Microtomographic analyses
Hydrological data provided an indirect evaluation of the pore organisation through the equivalent pore size distribution; however a direct evaluation of the real pore size distribution was achieved by the image analysis.The inter-aggregate and the intraaggregate porosity were investigated using different image resolution limits, 30 and 1.5 µm, respectively (pores smaller than pixel resolution cannot be detected).These values roughly correspond to −100 and −2000 cm of pressure head, according to the capillary law.In Fig. 5b-1 the inter-aggregate PoSD (> 30 µm) of three significant P2 soil horizons (Bw 4 , 2CB, 2C) was reported.The three curves exhibited a very similar shape, asymmetric unimodal to the right.The Bw 4 was characterised by the highest porosity (36.2 %), which was calculated by integrating the area under the curve, by the highest modal value (210 µm) of pore size and the widest range of pore size variability, with the largest pores reaching a size of 2 mm; on the contrary, the 2C had the lowest porosity (only 6.7 %), the lowest modal value of pore size (150 µm) and the largest pore size reached 1 mm, even if the pore sizes most frequently found were below 550 µm (Fig. 5b-1).Measurements of inter-aggregate connected porosity were reported vs. increasing "occluded" pore size, in the three spatial directions by the percolation curves 14 Introduction

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Full (Fig. 5b-2).They represent the percentage of pore volume which connects opposite faces of the ROI after virtual occlusion of pores of increasing size.The curves of the 3 selected soil horizons exhibited different values of initial (no pore occlusion) connectivity and percolation thresholds (pt).The pt represented the maximum diameter which guarantees the connectivity in a defined space direction.Therefore, the percolation curves allowed to identify also critical sizes of pores limiting fluid and solute transport in any direction.All samples showed an isotropic initial connectivity with the highest and lowest value for Bw 4 and 2C, respectively.Results on the z axis (Fig. 5b-2) showed a pt occurring in correspondence of Bw 4 for higher (510 µm) values of pore size, compared with 2C (330 µm) and 2CB (270 µm).Since the 2C connected porosity was only 11 % at 270 µm, data of 2C and 2CB can be considered very similar.Not very different revealed the pore connectivity along the other two (x, y) directions (the horizontal plane), where the 2C horizon showed the lowest pt at 210 and 150 µm in x and y axis, respectively.Therefore, in the range of the detected pore size (> 30 µm), data seem to suggest occurrence in the soil profile of an isotropic highly permeable horizon (Bw) overlaying horizons with lower permeability (2CB and 2C).By the comparison of the porosity assessed by means of hydrological measurements and that obtained by inter-aggregate microtomographic analyses (pores larger than 30 µm), as expected the first data resulted higher for all the analysed horizons, with the largest difference found for the 2C (Table 5).The discrepancy of data is mainly due to the resolution applied and the largest difference was found for the 2CB and 2C horizons, because they were formed mainly by pore smaller than 30 µm.Therefore, in order to better investigate the porosity of the 2C horizon, due to its important role as sliding surface, data of intraaggregate porosity were successively acquired on 2C using a resolution limit of 1.5 µm (Fig. 5c-1).Results showed the occurrence of two main types of aggregates building this horizon: aggregate type I, with internal massive microstructure and aggregate type II, having a micro fibric structure; both were characterised by high porosity (43.9 and 60.7 %, respectively) made by small pores (15 and 30 µm modal class, respectively) (Fig. 5c-1).By the analysis of the pore connectivity (Fig. 5c-2), the 100 % of pores was Introduction

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Full interconnected in each direction but the percolation thresholds were very low (around 15-25 µm).The latter result, combined with the very high porosity values of small size, fully agree with the high water retention found by the hydrological analyses in correspondence of such horizon.In fact the full connectivity of the intra-aggregate pore network allows the complete water saturation of the 2C horizon, while the small size pores with low percolation thresholds require high energy to remove water from such a pore system.

The Ischia soil properties
As a whole, the results of the pedological study carried out on some detachment crowns of the Mt.Vezzi landslides show the occurrence of soils ranging from weakly to moderately developed in terms of soil structure, organic matter accumulation in the subsurface horizons, degree of weathering of the rock fragments and iron oxides formation.Despite these soils originate by volcanic parent material, they do not exhibit andic Introduction

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Full properties (WRB, 2014) but only incipient vitric properties in few horizons of the P1.As well as other volcanic soils, the Ischia soils exhibit both large -spatial (horizontal) and vertical variability between pedons, in terms of chemical and physical properties.The main abrupt variation of soil properties is identified at variable depth between 70/90 and 190 cm, where pumice bearing soil horizons (A and Bw) overly fine ashes (2CB and/or 2C) with a silt content increased by the 20 %.Accordingly, also the physical, micromorphological and mineralogical properties change.Indeed, the inter-aggregate porosity, investigated by means of the microtomographic analyses, shows significant differences between the Bw and 2C horizons in terms of: (i) total porosity larger than 30 µm (36.2 and 6.7 %, respectively), (ii) mode of pore size distribution (210 and 150 µm, respectively), (iii) pore size, which is larger in Bw (till 2 mm) than in 2C (till 1 mm) and (iv) connected porosity (percolation threshold at around 500 µm and 150-225 µm pore size, respectively).Moreover, the intra-aggregate (in the range 30-1.5 µm) connected porosity, measured only in the 2C, shows the occurrence of aggregates characterised by high total porosity (from 60.7 to 43.9 %) made by small pores (15 to 30 µm modal class) and very low percolation threshold (around 15-25 µm), which cause a lower permeability of the pore network.These data are consistent with the general decrease of saturated hydraulic conductivity (k s ) also observed with depth, where the lowest values are found in correspondence of 2C.Nevertheless, the k s value of 2C is still high and the authors believe that this horizon cannot be considered an impeding layer (i.e.horizon having a low k s value) or a waterlogging plane.By the comparison of the k s by the laboratory (in this paper) and those by the field (De Vita et al., 2007) on the same soil horizons (the Bw are named B, the 2CB is C 1 and the 2C is C 2 in De Vita et al., 2007), it was observed: (i) generally lower values of k s measured in field (1 × 10 −2 cm s −1 measured for B and 1 × 10 −5 cm s −1 for 2C) than those measured in laboratory (see Table 3), but  , 2007), the field data of 2C are two orders of magnitude lower than the laboratory (10 −5 and 10 −3 cm s −1 , for the first and latter data, respectively).Therefore, if we only look at the field data, the very low values of k s really indicate 2C as impeding layer, but this is in opposition to what reported above by the evaluation of the laboratory k s measurements.The results of the micromorphological analyses enabled to clarify this incongruity.Indeed, the occurrence of the thin (6.5 mm) slightly porous layer, made by finely stratified ashes, at the interface 2CB/2C in P2, very likely represents an obstacle to vertical and horizontal water fluxes, as testified by the hydromorphic features (e.g.Fe/Mn concretions) found on the top and within the layer.The 2CB is interpreted as the upper part of 2C reworked by the pedofauna activity.Due to the very low thickness, the authors suppose that the stratified layer was not sampled for the hydrological laboratory measurements, therefore it did not affect the results.The presence of clay coatings in the pore space of 2CB and 2C very likely increased the conditions of waterlogging created by the thin ash layer above 2C, because the swelling clay particles, expanding during hydration, also reduce the pore space for water fluxes.This process is certainly increased by the occurrence of Na, which has been found in the soil exchange complex of the deepest horizons, because this cation causes clay hydration and deflocculation, which deteriorate the soil physical properties (Basile et al., 2012).The origin of the Na in these soils is likely related to the alkaline trachytic composition of the volcanic ash.

Comparison between the Ischia and the other Campania soils affected by debris-mudflows
Despite the common landform (northern facing slope with high gradient), the similar forestry (mixed chestnut woodland -coppice), the volcanic origin of the parent material, the same movement type of the triggered landslides, the results of this study evidence significant differences in terms of peculiar properties between the Mt.Vezzi (MV) and the other Campania (CA) soils involved in debris-mudflows.Indeed, as a whole, both MV and CA are characterized by shallow surface horizons, which store the most

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Full OC content of these soils (49-71 and 28-98 g kg −1 , respectively), and thick deep horizons, which generally show lower OC content in MV than in CA (2.3-12.3 and 2.2-32.7 g kg −1 , respectively).Similar soil reaction (pH ranging between 6.0 and 7.6 in MV and 6.5-8.3 in CA) and comparable saturated water content (0.44-0.64 and 0.50-0.75cm 3 cm −3 , MV and CA, respectively) are found in both soils, along with saturated hydraulic conductivity, which is generally high and decreasing with depth.The difference between MV and CA occurs essentially in terms of andic properties: the former soils do not exhibit andic properties, while the latter are well developed Andosols.As a consequence, also the thixotropy is absent in MV while is well development in CA, consistently with the degree of development of the andic properties.The difference in andic properties is probably due to the climatic conditions of the pedogenetic environments, which strongly affect the andosolization processes (Shoji et al., 1993).In particular, the average of the yearly rainfall between the 1960 and 1990 years (data from ISPRA -SINTAI, 2005) shows that Ischia (Ischia Ponte -Pilastri station registered an average annual rainfall of 850 mm) is dryer compared with the inland areas of the Campania Region (Montevergine station registered an average annual rainfall of 1857 mm).Therefore, the occurrence of debris and mud-flows in "non-andic" soils, like those of MV, enables to give more and stronger relevance, in terms of predisposing factor to landslides occurrence, to the abrupt even microscopic spatial variability of the soil (physical) properties.

Conclusions
The results of the study carried out on Mt.Vezzi enabled to identify generally young and deep volcanic soils showing incipient vitric properties but not andic properties.Therefore, within the landslide phenomena occurred in the Campania Region, the soil properties at Mt. Vezzi do not confirm the relationship between debris/mud-flows and Andosols previously found for the Campania Region (Terribile et al., 2007).In this re-

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Full spect the climatic conditions seem to play a key role in the differentiation of soils.Nevertheless, the Mt.Vezzi and the other CA sites have in common the significant vertical variability of soil properties, besides environmental factors.Specifically, at Mt. Vezzi a significant discontinuity was identified at variable depths (70/90-190 cm) at the interface B-2C horizons, in terms of physical (i.e.different particle size distribution, pore size and connected porosity, anisotropy of hydraulic conductivity), chemical (i.e.Na enriched deep horizon) and mineralogical (i.e.swelling clays only in the deep horizons) properties.Moreover, the identification of the thin finely stratified ashy layer above 2C, by means of the optical microscopy, emphasized the role of the 2C as sliding surface.Indeed, this layer further reduces vertical and horizontal water fluxes and lead to the increase of soil water storage in the overlying soil, when heavy rainfall events occur and produce an increased instability of the slope because of the greater soil weight.
As resulting by the observations on Mt.Vezzi landscape and on the other CA, these are very fragile ecosystems due to the frequent human-induced disturbances, consisting of soil mantle cutting due to track ways and unplanned cuttings of woodlands, which could increase the water stored in the profile from about 30 to 45 % (Basile et al., 2003).Therefore, with the aim to safeguard these precious (for example in terms of biomass production) and highly vulnerable forest ecosystems, it is emphasized the must for further investigations on the spatial distribution of both (i) high soil thickness above "impeding layers" and (ii) road cut/natural steep walls, in order to identify the "highest risk areas" to landslides.

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Full  Full 1 , G. Mele 2 , R. De Mascellis 2 , F. Terribile 1 , and A. Basile 2 Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Screen / Esc Printer-friendly Version Interactive Discussion Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | likely neogenetic clays in pumice fragments (Fig. 4d 1 and d 2 ), both indicating weathering processes in place.
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ii) consistent trends between laboratory and field data, both characterised by decreasing values with depth.Despite the field measurements of k s by the literature are lower than those obtained by the laboratory, due to the different methodologies applied and the experimental procedures carried out in different environmental conditions (Basile Discussion Paper | Discussion Paper | Discussion Paper | et al.
Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Discussion Paper | Author contributions.S. Vingiani, R. De Mascellis and F. Terribile sampled the soils in the landslide detachment crowns.S. Vingiani designed the experiments and carried out the chemical and the mineralogical analyses.S. Vingiani and F. Terribile analysed soil thin sections by means of the micromorphological methodologies. A. Basile and R. De Mascellis designed and performed the physical measurements.G. Mele planned and carried out the microtomographic analyses.S. Vingiani and A. Basile prepared the manuscript with contributions from all coauthors.
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Figure 5 .Figure 6 .
Figure 1.(a) Topographic map in scale 1 : 50 000 of Ischia, (b) the Northern slope of Mt.Vezzi, in which both positions and names (by Ascione et al., 2007) of the 5 detachment crowns have been reported, (c) location of the Ischia island ahead the Golfo di Napoli (southern Italy).

Table 2 .
Soil chemical properties.Exchangeable Sodium Percentage; SAR = sodium adsorption ratio; ∆pH = calculated as difference between pH H 2 O and pH KCl.Introduction BS = base saturation calculated as bases • 100/CEC; ESP =