Pedogenesis in disturbed alpine soils (NW Italy)

Pedogenesis in disturbed alpine soils (NW Italy)

Riccardo Scalenghe a, Eleonora Bonifacio b, Luisella Celi b,*,Fiorenzo C. Ugolini c, Ermanno Zanini b

aDipartimento di Agronomia, Coltivazioni Erbacee e Pedologia, Universita di Palermo, viale delle Scienze,

I-90128 Palermo, ItalybDipartimento di Valorizzazione e Protezione delle Risorse Agroforestali, Chimica Agraria,

Universita degli Studi di Torino, 44, via Leonardo da Vinci, I-10095 Grugliasco, ItalycDipartimento di Scienza del Suolo e Nutrizione della Pianta, Universita di Firenze, 28, p.le Cascine,

I-50144 Florence, Italy

Received 17 July 2001; received in revised form 14 February 2002; accepted 17 May 2002

Abstract

In alpine environments, natural or man-induced disturbances are fairly common and

acknowledged as determining factors in pedogenesis and soil distribution. We have selected a

representative alpine valley in the Italian Northwestern Alps to evaluate the effects of perturbations

of different intensity and frequency on soil development and weathering during the last few

centuries. In order of decreasing disturbances, we selected: (i) an active avalanche shoot; (ii) a man-

built terrace; (iii) a park-like forest; and (iv) a coniferous forest. In the two most disturbed sites,

independently of the intensity and frequency of the disturbances, the active pedogenic processes are

restricted to the topsoil. At site 1, the acidity induced by the invading pioneer N2-fixing tree species

determines a certain degree of mineral dissolution, while at site 2, plowing and manuring influence

the properties of organic matter and limit its mobility. At site 3, where coniferous species are

associated to the herbaceous cover, the weathering proceeds further and a cambic horizon was

detected in the field and supported by evidences of K depletion from illite. The higher polarity of

the organic substances of mixed origin is certainly responsible for this more intense weathering. In

the coniferous forest, the least disturbed situation, the vegetation is more representative of the

climax in the area and pedogenesis further proceeds leading to the appearance of podzol-like

features. These, even if scarcely displayed by a very shallow and discontinuous E horizon, are

clearly manifested by the formation of interstratified minerals and by the amount of pedogenic iron

oxides. This can be related to the organic matter dynamics, with a high FA/HA ratio and

accumulation in the Bw horizons of acidic and highly polar aromatic molecules. However, podzols

0016-7061/02/$ - see front matter D 2002 Elsevier Science B.V. All rights reserved.

PII: S0016 -7061 (02 )00176 -3

* Corresponding author. Tel.: +39-011-6708515; fax: +39-011-4031819.

E-mail address: [email protected] (L. Celi).

www.elsevier.com/locate/geoderma

Geoderma 109 (2002) 207–224

are not common at this elevation and this steady state condition seems to be very vulnerable to

disturbances even of low intensity. In conclusion, in an alpine valley, the impact of different

disturbances on soil development, independently from their intensity and frequency, leads to a

pattern of development of soils in which the vegetation plays a key role.

D 2002 Elsevier Science B.V. All rights reserved.

Keywords: Avalanches; Anthropic disturbances; Organic matter; Podzolisation

1. Introduction

Disturbances, either natural or man-induced, are common and help to determine the

genesis and distribution of soils (e.g. Stewart et al., 1993; Little et al., 1996).

Disturbances interrupt the progression of soil development or may provide fresh

material for initiating a new pedogenic event; in alpine environments, disturbances

are not the exception but rather the rule. During the complex history of the Holocene,

disturbances to pedogenesis have been related to the intermittent periods of glacial

advance and retreats (Karlstrom and Osborn, 1992). Today, on the valley bottoms,

natural perturbations are caused mainly by catastrophic events, such as avalanches and

landslides. In the valleys of the Northwestern Alps, a lower impact is that of the human

perturbances caused by traditional agriculture, logging, grazing and recreational

activities.

Under undisturbed conditions in the cold-wet dominant climate of an alpine degla-

ciated valley, a continuum of soils derived from the interaction among parent material,

vegetation and time is expected (Legros, 1992). However, this continuity may be

interrupted abruptly by disturbances of high intensity that trigger vegetation changes

and disrupt soil evolution or by lower intensity perturbations that influence the dynamics

of organic matter (Richter et al., 1989). The result of these events brings about a mosaic

of environments that appear to have an element of predictability in the valleys of the

Italian side of the Northwestern Alps. We have selected one of these valleys, the Lys

Valley, to examine the different environments created by a range of perturbations and the

attendant evolutionary recovery.

The Lys Valley, a longitudinal south-facing area, typifies the physical and social

environment of the Northwestern Italian Alps and can serve as a pattern for these regions.

It has frequent spring snow avalanches (Capello, 1962; CSMA, 1966; Simonetta and

Negro, 1972) and a long history of human settlements and, therefore, of anthropic impacts

(Grotzbach and Stadel, 1997). More recently, winter sports with modern installations have

been present since the middle of the 20th century.

This work deals with an alpine valley whose upper reaches is still occupied by a

receding glacier, and where the slopes and valley bottom are affected by disturbances of

different levels of intensity and frequency.

The aim of the study is to describe and characterise benchmark sites expressing either

catastrophic events or anthropic disturbances and to relate soil properties to the frequency

and intensity of the disturbances.

R. Scalenghe et al. / Geoderma 109 (2002) 207–224208

2. Materials and methods

2.1. Study area

The Lys Valley (45j50VN and 7j49VE) lies in the Northwestern Alps ranging in

elevation from 342 to 4673 m a.s.l. The climate of the upper reaches of the valley is

strongly influenced by the presence of Mount Rose Glacier. Historically, after deglacia-

tion, the temperature increased to reach a maximum 5000 years ago, then decreased and,

once became relatively stable, it determined the present vegetational zones. At 1850 m

a.s.l., current mean annual air temperature is 4.0jC and annual precipitation is 1069 mm.

Annual precipitation (including water equivalent snow) increases with altitude being 736

mm at 342 m a.s.l. and 3000 mm at 3200 m a.s.l. Mean annual snowfall is 6.5 m and

represents up to 50% of total precipitation: snowfall days per year are 30, while the snow

cover extent is 180 days year � 1. Below 3300 m a.s.l., there is no permafrost but the soils

are subjected to considerable fluctuations in daily temperature across the freezing point.

In the upper reaches of the valley, the vegetation consists of hygrophiles pioneer tree

species such as the alder (Alnus incana and Alnus viridis) growing on the highly eroded

small alluvial fans as well as on the coarse grained young colluvial debris. On the

footslopes and toeslopes, where erosional events are less intense, man has developed

agriculture and built houses. As agricultural crops are not any longer profitable, the

terraces are now occupied by meadow, normally grazed and manured. Meadows are

normally associated with larch (Larix decidua) on shoulder and footslope positions, where

these trees provide a certain degree of slope stabilisation. The climax vegetation consists

of Norway spruce and Silver fir (Picea excelsa and Abies alba) colonising mid slope

positions up to about 1700 m a.s.l. Stone pine (Pinus cembra) forests occur at higher

elevations, often associated with larch. The timberline delineated by Stone pine is at

approximately 2300 m a.s.l. Modern recreational activities, such as winter sports, have led

to the logging of fir-spruce forests to open ski trails, and to the substitution of meadow for

forests at mid slope positions.

Four benchmark sites (Fig. 1), where perturbations range from low frequency and high

intensity to high frequency and low intensity, were selected in two different areas of the

valley, as representative by the following.

(1) Active avalanche shoot (E–NE, 1480 m a.s.l., slope 20%), located at the footslope

position, where frequently more than six avalanches tracks per kilometer occur. The last

high intensity event occurred approximately 20 years ago, as revealed by tree ring

analysis. The vegetation is alder (A. viridis and A. incana) and Eurasian maple (Acer

pseudoplatanus). The soils developed from clasts of mixed lithology with gneiss

prevailing.

(2) Man-built terraces (E, 1540 m a.s.l., slope 15%), located on the colluvial footslope,

but higher in the valley; they are grazed and manured, but not disturbed by avalanches.

The vegetation are Poa alpina, Festuca ovina, Phleum alpinum, Luzula lutea, Anthox-

anthum odoratum and Geranium sylvaticum. The lithology consists mainly of calcschists,

serpentinites, serpentinoschists and prasinites.

(3) Park-like forest (E–SE, 1560 m a.s.l., slope 50%), located on the shoulder position,

not disturbed by avalanches. The site is presently utilised for grazing and wood collection.

R. Scalenghe et al. / Geoderma 109 (2002) 207–224 209

The forest is represented by L. decidua, the herbaceous cover is dominated by F. ovina.

The lithology is the same as described for site 2.

(4) Coniferous forest (NE, 1600 m a.s.l., slope 60%), located on the backslope,

occasionally disturbed by falling blocks. The tree cover is made of A. alba and P. excelsa,

while the shrub and herb layer is dominated by Vaccinium myrtillus, Politrichum

formosum and Luzula sylvatica. Gneiss is the dominant rock at this site.

Fig. 1. The study area.


Table 1

Morphology of the pedons

Soil Horizon Depth Munsell color Structurea Consistenceb Rootsc Skeletond Boundarye

(cm)Dry Moist

1 Oi 10–2 5YR 3/1 5YR 2/1 2, f, gr fri cvf, ffi f, an s, c

Oe 2–0 5YR 3/1 5YR 3/1 2, f, gr fri ffi f, an s, c

A1 0–10 10YR 4/2 7.5YR 3/2 2f, 2gr fri ffi c, an s, c

A2 10–20 10YR 4/3 10YR 3/3 2m, abk fri afi c, an,

san

w, c

AC 20–50 10YR 6/3 2.5Y 4/4 vw, m,

abk

l, ns, np ffi, fm la, an w, d

C1 50–70 10YR 6/3 2.5Y 5/3 vw, m,

abk

l, ns, np a a, an w, a

C2 70–120 + 10YR 6/2 2.5Y 4/4 vw, abk,

sg

l, ns, np a a, an b

2 Oe 1–0 7.5YR 5/2 10YR 3/2 vf, f, gr fri avf, fco f, an s, c

A 0–15 7.5YR 5/1 10YR 3/2 1, m, gr ss, sp af, cco f, san w, c

AC 15–40 2.5YR 4/1 2.5YR 3/1 1, m/c,

san

l afi, cco c, san w, c

C1 40–75 10YR 5/1 10YR 3/2 w, m, an l vf, fi c, san w, c

C2 75–120 + 2.5YR 5/1 2.5YR 3/1 vw, abk,

sg

l, ns, np a a, an b

3 Oi 3/1–0 7.5YR 5/2 10YR 3/2 2, vf, gr fri avf, fco f, an s, a

Oe 1–0 7.5YR 5/1 10YR 3/1 2, vf, gr fri avf, fco f, an s, a

A 0–7 10YR 3/1 2.5Y 3/1 w, f, gr fri, ss/sp avf, cco f, an w, c

Bw 7–40 10YR 3/1 2.5Y 3/2 1, m/c,

sbk

fri, ss/sp c, fi f, me w, a

C 40–80 + 10YR 5/1 2.5Y 3/2 vw, abk,

sg

l, ns, np a a, an b

4 Oi 8–4 5YR 2/1 5YR 2/1 f, gr fri avf, fco a s, c

Oe 4–2/0.5 5YR 2/1 5YR 2/1 f, gr fri cme, fco a w, c

Oa 2/0.5–0 5YR 5/2 7.5YR 3/2 m, gr fri mvf, cco f, an w, a

E 0/2–3 10YR 7/4 10YR 6/6 2vf, fabk fri, np, ns fvf bsg w, a

Bw1 0/3–30 10YR 5/5 10YR 4/4 w, m,

sbk

fri, ss, sp fvf Bsg w, s

Bw2 30–50 7.5YR 5/4 10YR 4/4 w, m,

abk

fri, ss, sp c, mco c, co,

an

w, s

Bw3 50–90 7.5YR 6/4 10YR 4/4 w/vw,

m, sbk

fri, ss, sp ffi va, co,

an

w, s

BC 90–120 10YR 5/4 10YR 4/6 w/vw,

m, sbk

fri, ss, sp a va, co,

an

w, c

C 120–140 + 7.5YR 6/4 10YR 4/6 vw, abk,

sg

l, ns, np a va, m/la,

an

b

a STRUCTURE 1= strong, 2 =moderate; w =weak, vw= very weak; m=medium, f = fine, vf = very fine,

sg = single grain; gr = granular, abk = angular blocky, sbk = subangular blocky.b CONSISTENCE l = loose, fri = friable, fm = firm, s = sticky, p = plastic, ss = slightly sticky, sp = slightly

plastic, np = nonplastic, ns = nonsticky.c ROOTS a = absent, f = few, c = common, m=many; vf = very fine, fi = fine, me =medium, co = coarse.d SKELETON a = absent, f = few, c = common, ab = abundant; vb = very abundant, fi = fine, me =medium,

la = large; an = angular, san = subangular, sr = subrounded, bsg = bleached sand grains.e BOUNDARY a= abrupt, s = smooth, g = gradual; w =wave, c = clear, d = diffuse, b = bottom.


The soils were described (Table 1) (Soil Survey Staff, 1951), sampled and classified

according to the USDA Soil Taxonomy (Soil Survey Staff, 1998).

2.2. Chemical and mineralogical analyses

All analyses were carried out on the fine earth fraction ( < 2 mm). Soil pH was

measured potentiometrically in H2O (1:2.5 w/v). When present, carbonates were detected

volumetrically (Ministero delle Politiche Agricole e Forestali, 2000). Particle-size analysis

was performed by the pipette method after dispersion with Na–hexametaphosphate.

Organic carbon (Org C) was determined by the Walkley–Black technique and total N

by standard Kjeldahl method (Bremner and Mulvaney, 1982). The cation exchange

capacity (CEC) and the exchangeable cations were determined in a BaCl2 solution

buffered at pH 8.1 (Rhoades, 1982). Available P (Pols) was extracted according to Olsen

et al. (1954) and determined colorimetrically by the ascorbic acid molybdate blue method

(Murphy and Riley, 1962).

Total iron (Fet) was determined after HCl–HNO3 digestion. The Fe extracted by the

DCB method (Fed) (Mehra and Jackson, 1960) was taken as representative of pedogenic

iron oxides, while the amorphous oxides were estimated from the extraction of Fe with an

ammonium oxalate solution buffered at pH 3 (Feo) (Schwertmann, 1964). In addition, Fe

was also extracted by 0.1 M Na–pyrophosphate (Fep). Aluminium extracted with the

oxalate solution (Alo) was also evaluated. Both elements were determined by atomic

absorption spectroscopy.

Humic acids (HA) and fulvic acids (FA) were extracted from the fine earth fraction and

purified as described by Schnitzer (1982). HA and FA were then freeze-dried and their

elemental composition (C, H and N) was determined by a CHN LECO 600 combustion

analyzer; the content of oxygen was calculated by difference.

The clay fraction ( < 2 Am) was separated by sedimentation and freeze-dried for the

mineralogical analyses. The X-ray diffraction (XRD) analyses were carried out using a

Philips PW 1710 diffractometer (40 kV and 20 mA, Fe-filtered Co–Ka radiation) on air-

dried (AD), ethylene glycol-solvated (EG) and heated (773 K) oriented mounts. Scans

were made from 3 to 35j2h at a speed of 1.5j2h min� 1. The amounts of phyllosilicates in

the oriented clay samples were estimated from the XRD peak height without any

correction factor.

3. Results and discussion

3.1. Site 1: active avalanche shoot

At site 1 (Fig. 1), the ‘‘spring-bottom avalanche’’ includes a 700-m-long sliding and a

500-m-long accumulation zone. The soil, located within the accumulation zone, is scarcely

developed and shows only an O–A–C horizon sequence (sandy, paramicaceous, acid,

mesic Typic Udorthent). The chemical data (Table 2) indicate an irregular distribution of

organic C, CEC and available P with a relative increase in the C2 horizon, suggesting the

burial of a preexisting soil surface by a layer of 50 cm of sediments. In the present scarcely


Table 2

Selected physical and chemical soil properties and clay mineral species

Soil Horizon Clay mineralsa pH CaCO3 Particle size distribution Org C C/N Exchangeable cations BS (%) Polsb

(H2O) (g kg� 1)Cc

(%)

FSic

(%)

CSic

(%)

FSac

(%)

CSac

(%)

(g kg� 1) CEC

(cmolc +

kg � 1)

Ca

(cmolc +

kg� 1)

Mg

(cmolc +

kg� 1)

K

(cmolc +

kg� 1)

(mg kg� 1)

1 A1 I–C–ML 4.4 nd 2 7 12 24 55 103 14 26 4.0 1.2 0.6 22 60

A2 I–C–ML 4.1 nd 2 8 13 29 48 47 12 14 1.5 0.4 0.2 14 40

AC I–C–ML 4.1 nd 2 11 12 22 53 8 9 3 0.3 0.2 0.1 15 15

C1 I–C–ML 4.7 nd 3 8 8 22 59 2 8 1 0.1 0.1 0.1 28 3

C2 I–C–ML 4.5 nd 2 8 11 29 50 6 10 3 0.4 0.2 0.1 22 8

2 A C–I–Sr–T 6.7 0 2 8 13 28 49 32 10 13 11.6 1.1 0.1 100 7

AC C–I–Sr–T 7.7 20 1 11 13 31 44 12 8 10 9.8 0.3 0.1 100 3

C1 C–I–Sr–T 7.9 20 2 11 15 31 41 9 8 10 10.1 0.3 0.0 100 2

C2 C–I–Sr–T 7.8 20 2 13 15 25 45 9 8 8 7.5 0.2 0.0 100 2

3 A C–I–Sr–T 6.5 0 3 13 18 25 41 63 12 24 20.5 1.9 0.2 93 17

Bw C–I–Sr–T 7.1 0 2 10 14 22 52 27 9 19 17.0 1.8 0.1 100 7

C C–I–Sr–T 8.1 20 2 11 14 21 52 4 7 5 5.1 0.1 0.0 100 1

4 E C–I–ML–Sr–T–A 4.8 nd 3 14 15 20 48 95 19 10 1.1 0.2 0.1 14 7

Bw1 C–I–ML–Sr–T–A 4.9 nd 5 12 17 19 47 17 11 12 1.4 0.3 0.1 15 10

Bw2 C–ML–I–Sr–T–A 4.9 nd 5 14 14 27 40 9 11 7 0.6 0.2 0.1 12 6

Bw3 C–I–ML–Sr–T–A 5.1 nd 2 9 13 31 45 7 10 5 0.5 0.1 0.1 13 6

BC C–I–ML–Sr–T–A 5.4 nd 2 8 15 27 48 8 11 7 0.7 0.2 0.1 13 6

C C–I–ML–Sr–T–A 5.6 nd 2 6 14 28 50 3 14 5 0.5 0.1 0.1 12 5

a Clay minerals listed in order of importance; C = chlorite, I = illite, Sr = serpentine, ML=mixed layers phyllosilicates, T = talc, A= amphiboles.b Available phosphorus = Pols.c C = clay, FSi = fine silt, CSi = coarse silt, FSa = fine sand, CSa = coarse sand.

R.Scalen

gheet

al./Geoderm

a109(2002)207–224

213

developed soil, the pH is low and increases slightly with depth; the acidity is probably

enhanced by the presence of the alder, its symbionts and the attendant process of

nitrification. There is little variation in particle size distribution throughout the profile,

and CEC values show a sharp decrease below the A horizons reflecting the organic carbon

distribution. The Fed/Fet ratio indicates that a maximum of 20% of total iron is in the

pedogenic forms. The Feo/Fed ratio shows that the oxides are mainly amorphous (Table 3).

Humification occurs in the Oe and A horizons, as shown by the elemental analysis and

distribution of HA and FA (Table 4). The C/N ratio in the Oi horizon is clearly influenced

by the N2-fixing species. In the HA, the C/N values range from 14.2 in Oi to 12.7 in A2

while the H/C values are higher than unity (Table 5), indicating a good incorporation of N

and a considerable aliphaticity, respectively. With the exception of the A2 horizon, O/C

ratios decrease along the profile probably due to the loss of the carbohydrate residues. As

suggested by Zech and Guggenberger (1996), the expected O-carboxyl increase is

probably masked by the high content of O-cellulosic residues. The FA shows higher C/

N and lower H/C ratios than HA (Table 6). In the A2 horizon, the FA is characterised by a

particularly high degree of aromaticity and low N incorporation, supporting the hypothesis

of a limited mobility of the humified organic matter along the profile. Moreover, the

anomalous characteristics of HA and FA in the A2 horizon can be due also to the

Table 3

Iron and aluminium forms: selective extraction data

Soil Horizon Fet(g kg� 1)

Fed(g kg� 1)

Feo(g kg� 1)

Fep(g kg� 1)

Alo(g kg� 1)

Fed/Fet Feo/Fed

1 Oi n.d. 5.3 2.7 3.0 1.0 n.d. 0.51

Oe n.d. 4.2 2.3 1.0 0.7 n.d. 0.55

A1 20.8 5.0 2.4 2.0 0.7 0.24 0.48

A2 23.3 5.4 2.8 2.0 0.7 0.23 0.52

AC 27.4 4.0 2.2 1.3 0.6 0.15 0.54

C1 26.8 3.3 1.1 0.5 0.4 0.12 0.32

C2 27.7 4.0 1.6 0.8 0.5 0.14 0.40

2 Oe n.d. 9.0 3.7 1.5 0.9 n.d. 0.41

A 35.9 10.7 4.5 1.7 0.9 0.30 0.42

AC 39.1 10.2 3.7 0.7 0.7 0.26 0.36

C1 39.2 11.0 3.7 0.9 0.7 0.28 0.33

C2 38.6 12.9 5.5 1.1 0.8 0.33 0.43

3 Oi n.d. 7.3 2.4 0.8 0.4 n.d. 0.32

Oe n.d. 7.8 2.6 1.1 0.3 n.d. 0.33

A 37.8 16.6 8.8 3.4 0.8 0.44 0.53

Bw 38.0 17.3 8.7 3.4 1.4 0.46 0.50

C 41.8 12.3 4.5 0.8 0.5 0.29 0.37

4 Oi n.d. 4.0 1.8 0.7 0.6 n.d. 0.45

Oe n.d. 7.7 3.3 2.3 0.8 n.d. 0.42

Oa n.d. 9.6 4.3 3.4 1.3 n.d. 0.45

E 27.0 11.0 4.3 4.4 1.7 0.41 0.39

Bw1 27.2 11.2 5.1 4.7 1.7 0.41 0.46

Bw2 28.0 11.0 4.6 3.3 2.2 0.39 0.41

Bw3 27.2 9.6 4.9 3.4 3.1 0.35 0.51

BC 26.7 8.5 2.4 1.3 2.2 0.32 0.28

C 30.1 9.8 4.2 3.9 1.5 0.33 0.43


contribution of the removal of components through the migration of particulate organic

matter and/or even raw residues to the deeper horizons (Kogel-Knabner, 1993).

The main phyllosilicates in this profile are illite and chlorite (Table 2). The chlorite is

relatively Fe-rich as deduced from the higher intensity of the even order reflections (data

not shown). Its estimated content decreases from top to bottom: from 35% in the A1 to

25% in the C1 horizon. An opposite trend is found for illite (from 50%, A1, to 70%, C1).

Appreciable amounts of interlayered phyllosilicates are also found in the upper two

horizons. The mineralogy of phyllosilicates in the ‘‘supposed buried’’ C2 horizon does not

show any difference when compared to the overlaying C1.

Table 4

Carbon and nitrogen contents of the four profiles and distribution of the two elements in the HA and FA

Soil Horizon Org C

(g kg� 1)

Ntot

(g kg� 1)

C/N FA/HA CHA/Ctot

(%)

NHA/Ntot

(%)

CFA/Ctot

(%)

NFA/Ntot

(%)

1 Oi 245 18.8 13 0.19 9.5 4.9 1.8 2.6

Oe 94 7.2 13 0.25 30.0 18.5 7.4 1.7

A1 103 7.4 14 0.46 12.0 8.6 5.5 1.6

A2 47 3.9 12 0.20 18.3 10.6 3.7 0.2

2 Oe 150 11.5 13 0.30 14.0 8.5 4.2 2.6

A 32 3.2 10 1.23 7.7 4.8 9.5 0.5

3 Oi 145 7.3 20 0.41 10.0 9.2 4.1 1.3

Oe 167 9.3 18 0.53 9.6 7.2 5.1 2.3

A 63 5.3 12 0.55 11.0 6.5 6.0 0.7

4 Oi 299 9.6 31 0.28 6.0 6.7 1.7 1.3

Oe 230 10.0 23 0.06 20.0 15.4 1.7 0.7

Oa 184 9.2 20 0.25 13.0 9.9 3.2 1.0

E 95 5.0 19 0.13 33.0 25.8 4.2 0.4

Bw1 17 1.5 11 0.94 23.5 12.4 22.0 0.1

Bw2 9 0.8 11 1.00 11.5 6.2 11.5 0.01

Table 5

Elemental composition (%) and atomic ratios of HA

Soil Horizon C H N O C/N H/C O/C

1 Oi 47.41 5.44 3.97 42.80 14.22 1.38 0.68

Oe 52.06 5.67 4.75 37.01 13.23 1.31 0.53

A1 53.77 5.04 5.11 35.29 12.29 1.12 0.49

A2 52.88 5.12 4.85 37.15 12.71 1.16 0.53

2 Oe 50.28 5.75 4.66 38.77 12.95 1.37 0.58

A 55.97 4.60 6.20 32.06 10.53 0.99 0.43

3 Oi 52.89 5.97 4.60 36.12 13.74 1.36 0.51

Oe 52.89 5.35 4.18 37.12 16.03 1.21 0.53

A 53.00 4.81 4.95 36.34 12.49 1.09 0.51

4 Oi 55.19 6.41 3.59 34.56 18.66 1.39 0.47

Oe 51.63 5.60 3.35 39.14 18.42 1.30 0.57

Oa 52.20 5.64 3.79 38.10 16.87 1.30 0.55

E 51.33 5.82 4.11 38.43 15.39 1.36 0.56

Bw1 55.19 5.03 4.79 34.32 13.45 1.09 0.47

Bw2 55.08 4.42 4.90 35.61 13.12 0.96 0.48


From these results, it appears that during the short interval of time since the last

avalanche, the genesis of this profile has been mainly influenced by the invasion of

pioneer tree species. The presence of humic materials and the acidity induced by N2-fixing

species cause the horizons at the surface to be more weathered than the deepest ones. In

fact, these factors may be responsible for the weathering of primary chlorite and for the

more pronounced Fe released from silicates in the A horizons. The iron-rich chlorites are

in fact reported to weather easily in acid environment (Ross, 1969), acting as source of

iron for the formation of oxides (Bain, 1977). The presence of humic substances, together

with the short time of soil development, favours the formation of amorphous Fe oxides

(Schwertmann, 1985). Moreover, the rate of mineralisation and the prevalence of humic

over fulvic acids limit the downward leaching of iron (Table 4).

The presence of interstratified minerals of the illite-vermiculite type, more abundant in

the surface horizons, suggests the transformation of illite through the loss of interlayer K, a

process which is well known to be enhanced by soil acidity (Norrish, 1973) and roots

(Mortland et al., 1956). Hence, although the last high intensity perturbation has destroyed

the existing soils, the colonisation by acidifying pioneer tree species has allowed to initiate

a certain degree of development, even if limited to soil surface horizons. The similarities in

the phyllosilicates of C1 and C2 horizons might indicate that the C2 horizon had been

present at the soil surface for a short time before its burial by the last catastrophic event,

not long enough to allow the colonisation of the surface by the alder.

3.2. Site 2: man-built terraces

At site 2 (Fig. 1), soil developed on downwasted material that has covered a preexisting

surface, including anthropic features. In fact, on the left side of the trench, we observed a

buried accumulation of poorly sorted fragmental clasts (the voids are empty) derived from

a former human stone-removal operation by Walser population. The collection of clasts

may have occurred during the 14th century when the valley was colonised. According to

Table 6

Elemental composition (%) and atomic ratios of FA

Soil Horizon C H N O C/N H/C O/C

1 Oi 50.01 4.31 3.18 41.81 20.33 1.04 0.63

Oe 50.80 4.15 3.41 41.19 19.96 0.98 0.61

A1 50.36 4.45 3.88 40.74 15.14 1.06 0.61

A2 52.18 2.98 2.47 42.37 24.62 0.68 0.61

2 Oe 50.41 4.65 3.56 40.81 19.22 1.11 0.61

A 50.63 4.36 4.71 39.57 12.53 1.03 0.59

3 Oi 51.04 4.37 2.95 41.26 23.08 1.03 0.61

Oe 50.56 4.29 2.89 41.87 23.69 1.02 0.62

A 50.83 4.25 3.47 40.90 17.06 1.00 0.60

4 Oi 53.15 4.61 2.56 39.40 26.84 1.04 0.56

Oe 51.38 4.12 1.87 42.30 35.06 0.96 0.62

Oa 49.49 3.95 1.84 44.54 34.57 0.96 0.68

E 50.25 4.10 1.98 43.47 32.94 0.98 0.65

Bw1 49.37 3.66 1.32 45.30 43.70 0.89 0.69

Bw2 43.69 2.47 1.50 52.34 33.92 0.68 0.90


this interpretation, the landslide that buried the rock mound could not be more than 600

years old, while the colluvial surfaces have been cultivated with traditional agriculture

until the present. Plowing and manuring are yearly performed. The soil (sandy, magnesic,

nonacid, frigid Typic Cryorthent) shows an A horizon followed by an AC transition

horizon. Below there are two C horizons enriched in coarse fragments. The pH increases

with depth and values below 7 were observed only in the surface horizon (Table 2).

Carbonates are present in the fine earth fraction in all horizons except in the A. The clay

fraction is at low level. The CEC is only slightly variable through the profile, while the

organic C decreases below the topsoil. The presence in the A horizon of easily degradable

organic matter, such as roots and rootlets of grasses, might be inferred from the

percentages of HA and FA (Table 4). Furthermore, the HA elemental composition (Table

5) shows low C/N, H/C and O/C ratios due, probably, to rapid mineralisation of degradable

compounds with associated accumulation of the more recalcitrant aromatic moieties. The

high incorporation of N can be influenced also by manure application associated to a

consistent biological activity.

The proportion of pedogenic iron oxides (Table 3), indicated by the Fed/Fet ratio, is

homogeneous throughout the profile. This homogeneity is also shown by the minera-

logical analyses that indicate little changes in the phyllosilicate types or in their relative

amounts (data not shown). Chlorite prevails in all horizons; illite and serpentine are also

present and approximately in the same amounts. Traces of talc are also found, hence

providing the only evidence of mineral weathering. In fact, the ratio between the height of

the peaks of talc and illite decreases from the bottom to the top of the profile (57 in C

horizons and 36 in the A one).

Since the building of the terrace, the major pedogenic processes are restricted to the

surface horizon, as manifested by the decrease in pH and the consequent removal of

carbonates and weathering of Mg-bearing minerals. Man-induced disturbances, connected

to land use, even though of low intensity, have limited the development of pronounced

morphological differentiation. Cultivation and manuring favoured the presence of easily

decomposable organic matter with low acidity. Moreover, the incorporation of N into the

humic materials may further decrease their complexing ability (Stevenson, 1994), limiting,

therefore, their effect on the mineral surface horizon.

3.3. Site 3: park-like forest

At site 3 (Fig. 1), the soil has developed on a former landslide material that includes

large boulders. The steepness and the surficial pietrosity have conditioned its use:

presently, a park-like association of larch and meadow provides wood for cutting and

herbage for grazing. The steep slope has determined the scarce depth of this soil that,

nevertheless, shows an A–B–C profile (sandy, magnesic, nonacid, frigid Mollic Eutroc-

ryept).

The pH increases with depth and carbonates were found only in the fine earth fraction

of the C horizon. However, boulders in the field and cobbles in all horizons contain

carbonates. As found by others, the relative low rainfall on the Italian side of the Alps

prevents the leaching of the carbonates from the coarse fraction (Legros, 1992). In this

profile, the organic C (Table 2) shows the influence of the arboreal species, with higher


values either in the A1 and Bw horizons compared to the profile under herbaceous species

(site 2). The association of herbaceous and arboreal species at the same site results in a

higher C/N ratio, particularly in the organic horizons, demonstrating the accumulation of C

in the litter due to a low degradability of lignin-rich woody debris. This is also evident by

comparing the HA and FA contents (Table 4). Moreover, HA and FA show high C/N ratios

and H/C close to unity, indicating that material only partially degraded.

The fractionation of iron indicates that more than 40% of total Fe is in the pedogenic

oxides in the A and Bw horizons and that less than half of them are amorphous. The

mineralogy of the clay fraction of this profile, consisting of chlorite, illite and serpentine,

is very similar to that of the soil described at site 2. A trend towards a more weathered

stage could, however, be observed in the illite of the Bw horizon, as manifested by some

degree of asymmetry in the air-dried sample, indicating a loss of K from the interlayer.

This depletion is confirmed by the collapse of the peak when heated to 500jC (Fig. 2).

Fig. 2. XRD patterns of the clay fractions of Profile 3.


In the disturbance scale (Fig. 1), this site is relatively stable; this fact, in addition to the

presence of a more recalcitrant humic material and its low N content, is probably

responsible for mineral weathering not limited to the surface horizon.

3.4. Site 4: coniferous forest

At site 4 (Fig. 1), soil has developed on a stabilised slope wash covered by large blocks;

it shows an O–E–B–C sequence of horizons (sandy, micaceous, acid, mesic Spodic

Dystrudept). The E horizon is, however, very shallow and discontinuous; in some cases, it

can be visualised only by the presence of a ‘‘salt and pepper’’ effect.

Soil pH is low and increases downward the profile, while organic C decreases

abruptly below the E horizon. In the E, Bw1 and Bw2 horizons (Table 4), a higher

proportion of organic C is extractable as HA and FA. The FA/HA ratio increases through

the profile (Table 4), indicating an incipient podzolisation (Andreux, 1996). Moreover,

the extraction procedure of FA and HA shows that the organic matter is strongly bound

to the mineral fraction and the removal of it was more difficult in the Bw1 than in Bw2

and E horizons. This, together with the accumulation of FA in the Bw1 horizon, further

supports the on-going podzolisation process. HA show a more condensed and aliphatic

structure compared to FA throughout the profile, as deduced by the O/C and H/C values

(Tables 5 and 6). Conversely, FA have high C/N and O/C and low H/C ratios, indicating

a high accumulation of acidic aromatic rich molecules with high polarity and, therefore,

great mobility and high ability to interact with Fe and Al (De Coninck, 1980). The

surprisingly high O/C and low H/C values registered in the Bw2 horizon of this profile

are probably due to a selective translocation of the more polar fraction of FA below the

Bw1 horizon.

The iron fractionation (Table 3) indicates that approximately 40% of total iron is in the

form of pedogenic iron oxides and that half of these are amorphous. In the Bw1 horizon, a

relative maximum of organic-complexed Fe (pyrophosphate-extractable Fe) was found.

Appreciable amounts of interstratified minerals were observed in all horizons, together

with chlorite, illite and serpentine. The interstratified minerals show a higher crystallinity

in the upper horizons of the profile, as evidenced by the different behaviour of the peaks in

the 1.0-nm zone after heating to 773 K (Fig. 3). The interstratified minerals in the E

horizon show the complete collapse to 1.0 nm when heated, a d-spacing of about 1.1 when

air-dried and some swelling behaviour upon EG solvation (Fig. 4), indicating the presence

of some low-charge layers in an illite-vermiculite-type structure. The Bw1 and Bw2

horizons show instead a peak at 1.02 nm following heating, a peak at 1.2 nm when air-

dried and no swelling behaviour upon solvation. These evidences indicate a mixed layer

mineral of illite-vermiculite-type with some degree of Al hydroxy interlayering or of

chlorite-type layers.

The presence of swelling minerals in the E horizon again confirms the tendency

towards podzolisation. In fact, in the eluvial horizons of spodosols, smectites are often

found (e.g. Zabowski and Ugolini, 1992; Wang et al., 1989). These minerals derive from

preexisting micaceous materials (Righi et al., 1999) or from chlorites through the

depletion of Mg in interlayer, followed by the migration of structural Al (Carnicelli et

al., 1997).


This soil has experienced the least disturbances compared to the others, showing not

only an evident development of an inceptisol, but also by an evolution towards podzol

state occurring at an elevation where podzols are seldom found in the area, i.e. below 1700

m a.s.l. This condition can be related to the lack of major disturbances that allow the

accumulation and transformation of the conifer litter. The stability of this site favours the

distribution along the profile of a very stable organic matter with a high percentage of FA

accumulated in the Bw1 horizon (Table 4). However, the shallowness of the E horizon

makes this soil particularly vulnerable to minor disturbances: for example, the blowdown

Fig. 3. XRD patterns of the clay fractions of Profile 4 after heating to 773 K.


of trees (e.g. Brown, 1977). The human impacts due to logging for building ski trails can

also rapidly disrupt this pedoenvironment by enhancing the disappearance of E horizons,

as described by Ugolini (1982).

4. Conclusions

In the Lys Valley, a representative of high alpine valley of the Northwestern Italian

Alps, we have invoked the concept of natural and human perturbations to explain the

pedogenesis of soils developed on a sequence of disturbed sites (Fig. 1).

Fig. 4. XRD patterns of the clay fractions of E and Bw1 horizons Profile 4.


The starting member is the more disturbed situation where the avalanche event and the

consequent invasion of pioneer trees affect the composition of organic matter and,

therefore, the chemical and mineralogical soil characteristics. Thanks to the effectiveness

of the alder (Ugolini, 1968), the pedogenesis can proceed quickly as shown by the changes

that occur at least in the more surficial horizons in a time span of about 20 years. The

events of lower intensity, but higher frequency, as the cultivation of the land, seem to have

similar effects. In fact, in the second member of the disturbance sequence, the presence of

the herbaceous species, with easily decomposable organic matter, affects the profile

development keeping it in juvenile stage even if it is located in a rather stable geo-

morphologic position. In the third member, park-like forest, the larch associated to

herbaceous cover has increased the activity of the organic matter, which appears more

polar and more recalcitrant to degradation than under meadows. This has induced the

weathering of the clay minerals and the dissolution of carbonates from the fine earth

fraction. In the fourth member, there is a vegetation association that favours the

podzolisation process and, in fact, under undisturbed conditions, the soil tends to reach

a podzol-like state. In this case, the steady state is confirmed both by chemical and

mineralogical characteristics and widespread evidences of a shallow E horizon, which can

be easily destroyed even by low intensity disturbances.

The trend of these soils, from the most disturbed to the undisturbed, can be followed by

the Fed/Fet ratio that increases from site 1 to site 4. This clear trend can be also recognized

for clay minerals. The most disturbed situation (site 1) shows weathering only of the most

unstable minerals; at sites 2 and 3, the weathering proceeds through the depletion of K

from illite and little formation of mixed layers, while true transformations of the clay phase

are only visible in the undisturbed situation at site 4. The pedogenic processes are mainly

caused by the organic matter transformation. In soils with an easily degradable organic

matter, the effects are limited to the upper horizons, while the material under larch and

spruce, more recalcitrant and polar, manifests its effect also at depth.

In conclusion, in an alpine valley, the impact of different disturbances on soil

development, independently from their intensity and frequency, leads to a pattern of

development of soils in which the vegetation plays a key role.

Acknowledgements

We are grateful to Dr. Luca Mercalli (Societa Meteorologica Italiana) for his support on

glaciological and meteorological aspects and Dr. Stefania Santoni (Universita di Torino)

for drawing the soil profiles.

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Pedogenesis in disturbed alpine soils (NW Italy)

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