The contrasting effect of broom and pine on pedogenic processes in volcanic soils (Mt. Etna, Italy

Ž .Geoderma 102 2001 239–254www.elsevier.nlrlocatergeoderma

The contrasting effect of broom and pineon pedogenic processes in volcanic soils

ž /Mt. Etna, Italy

Giacomo Certinia,), Marıa J. Fernandez Sanjurjob,´ ´Giuseppe Cortic, Fiorenzo C. Ugolinic

a Istituto per la Genesi e l’Ecologia del Suolo, CNR, Piazzale delle Cascine 28,50144 Florence, Italy

b Departamento de Edafologıa y Quımica Agrıcola, Facultad de Biologıa, UniÕersidad de´ ´ ´ ´Santiago de Compostela, 15706 Santiago de Compostela, Spain

c Dipartimento di Scienza del Suolo e Nutrizione della Pianta, UniÕersita di Firenze,`Piazzale delle Cascine 28, 50144 Florence, Italy

Received 16 November 1999; received in revised form 8 September 2000;accepted 18 January 2001

Abstract

Ž Ž . . ŽEffects of Etnean broomGenista aetnensisBiv. DC. and Corsican pinePinus nigraArn..ssp. laricio Maire on the morphological, mineralogical and chemical properties of volcanic soils

Ž .from Mt. Etna Italy were compared and contrasted. For this purpose, we studied the rhizosphereand the bulk soils under adjacent 30 years old pure plantations of both species. Morphology of the

Ž .soil under broom differs from that under pine for i a higher accumulation of organic matter inŽ . Ž .the topsoil, ii an incipient formation of E material around the base of the stem, and iii the

presence of yellowish collars around the primary roots. Mineral horizons of the two soils are madeof plagioclases, pyroxenes, magnetite and glass. The yellowish colour of the collars is attributed toa root effect that results in a confined alteration of primary volcanic glass and also iron-bearingminerals, leading to the precipitation of amorphous Fe-oxides. Under pine, we observed a morewidespread weathering of primary minerals throughout the profile, and a depletion of base cationsand a release of Al in the topsoil. On the whole, therefore, Corsican pine—commonly planted in

) Corresponding author. Fax:q39-55-33-3273.Ž .E-mail address:[email protected] G. Certini .

0016-7061r01r$ - see front matterq2001 Elsevier Science B.V. All rights reserved.Ž .PII: S0016-7061 01 00017-9

( )G. Certini et al.rGeoderma 102 2001 239–254240

the last decades on the pyroclastic deposits and lava flows of the Etna volcano—seems to play adetrimental role on these soils.q2001 Elsevier Science B.V. All rights reserved.

Ž . ŽKeywords:Corsican pine Pinus nigraArn. spp. laricio Maire ; Etnean broomGenista aetnensisŽ . .Biv. DC. ; Mineral weathering; Rhizosphere; Volcanic soil

1. Introduction

Pyroclastic deposits are unconsolidated, comminuted, glassy and vesicularejecta produced during volcanic activity. When climate conditions are suitable,vegetation easily colonises this substratum that appears vulnerable to chemicalweathering and, therefore, favourable for plant growth. Even though in the last

Ždecades the genesis of volcanic soils has been widely investigated e.g. Ugolini.and Zasoski, 1979; Lowe, 1986; Shoji et al., 1993; Dahlgren et al., 1997 , the

way the biota affects the soil formation has not been sufficiently emphasised.Ž .Taylor 1933 was one of the first authors to relate the degree of mineral

alteration to the vegetative cover in volcanic soils. He noticed that the clayfraction of the superficial horizons of forest soils with acid litter showed a larger

Ž .SiO rAl O ratio than that under fern or scrub. McIntosh 1980 obtained2 2 3Ž .similar results also in Vitrandepts from New Zealand. Ward 1967 found a

relation between the presence ofAgathis australis and the formation ofŽ .halloysite in volcanic soils. Ugolini et al. 1988 , studying volcanic soils from

Japan, showed how the type of vegetation is fundamental in promoting certainpedogenic processes on the same original substratum: conifers favour Podzol,

Ž .while pampas grass was associated with Andisol. Dahlgren et al. 1991 studiedthe impact of species substitution on current soil-forming processes on volcanicsoils from Japan and found that, after 50 years of oak invasion in a Pampasgrass field, the soil had been acidified in the rooting zone.

On Mt. Etna, the largest active volcano in Europe, soils derived frompyroclastic deposits have been scarcely investigated, especially with regard tothe influence of endemic plant species. Up to 1900 m above sea level, Etnean

Ž Ž . . Žbroom Genista aetnensisBiv. DC. and Corsican pinePinus nigraArn. ssp..laricio Maire prevail in uncultivated areas. In the past, the natural distribution

of the Etnean broom, one of the most common species colonising pyroclasticdeposits and lava flows of Mt. Etna, has been favoured by man, who used it toproduce high value charcoal. Lately, the production of charcoal has beenabandoned and Corsican pine has become the chosen species for afforestation.Presently, in the Italian ecological circles, extending the plantations of Corsicanpine is not encouraged, because of the widespread coverage that has often led toa monotonous coloured and textured landscape and the fire hazard to which this

Ž .species is subjected. From the chemical point of view, Certini et al. 1998demonstrated that Corsican pine, on sandstone-derived soils, over a few decadescan cause acidification around the trunk, and promote weathering.

( )G. Certini et al.rGeoderma 102 2001 239–254 241

The purpose of this work is to compare and contrast soil profiles developedon pyroclastic deposits of Mt. Etna under broom and neighbouring pine stand,so as to understand the role of the two species in the genesis of the soils. Weintend also to ascertain if the indiscriminate planting of the pine during the lastdecades has had effects on the mineralogical and chemical properties of thesoils.

2. Studied area

Ž .The study was conducted on Mt. Vetore Sicily, Italy , an extinct basalticŽ . Žpyroclastic cone of Mt. Etna Fig. 1 , dating back from 2 to 15 ka Del Carlo

. Ž X X . Žand Branca, 1998 . Mt. Vetore 37841 N, 14859 E is about 1800 m above sea.level and is around 7 km south from the summit of the active craters. The area

Ž . Žof Mt. Vetore periodically receives ash particles-2 mm and cinders particles.from 2 to 64 mm ejected by these craters. The climate of the area is Montane

Ž .Mediterranean Bagnouls and Gaussen, 1957 , characterised by a mean annualprecipitation of 1950 mm and a mean annual air temperature of 9.38C. February

Ž . Ž .is the coldest month 1.88C , August the warmest 18.38C . The forest coverconsists mostly of pure plantations of Etnean broom and Corsican pine, with

Ž . Žsmall areas occupied by beechFagus sylÕatica L. , black locust Robinia. Ž .pseudacaciaL. and planetree mapleAcer montanumLam. . Etnean broom

and Corsican pine are the species considered for this study. Under broom, the

Fig. 1. Map of Sicily and location of Mt. Etna.


Žunderstory is composed by a thick carpet of gramineous plants chieflyPoaŽ . .Õiolacea Bell. spp. aetnensis C. Presl Ciferri et Giac. , while under pine it is

virtually absent. Both the broom and the pine were introduced as pure planta-tions about 30 years ago on a semi-barren eroded substratum; thus, we assumedthat the soils were formed by the same forming factors till the present standswere planted.

3. Materials and methods

Under both Etnean broom and Corsican pine several profiles were opened andŽ .described according to the Soil Survey Division Staff 1993 . The description of

a typical profile for each site is reported in Appendix A. The soils studied wereŽclassified as Vitrandic Udorthents, sand-skeletal, mixed, frigid Soil Survey

.Staff, 1999 . Three profiles were sampled and analysed for each species. Wecollected bulk samples and, from the lowest horizons, also the rhizosphere

Ž .according to Courchesne and Gobran 1997 . Samples were air-dried and sievedŽat 2 mm. Textural analyses were carried out by theApipette methodB Day,

.1965 on samples treated withf3 M H O solution to remove the organic2 2

matter. The sand was separated using a 53-mm sieve, and the clay from silt bysedimentation after dispersion in 0.01 M NaOH. Subsamples were treated withH O , washed with deionised water, sonicated at 15 kHz for 15 min, and2 2

repeatedly suspended both in HCl solution at pH 4 and in NaOH solution at pHŽ .10 Wada, 1989 . Clay-size materials was treated with NH -oxalateroxalic acid4

Ž .at pH 3 Blakemore et al., 1981 and the extracted Al, Fe, and Si determined onŽ .a Varian Spectraa 250 Plus, equipped with graphite furnace GTA 97 . Speci-

mens heated at 4808C for 2 h were submitted to repeated dissolution withboiling 0.5 M NaOH for 15 min to dissolve the glass and determinate itgravimetrically. Investigation of crystalline minerals was accomplished on man-ually compressed powdered specimens of sand, silt and clay fractions from bulkand rhizosphere samples by a Philips PW 1710 X-ray diffractometer, using theFe-filtered Co-Ka radiation and operating at 35 kV and 25 mA. The pH in1

water and in 0.01 M CaCl was determined potentiometrically using a solid:liquid2

ratio of 1:2.5. Organic C and total N were measured by dynamic dry flashŽ .combustion 10308C under O flow , using a Carlo Erba NA 1500 NrCrS2

Ž .Analyser. Available P was determined according to Watanabe and Olsen 1965on extracts filtered through a 0.45-mm micropore membrane filter. Exchange-

Ž .able base cations Ca, Mg, K and Na were displaced by a 0.2 M BaCl solution2

and measured by a Perkin Elmer 1100B atomic absorption spectrophotometer.

Ž . Ž .Fig. 2. Schematic representation of two profiles under Etnean broom a and Corsican pine b ,with details of the rooting zone. Collars around the roots represent portions of yellowish soil.



Ž .Exchangeable acidity Al and H was measured by titrating the M KCl solutionŽ .used for the displacement Yuan, 1959 . Effective cation exchange capacity

Ž .ECEC was obtained by the summation of the basic and the acidic cations. Onthe humic and fulvic acids collected by repeated extractions with a 0.1 MNaOHy0.1 M Na P O solution, the C content was measured by dynamic dry4 2 7

flash combustion with the NrCrS Analyser. All the analyses were carried outat least in duplicate.

4. Results and discussion

4.1. Field data

The broom plants have an average height of about 2.5 m and an average baseŽ .diameter of about 10 cm. Profiles under broom Fig. 2a consist of two organic

Ž .horizons Oi and Oe underlain by an ArC horizon formed by the addition ofash and cinders produced during the eruption of 1986. Below the ArC, there are

Ž . Ž .two C horizons 2C1 and 2C2 . Patches of bleached material E are presentwithin the 2C1 horizon, at the base of the stem. In the C horizons, a collar of

Ž . Žsoil more yellowish 7.5YR 5r6 than the matrix 5YR 4r6 for the 2C1 horizon. Ž .and 5YR 5r4 for the 2C2 horizon surrounds all the primary roots Fig. 2a . The

yellowish collars are about 3–5 cm in thickness, and their inner portioncoincides with the rhizosphere. Even though the broom is a leguminous species,we did not observe nodules of symbionts on the roots.

The pines have an average height of about 8 m and an average base diameterŽ . Ž .of about 28 cm. Profiles under pine Fig. 2b have a thin organic horizon Oi ,

two superficial horizons formed by the addition of ash and cinders in 1986Ž . Ž .ArC1 and ArC2 , and three C horizons 2C1, 2C2, and 2C3 .

The different horizon organisation of the two soils is mostly due to thedifferential organic matter accumulation, high under broom, due to the abun-dance of graminae as understory.

4.2. Textural and mineralogical characterisation

Both profiles show a sandy texture; silt tends to decline downwards with aŽ .relative enrichment of coarse sand Table 1 . The amount of clay is very small

in all the horizons, reaching 1% only in the superficial ones. However, under thebroom, in the 2C horizons, we observed a slight increase in the clay contentfrom the bulk soil toward the outer part of yellowish collars that continues into

Ž .the rhizosphere data not shown . The dispersion of the bulk samples did notproduce any suspended material, neither at pH 4 nor at pH 10. From therhizospheres of broom and pine, we extracted small quantities of suspended

Ž y1.material only at pH 10 0.1 to 0.5 g kg . This material was always more

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Table 1Ž .Particle size distribution, glass content, pH, organic C, total N, available P, content of C as humic and fulvic acids expressed as g of C per kg of soil ,

Ž . Ž .and humic Crfulvic C ratio ChrCf for soil under broom and pine Mt. Vetore, Italy

Ž .Particle size Glass % pH Organic C Total N Available P Humic C Fulvic C ChrCfa y1 y1 y1 y1 y1Ž . Ž . Ž . Ž . Ž . Ž .distribution % g kg g kg mg kg g kg g kgH O CaCl2 2

Broomb ( ) ( ) ( ) ( ) ( ) ( )Oi 59-22-18-1 8.90.6 4.9 4.2 286.2 5.1 16.5 0.7 97 4 87.1 3.9 65.4 2.4 1.3

b ( ) ( ) ( ) ( ) ( ) ( )Oe 1-3-94-2 6.30.3 5.8 5.7 155.02.8 10.3 0.3 34 2 74.8 2.9 45.5 2.6 1.6( ) ( ) ( ) ( ) ( ) ( )ArC 57-28-14-1 6.30.5 6.0 5.7 53.3 1.6 4.2 0.2 12 2 18.8 0.8 10.9 1.3 1.7( ) ( ) ( ) ( ) ( ) ( )2E 78-15-6-1 7.11.0 6.3 5.9 12.9 0.3 0.9 0.2 6 1 5.7 0.5 3.9 0.2 1.5

c ( ) ( ) ( ) ( ) ( )2C1 87-11-2-r 8.2 0.5 7.0 6.2 1.8 0.1 0.1 0.0 3 0 – 1.3 0.6 0.0c ( ) ( ) ( ) ( )2C2 83-16-1-r 8.7 1.1 7.0 6.3 0.5 0.0 – 1 0 – 0.4 0.1 0.0

PineOi nd nd 5.5 4.6 nd nd nd nd nd nd

( ) ( ) ( ) ( ) ( ) ( )ArC1 48-37-14-1 6.40.8 5.5 5.0 24.11.1 2.6 0.2 20 3 6.2 0.8 9.4 1.4 0.7( ) ( ) ( ) ( ) ( ) ( )ArC2 65-23-11-1 10.40.4 5.8 5.1 14.4 0.9 1.8 0.1 13 4 4.1 0.5 5.2 1.0 0.8

c ( ) ( ) ( ) ( ) ( ) ( )2C1 69-20-11-r 8.4 0.2 6.2 5.4 9.6 0.9 0.7 0.1 10 4 2.2 0.3 3.7 0.4 0.6c ( ) ( ) ( ) ( ) ( ) ( )2C2 78-15-7-r 9.9 0.7 6.4 5.7 6.2 0.6 0.7 0.0 9 2 0.7 0.3 2.7 0.2 0.3c ( ) ( ) ( ) ( ) ( ) ( )2C3 80-11-9-r 11.10.2 6.5 5.8 3.6 0.4 0.3 0.0 7 2 0.4 0.1 1.4 0.3 0.3

Values in parentheses are the standard deviations. ndsnot determined; –snot detected.aRespectively: coarse sands2–0.25 mm, fine sands0.25–0.05 mm, silts0.05–0.002 mm, clays-0.002 mm.bReferred to the mineral fraction contained in the organic horizons.cClay present in amount less than 0.5%.


abundant in the rhizosphere of the broom than in that of pine and appeared to beŽ .amorphous to X-ray diffraction patterns not presented . Dissolution with NH -4

oxalateroxalic acid at pH 3 showed that the suspended material had an AlrSiŽratio ranging from 0.3 to 0.9, not typical of allophane or imogolite Nanzyo et

.al., 1993 . In addition, observations under electron scanning microscope did notshow the presence of opaline silica. Consequently, we attributed the low valuesof the AlrSi ratios to the dissolution of portion of glass. The absence ofallophane, imogolite, and opaline silica agrees with the results of a previous

Ž .mineralogical work on soils near Mt. Vetore Agnelli, 1999 . Under broom, theglass ranges from 6% to 9%, whereas under pine it is slightly higher, from 6%

Ž .to 11% Table 1 . Sand, silt and clay fractions of bulk samples from both soilsare constituted of plagioclases, mostly of anorthitic composition, pyroxenes andmagnetite. In the superficial horizons of both soils, the clay fraction shows amineralogical assemblage different from that of deeper horizons. In fact, in theO and ArC horizons, we detected the presence of kaolinite, mica and traces of

Ž .interstratified mica-vermiculite Fig. 3 . The lack of the diffraction line at 0.50Ž .nm indicates the ferric nature of mica Fanning et al., 1989 . The interstratified

mica-vermiculite was identified by the fact that the peak at 1.23 nm did not shiftafter glycerol solvation of the Mg-saturated sample and collapsed at 1.01 nm

Ž .after heating to 5508C diffractograms not shown . Mica, kaolinite and interstrat-ified mica-vermiculite could be of eolian origin. On the other hand, kaolinitecould derive from the alteration of plagioclases, but also from the alteration ofthe mica, as in the case of the interstratified mica-vermiculite. Alteration in situof minerals and formation of kaolinite has been shown to occur in a pyroclastic

Ž .deposit in about 6 years Ugolini et al., 1991 . Under pine, the clay fractionŽ .shows diffraction lines of low intensity in all the horizons Fig. 3 . Under

broom, the clay fraction from the 2C horizons shows a peak with an intensityŽ .higher than that from the superficial horizons Fig. 3 . The rhizosphere clay of

Ž .the 2C1 and 2C2 horizons of both soils Fig. 4 has a composition similar to thatŽ .from the bulk samples Fig. 3 . However, under broom, clay fraction from

rhizosphere of 2C2 horizon show a general decrease in peak intensity respect tothat from bulk soil. These results suggest the existence under the broom of aAroot effectB able to produce a mineral alteration more intense than in the bulksoil. This alteration could be responsible for the yellowish colour of the materialsurrounding the primary roots. By observing this material under an opticalmicroscope, we ascertained that the pigmentation is imparted by yellow fineparticles that, even if in scarce amount, coat the sand grains. The colour persistsafter the sample is treated with 8 M H O solution, hence, it is not due to2 2

organic exudates adsorbed on the minerals. On the contrary, when the sample istreated with acid NH -oxalate, it acquires a shade similar to that of the bulk soil.4

Yet, no significant differences in NH -oxalate extractable Fe were found4

between the clay fraction from both rhizosphere: 7.6"0.8% Fe in the rhizo-sphere of broom, 6.6"0.2% in that of pine. On the other hands, it was expected


Fig. 3. XRD patterns of Mg-saturated, air-dried clay fraction from bulk samples manuallycompressed into plexiglass slides. Msmica; Kskaolinite; M-Vs interstratified mica-vermicu-lite; Plsplagioclases; Masmagnetite; Pyspyroxenes.

that, in these soils, the finding of small differences of Fe oxyhydroxides couldhave been overshadowed by the presence of magnetite that, according to

Ž .Schwertmann and Taylor 1989 , partly dissolves in acid oxalate. We assumethat the yellowish colour of the collars is due to a minimal difference inconcentration of secondary amorphous Fe-oxides. These latter would come fromthe weathering of volcanic glass and, in addition, to iron-bearing minerals such


Fig. 4. XRD patterns of Mg-saturated, air-dried fraction clay from rhizosphere samples manuallycompressed into plexiglass slides. Masmagnetite; Kskaolinite; Plsplagioclases.

Ž .as pyroxenes and magnetite Dahlgren et al., 1993 . A similar phenomenon wasŽ .reported by Schwertmann 1971 for old red soils. In that case, however, the

yellowish colour of the material around the roots was assessed to be due to thepresence of goethite originated by the transformation of hematite.

4.3. Chemical properties

In both soils, pH increases with depth, reaching neutrality in the deepestŽ .horizons Table 1 . Comparing the corresponding horizons of the two soils, it

appears that those under pine are more acid. Under broom, the E material ismore acid than the surrounding 2C1 horizon, indicating a higher bleachingexperienced by this soil portion. The rhizosphere has a pHf0.5 units lowerthan that of the bulk samples. In both soils, organic C, total N, available P, and

Ž .ECEC decrease with depth Table 1 . In the topsoil under broom, the organic Cis considerably more abundant than under pine. In spite of being a leguminousspecies, the broom contributes to the soil N at a similar level than the pine, soconfirming the apparent absence of N -fixer symbionts on the roots of broom.2

High amounts of available P are in the organic horizons under broom. In theArC horizons, the concentrations of this element are similar in the two soilsŽ .Table 1 . However, on a surface basis, the amount of available P in ArChorizons, considering their thickness and the contribution of both fine earth and

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Table 2Ž . Ž . Ž .Exchangeable cations, effective cation exchange capacity ECEC and base saturation BS for soils under broom and pine Mt Vetore, Italy

a Ž .Exchangeable ECEC BS %y1Ž Ž . .cmol q kgCa Mg K Na Al H

y1 y1 y1 y1 y1 y1Ž Ž . . Ž Ž . . Ž Ž . . Ž Ž . . Ž Ž . . Ž Ž . .cmol q kg cmol q kg cmol q kg cmol q kg cmol q kg cmol q kg

Broom( ) ( ) ( ) ( ) ( ) ( )Oi 19.4 0.8 3.1 0.2 1.1 0.2 6.6 0.4 – 1.1 0.2 31.3 1.0 97( ) ( ) ( ) ( ) ( ) ( )Oe 18.3 1.3 3.0 0.2 0.8 0.1 7.2 0.2 – 0.3 0.0 29.6 1.3 99( ) ( ) ( ) ( ) ( ) ( )ArC 13.2 0.5 2.2 0.4 0.3 0.0 7.2 0.5 – 0.1 0.0 23.0 0.8 99

( ) ( ) ( ) ( ) ( ) ( )2E 3.3 0.4 0.6 0.0 0.2 0.0 4.4 0.2 – 0.1 0.0 8.6 0.4 98( ) ( ) ( ) ( ) ( ) ( )2C1 2.8 0.5 0.5 0.0 0.2 0.1 4.5 0.1 – 0.1 0.1 8.1 0.5 99( ) ( ) ( ) ( ) ( ) ( )2C2 2.2 0.3 0.4 0.1 0.1 0.0 4.4 0.4 – 0.1 0.0 7.2 0.5 99

PineOi nd nd nd nd nd nd nd nd

( ) ( ) ( ) ( ) ( ) ( ) ( )ArC1 1.1 0.4 0.6 0.0 0.3 0.0 1.6 0.2 0.6 0.1 0.4 0.1 4.6 1.0 79( ) ( ) ( ) ( ) ( ) ( ) ( )ArC2 4.0 0.6 0.7 0.2 0.3 0.1 5.5 0.6 0.3 0.0 0.3 0.1 11.1 0.9 95( ) ( ) ( ) ( ) ( ) ( )2C1 3.7 0.1 0.6 0.1 0.2 0.0 4.8 0.3 0.1 0.0 – 9.4 1.3 99( ) ( ) ( ) ( ) ( ) ( )2C2 4.4 0.3 0.8 0.1 0.2 0.1 5.5 0.6 – 0.1 0.1 11.0 0.9 99( ) ( ) ( ) ( ) ( ) ( )2C3 2.9 0.3 0.5 0.0 0.2 0.0 4.5 0.4 – 0.1 0.0 8.3 0.5 98

Values in parentheses are the standard deviations. ndsnot determined; –snot detected.a w Ž 3q q .x100P ECECy Al qH rECEC.


rock fragments, is 609"188 mg my2 under broom and significantly lowerunder pine, 219"80 mg my2. Under broom, the amount of C as humic andfulvic acids is considerably higher in the organic horizons, then it decreases

Ž .gradually with depth Table 1 . A similar decrease occurs under pine, even if thequantities of humic and fulvic C are tendencially lower than under broom. The

Ž .humic Crfulvic C ratio ChrCf can be used as a suitable index for the degreeof organic matter degradation. Under broom, except the 2C1 and 2C2 horizons

Ž .where humic acids are absent, the values of the ratio Table 1 are typical ofŽ .grassland soils Stevenson, 1994 , indicating a sensible contribution of the

carpet of graminae to the formation of humic substances. Under pine, where theunderstory is virtually absent, the ChrCf ratio is smaller. In both soils, theprevalence of fulvic acids below 20–30 cm of depth can be explained by thehigher solubility of the fulvic respect to the humic acids and, maybe, the earlystage of humification of the organic compounds released by the roots.

Calcium and Na are the more abundant exchangeable cations in both soilsŽ .Table 2 . Under broom, Ca prevails on Na in the organic and ArC horizons, inthe lowest horizons the trend is reversed. Under pine, Na is always theprevailing cation. Noteworthy is the amount of Al in the two superficial mineralhorizons under pine: in the ArC1 horizon, for example, this element represents

Ž .about 12% of the ECEC Table 2 . Exchangeable Al is virtually absent underbroom. This difference between the two soils is consistent with their pH values.

5. Conclusions

Despite the short period of establishment on the same mineral substratum,about 30 years, the soils developed under Etnean broom and Corsican pine haveacquired differences in morphological, mineralogical and chemical properties.

Ž .The sequumunder broom shows the presence of i an accumulation of organicŽ .matter in the topsoil, ii an incipient E horizon around the base of the stem, and

Ž .iii a yellowish collar around the primary roots. The organic matter accumulatesin the soil under broom because of the abundant carpet of graminae that, onlyunder this species, finds good conditions for growth. The formation of Ematerial seems to be due to the bleaching effect exerted by the stemflow. Theyellowish pigmentation around the roots of the broom could be due to confinedenrichment of secondary amorphous Fe-oxides mostly formed from the alter-ation of the glass. The occurrence of the yellowish collars leads to hypothesise aAroot effectB of G. aetnensison pyroclastic deposits from Mt. Etna. Under pine,the weathering has, instead, equally affected the entire profile. Evidently, underthis species the organic acids released by the litter have more weathering impactthan the root exudates. The litter-released acids impart to the ArC horizons anacidity that favours the release of Al. From the chemical point of view, the pinehas inaugurated a depletion regime of base cations, which involves the entire

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( )Appendix A. Descriptions of typical profiles under broom and pine on Mt. Vetore Sicily, Italy

Ž .Profile underG. aetnensisBiv. DC.Elevation: 1820 m a.s.l.; Exposure: E; Slope: about 15%; Parent material: basaltic pyroclastic material

Ž .Understory: diffusedPoaÕiolaceaBell. ssp.aetnensisC. Presl Ciferri et Giac., with some loose plants ofRosa caninaL. var. canina

Ž .Soil: Vitrandic Udorthent, sand-skeletal, mixed, frigid Soil Survey Staff, 1999c d e fHorizons Depth Munsell Texture Structure Consistency Plasticity Roots Boundary Other

a bŽ . Ž . Ž .cm colour USDA USDA observationsŽmoist,

.crushed

Oi 6–3 slightly decomposedleaves ofGenistaand Fagus

Oe 3–0 partially decomposedleaves ofGenistaand Fagus

ArC 0–10 5YR 3r2 g cos 1fcr dlo, wso wpo 3mi and vf a, iand f andm and co

2E 10–20 7.5YR 5r6 v cos 1fcr dlo, wso wpo 3mi and vf a, band f andm and co

2C1 20–93 5YR 4r6 vg cos 0 dlo, wso wpo 3mi and vf c, s root collars ofand f and yellowish colour

Ž .m and co 7.5YR 5r62C2 93–160q 5YR 5r4 vg cos 0 dlo, wso wpo 3mi and vf root collars of

and f and yellowish colourŽ .m and co 7.5YR 5r6

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Ž .Appendix A. continued

Profile underP. nigra Arn. ssp.laricio MaireElevation: 1820 m a.s.l.; Exposure: E; Slope: about 15%; Parent material: basaltic pyroclastic materialUnderstory: virtually absent

Ž .Soil: Vitrandic Udorthent, sand-skeletal, mixed, frigid Soil Survey Staff, 1999Oi 1–0 partially decomposed needles of pine

Ž .ArC1 0–2 7.5YR 4r2 g s 0 dlo, wso wps 3mi and vf g, s many altered clasts 7YR 4r4and f ixed with abundant hyphae

pulverulent organic matterŽ .ArC2 2–4 7.5YR 5r4 g s 0 dlo, wso wps 3mi and vf c, s many altered clasts 7YR 4r4

and f, mixed with abundant hyphae andpulverulent organic matter

2C1 4–16 10YR 5r3 g s 0 dlo, wso wpo 3mi and vf c, w clasts more altered and less angularand f, than those of the upper horizons;

presence of hyphae2C2 16–30 10YR 5r6 g s 0 dlo, wso wpo 2f and m c, w presence of hyphae2C3 30–160q 7.5YR 5r7 g s 0 dlo, wso wpo 2f and m, presence of hyphae

1co

avgsvery gravelly, gsgravelly, cosscoarse sand, sssand.b0sstructureless, 1sweak; fs fine; crscrumb.cdsdry, wswet; los loose; sosnonsticky.d wswet; pssslightly plastic, posnonplastic.e1s few, 2splentiful, 3sabundant; mismicro, vfsvery fine, fs fine, msmedium, coscoarse.fgsgradual, csclear, asabrupt; sssmooth, wswavy, is irregular, bsbroken.


soil, and an incipient release of Al in the superficial horizons. In contrast, topsoilunder broom shows high contents of organic C and base cations, and the lack ofexchangeable Al.

From the study of volcanic soils formed under two commonly occurring plantspecies on Mt. Etna, we infer that the presence of Corsican pine, in time, couldproduce a series of detrimental effects, such as loss of nutrients, increased ratesof acidification, and liberation of Al. These effects presumably will not occurunder broom.

Acknowledgements

We thank C. Santi for the execution of the drawing, A. Dodero, A. Agnelli,and F. Berna for laboratory assistance, and A.C. Edwards and P. Cherubini forhelpful criticism on the manuscript. This research was partly supported by a

Ž .grant Marie Curie from theTMR ProgrammeEuropean Community .

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