Investigation of soils developed on volcanic materials in Nisyros Island, Greece

Catena 70 (2007) 340 – 349 www.elsevier.com/locate/catena

Investigation of soils developed on volcanic materials in Nisyros Island, Greece S. Drouza, F.A. Georgoulias, N.K. Moustakas ⁎ Agricultural University of Athens, Laboratory of Soil Science and Agricultural Chemistry, Iera Odos 75, Votanikos 118 55, Athens, Greece Received 16 May 2006; received in revised form 2 October 2006; accepted 9 November 2006

Abstract Soils that are forming on volcanic parent materials have unique physical and chemical properties and in most cases, on wet and humid climates, are classified as Andisols. The main purpose of this study is to examine if the soils that are forming on volcanic materials under a dry Mediterranean climate, in Nisyros Island (Greece), meet the requirements to be classified as Andisols. Soils from seven sites were sampled and examined for their main physico-chemical properties and selective dissolution analysis. Dithionite–citrate–bicarbonate (DCB) extractable Al and Fe (Áld, Fed), acid ammonium oxalate extractable Al, Fe, and Si (Álo, Feo and Sio), and sodium pyrophosphate extractable Al and Fe (Alp, Fep) were measured. In addition, Al and Si were determined after reaction with hot 0.5 M NaOH, (AlNaOH and SiNaOH) and with Tiron-(C6H4Na2O8S2), (AlT and SiT). P-retention was also measured. The soils are characterised by coarse texture, low organic matter content, low values of cation exchange capacity (CEC), and high pH values. Values of Sio, Alo and Feo are less than 0.022%, 0.09% and 0.35% respectively, highlighting the lack of noncrystalline components. The ratio (Fed–Feo)100/Fed is quite high expressing the degree of crystallisation of free iron oxides. For all samples tested, values of the Alo + 1/2Feo index are extremely low (b 0.24%). High SiNaOH and SiT (arising 2.76% and 2.18% respectively) indicate the presence of silica in amorphous forms. P-retention values are very low (b 12.6%). The results indicated the absence of noncrystalline minerals except for amorphous silica, and do not exhibit andic or vitric soil characteristics to be classified as Andisols. © 2006 Elsevier B.V. All rights reserved. Keywords: Mediterranean climate; Andisols; Noncrystalline minerals; Andic–vitric soil properties

1. Introduction Soils forming on volcanic parent material comprise 0.84% of the earth's surface (Leamy, 1984). These soils have been developed under a diverse range of climatic conditions and have been studied extensively worldwide. The majority of studies concern volcanic soils such as those of humid and tropical climates. Many studies have been reported, especially from Japan (Shoji et al., 1982), Indonesia (Tan, 1965), New Zealand (Parfitt and Wilson, 1985), Spain (Jahn, 1991), Italy (Quantin et al., 1985), France (Aran et al., 1998), United States (Mizota and Van

⁎ Corresponding author. E-mail address: [email protected] (N.K. Moustakas). 0341-8162/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catena.2006.11.001

Reeuwijk, 1989) and Mexico (Campos Cascaredo et al., 2001). The main characteristic of volcanic soils is the domination in the clay fraction of active forms of Al and Fe, such as noncrystalline minerals (allophane, imogolite, ferrihydrite) and Al/Fe humus complexes. The formation of noncrystalline minerals is the result of rapid weathering of volcanic glass (main constituent of volcanic ash parent material) that presents the least resistance to chemical weathering. The kind of noncrystalline minerals that will be formed is heavily dependent on climate, parent material and stage of weathering (Wada, 1989). The presence of high organic matter content and noncrystalline minerals gives volcanic soils unique physical and chemical properties, such as high water retention capacity, variable charge, high P-retention, and low values of bulk density (Tan, 1965; Shoji et al., 1993). Most

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soils that have developed on volcanic materials are classified as Andisols. Moustakas and Georgoulias (2005) reported that soils developed on volcanic parent materials in Thera Island (Greece) do not possess andic or vitric soil properties and hence cannot be classified as Andisols. Garcia-Rodeja et al. (1987) and Hunter et al. (1987) classified soils that are forming on nonvolcanic parent materials as Andisols. According to Soil Taxonomy (Soil Survey Staff, 2006) criteria for defining a soil characterised by andic or vitric soil properties and classified as Andisols are based on their chemical and physical properties. Alo + 1/2Feo and P-retention values are two of the main indices that are used to characterise a soil by andic or vitric soil properties. Alo + 1/2Feo must be greater than or equal to 2.0% and P-retention ≥ 85% for andic soil properties. Otherwise Alo + 1/2Feo has to be at least 0.4% and P-retention greater than 25% for vitric soil properties. The purpose of this study is the investigation of the physical and chemical properties of soils developed in Nisyros with emphasis on research into the classification of these soils. 2. Area description, materials and methods Nisyros is a member of the Greek Dodecanese island complex located in the south Aegean Sea, lying between 36°33′–36°40′ N and 27°08′–27°16′ E (Fig. 1). Nisyros is the Eastern part of the Aegean volcanic arc that is formed from the volcanic centres of Sousaki, Methana, Egina, Poros, Milos and Santorini which create a typical volcanic island arc (Fig. 1). The South Aegean volcanic arc is a zone of Pliocene to modern volcanism apparently related to the subduction of the African plate beneath Eurasia. Nisyros has a typical conical shape and covers a surface area

341

of 41.2 km2. The entire island is a typical caldera, with a perimeter of 25 km. The age of volcanic deposit is between 10,000 and 200,000 years old (Fytikas et al., 1976; Keller et al., 1989), which are Pleistocene deposits. Today, important hydrothermal activity continues to be observed, expressed mainly from sections of the caldera under the sea. Seymour and Vlassopoulos (1989) reported that active magma exists under the island. Volcanic rocks of Nisyros have important mineralogical similarities to other volcanic centres of the arc. All are included in the calc-alkaline trend and range from basaltic andesite to silicic rhyolith (Di Paola, 1974; Vougioukalakis, 1992). The main volcanic rocks of Nisyros (Fig. 2) are dacite, trachytes, rhyodacite, andesite, trachyandesite, trachyandesitic conglomerates, rhyolithe, pumice and volcanic glass (Davis, 1967). The soil parent material consists of Quaternary and Pleistocene deposits which are mostly intermediate. On the basis of meteorological data during 1982–1997, the average annual rainfall on Nisyros was 509 mm with 91% of rain falling between the middle of autumn and the end of winter. The mean annual temperature is 17.8 °C and relative humidity ranges from 59% in July to 74% in December. The climate is classified as Sub-Tropical Mediterranean (Papadakis, 1985), with a mean annual evapotranspiration of 905 mm and a dry season lasting 7 months (April–October). The soil temperature and moisture regime is classified as thermic and xeric respectively (Soil Survey Staff, 2006). 3. Soil analysis For the purposes of this study, soil samples were taken from seven sites at which soil profiles were taken (Fig. 2). The selection of these sites geomorphically was made in

Fig. 1. Location of the South Aegean Quaternary Volcanic Arc and the island of Nisyros.

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Fig. 2. Geological map of Nisyros and sampling sites.

areas where there was no evidence or likelihood of soil erosion or soil deposition, as enrichment of the soil with new material would result in slowing of soil-genesis and hence masking of actual developments. Profiles were examined, described and samples taken by horizon for physical and chemical analyses. Samples were air-dried, crushed and sieved to pass a 2 mm sieve. In the fine earth fraction, the sand fraction was determined by wet sieving and the silt and clay fractions using the pipette method. Organic matter was determined using the modified Walkley–Black method; carbonates were estimated by CO2 evolution with HCl; pH was measured in a suspension of soil in water (1:1), in 0.01 M CaCl2 (1:10) and in NaF (1:50). All the abovementioned procedures are detailed in the Soil Survey Laboratory Methods Manual (1996). Dithionite–citrate– bicarbonate (DCB) extractable Al and Fe (Áld, Fed) were measured using the method of Mehra and Jackson (1960). Acid ammonium oxalate (pH = 3.0–3.5) extractable Al, Fe,

and Si (Álo, Feo and Sio) were measured using the method of Blakemore et al. (1987). Sodium pyrophosphate extractable Al and Fe (Alp, Fep) were measured using the method of Bascomb (1968). In addition, after reaction with hot 0.5 M NaOH, Al and Si (AlNaOH, SiNaOH) were determined ( H a s h i m o t o a n d J a c k s o n , 1 9 6 0 ) . A l s o Ti r o n (C6H4Na2O8S2) extractable Al and Si (AlT and SiT) were measured (Kodama and Ross, 1991). Phosphate retention was determined using the method of Blakemore et al. (1987). Total elemental analysis was carried out in the sand fraction (Bernas, 1968). All elements were determined using atomic absorbance spectrometry, except for Na and K, which were determined using flame photometry. 4. Results and discussion A summary description of each soil profile is presented in Table 1. In all profiles, slope was approximately 0–2%.

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343

Table 1 Summary description of soil profiles Field texture

Structure a

Roots b (m)

Biological activity c

Boundary d

1st sampling site (trachyandesitic conglomerates) A 0–20 10YR 6/2 10YR 5/2 C1 20–60 10YR 6/2 10YR 5/2 C2 N60 10YR 6/2 10YR 5/2

LS LS LS

Weak coarse sbk Structureless Structureless

2 2 1

m w n

gs gs –

2nd sampling site (trachyandesitic conglomerates) A 0–30 10YR 5/4 10YR 4/4 C 30–60 10YR 5/3 10YR 5/2

LS LS

Weak coarse sbk Structureless

2 1

m n

gs –

3rd sampling site (andesites) A 0–30 10YR 7/2 C 30–60 10YR 6/3

10YR 6/2 10YR 5/2

LS LS

Weak coarse sbk Structureless

2 1

m n

gs

4th sampling site (pumice) A 0–30 C 30–60

10YR 6/2 10YR 7/3

10YR 5/3 10YR 6/2

LS LS

Structureless Structureless

2 1

m n

gs –

5th sampling site (volcanic ash) A 0–30 10YR 6/2 C 30–60 10YR 7/2

10YR 5/3 10YR 6/2

LS LS

Weak coarse sbk Structureless

2 1

m n

gs –

6th sampling site (volcanic ash) A 0–30 10YR 5/3 C1 30–60 10YR 5/6 C2 60–90 10YR 5/4

10YR 4/4 10YR 4/4 10YR 4/4

LS LS LS

Weak coarse sbk Structureless Structureless

2 1 1

m w n

gs gs –

7th sampling site (trachyandesitic conglomerates) A 0–30 10YR 5/4 10YR 4/4 C 30–60 10YR 7/3 10YR 5/3

LS LS

Weak coarse sbk Structureless

2 1

m n

gs –

Horizon

a b c d

Depth (cm)

Colour moist

Colour dry

Structure: sbk = Subangular blocky. Root abundance: 0 = none; 1 = few (2–20%); 2 = common (20–50%); 3 = many (N50%); root diameter size: f = 1–2 mm; m = 2–5 mm. Biological activity: n = none; w = weak ; m = moderate. Boundary: gs = gradual smooth.

Natural vegetation is composed mainly of brush-woods and annual herbs including Thymus capitatus, Salvia sp., Alcana tictoria, Phagnalon rupestre, and Fumaria officinalis. In sites 1, 2, 4, 5, and 6 the vegetation was abandoned vines (Vitis vinifera). Parent material for each sampling site is presented in Fig. 2. From the profile descriptions it is apparent that there is a lack of distinct soil horizons, with the exception of a weakly defined A horizon. The soils are characterised by coarse texture, weak coarse subungular blocky structure in A horizons (except the A horizon at site 4) and no structure in all other horizons. The soils are characterised by very good drainage, and the soil material in all profile locations is very friable and noncoherent. The colour hue is 10YR in all samples and is characterised by high value and intermediate colour. Slight variation was observed between the surface soil and that at greater depths due to the greater organic matter content of the surface horizon. There are no differences between profiles and the profiles are considered practically homogeneous. No reaction with HCl was observed in any horizon.

4.1. Physical properties Sand content ranged from 76% to 95%, silt from 1.1% to 20% and clay from 1.8% to 7.9% (Table 2). The soil texture is described as loamy sand (LS) indicating the low rate of weathering. Under the climatic conditions of Nisyros (long dry season, low precipitation) the weathering process is not easily facilitated. The rate of soil formation on volcanic parent materials has largely been studied in wet climates in Japan and New Zealand (Theng, 1980; Wada, 1985). Yamada (1968) reported that in wet climates in Japan, 100–500 years were necessary for the formation of A and C horizons and at least 1000 years for the formation of A, B and C horizons. Nieuwenhuyse et al. (1993) reported that in the wet tropical climate of Costa Rica, Andisols from sand of volcanic origin were formed in 2000 years. The age of the soils in the present study is approximately 10,000 to 200,000 years. According to the results from the sites examined, there is very low soil development and it is apparent that the major restricting factor is climate.

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Table 2 Particle size distribution, pH and P-retention values of the soils studied Horizon Depth (cm)

Sand Silt Clay Texture pHa pHb pHc Pretention % (%)

1st sampling site (trachyandesitic conglomerates) A 0–20 95 1.1 3.6 LS 6.6 C1 20–60 93 2.8 2.8 LS 7.3 C2 N60 92 5.9 1.8 LS 7.4

4.3 4.5 4.6

7.4 7.4 7.4

7.1 7.1 3.1

2nd sampling site (trachyandesitic conglomerates) A 0–30 89 4.2 6.9 LS 6 C 30–60 86 6.7 7.6 LS 6.3

4.2 4.2

7.4 7.4

10.6 10.2

3rd sampling site (andesites) A 0–30 90 6 C 30–60 76 20

3.9 4.4

LS LS

6.7 6.7

4.3 4.3

7.4 7.4

6.7 7.8

4th sampling site (pumice) A 0–30 91 6.3 3 C 30–60 87 5.7 6.9

LS LS

5.4 5.4

4.1 4.1

7.3 7.3

7.1 7.4

5th sampling site (volcanic ash) A 0–30 91 5.8 3.3 C 30–60 90 5.7 4.8

LS LS

5.4 5.5

4.2 4.1

7.2 7.3

7.8 7.4

6th sampling site (volcanic ash) A 0–30 94 2.4 3.9 C1 30–60 90 5.2 5.3 C2 60–90 89 5.7 5.5

LS LS LS

6.2 6.5 6.7

4.2 4.4 4.6

7.2 7.3 7.2

6.7 6.3 8.1

7th sampling site (trachyandesitic conglomerates) A 0–70 85 7.8 7.4 LS 6.7 C N70 78 14 7.9 LS 7.3

4.4 5.6

7.3 7.5

12.6 9.2

a

b

from organic matter can be posed if we assume that CEC value for organic matter is 200 cmol(+) kg− 1. Exchangeable bases are present in the order of abundance Ca N Mg N t Na N K. Base saturation is high and correlates with the high soil pH. The values of P-retention are very low confirming that active forms of Al and Fe have not accumulated in the soils (Table 2). The results of total sand elemental analysis are shown in Table 4. The SiO2/Al2O3 molar ratio is greater than 4.8 in all sites studied, with CV lower than 13%. Fytikas (1977) reported SiO2/Al2O3 molar ratios of 4.9, 5.3, 7.2 and 8.9 for andesites with low SiO2, andesites, dacites and rhyoliths, respectively. Wyers (1987) reported SiO2/Al2O3 molar ratios for andesites between 5.1 and 7.7, while Di Paola (1974) found SiO2/Al2O3 molar ratios of 4.9–5.3, 5.4–5.9, 7.1–7.4, and 9.5, for basic andesites, andesites, dacites, rhyodacites, respectively. From these data the parent materials of the island can be characterised as intermediate. Comparing the SiO2/Al2O3 molar ratios of total sand elemental analysis with those of the parent material, we can conclude that the studied soils are at an early stage of weathering. In addition, the values of SiO2/Al2O3 combined with low clay content indicate the low rate of weathering of the parent material. Total elemental analysis shows base cations are present in the order of abundance Na2O N K2O N CaO N MgO, indicating the high levels of Na in the parent material. 4.3. Selective dissolution analysis

c

soil: H2O 1:1; soil: 0.01 M CaCl2 1:10; soil: NaF 1:50.

4.2. Chemical properties The results of the chemical analyses are shown in Table 3. The organic matter content of the soils was low, between 0.2 and 2.3%. These low values, in combination with the low clay content, account for the lack of moderate or strong soil structure. The pH ranged from 5.4 to 7.3 (in most cases above 6.0), values which are relatively high considering the nature of the parent material and the absence of carbonates (Table 2). Exchangeable sodium (ESP) is relatively high, ranging from 1.4% to 10% (Table 3). Such high values may be the result of high Na levels in the parent material, which is rich in plagioclase (Davis, 1967) as can be seen from the total elemental analysis in the sand fraction (Table 4), and the addition of NaCl via transport and deposition of droplets of seawater. Also the high water deficit (approximately 400 mm H2O) and the long dry period (7 months) play a role in the accumulation of salts in the surface of the soils. Consequently, the relatively high pH may be due to the presence of high exchangeable Na. CEC values range from 5.5 to 8.1 cmol(+) kg− 1 and reflect the low content of these soils in aluminosilicate minerals of type 2:1 (Table 3). The hypothesis that CEC values arise

According to Wada (1989), acid oxalate extracts: 1) aluminium (Alo) from allophane, imogolite, allophane-like minerals and Al–humus complexes; 2) iron (Feo) from ferrihydrite and Fe–humus complexes; 3) silica (Sio) from allophane and imogolite. This extraction is considered to be selective for Sio. Sodium dithionite–citrate (DCB) extracts: 1) aluminium (Ald) from allophane, Al–humus complexes and noncrystalline oxides; 2) iron (Fed) from ferrihydrite, crystalline oxides, and Fe–humus complexes; 3) Na4S2O7 extracts aluminium (Alp) and iron (Fep) from organic complexes. The results of the selective dissolution analyses are presented in Table 5. The values for Fep and Alp are strikingly low in all samples and this is due to the low organic matter content of the soils as well as to the low solubility of Fe and Al at pH values greater than 5.5 (Lindsay, 1979). Shoji et al. (1982) found that at pH 5.0–7.0, Al and Fe form particular allophane, imogolite and crystalline phyllosilicate minerals, but not Al and Fe–humus complexes. The same researchers reported that in soils originating from volcanic ash in acidic environments, Al and Fe–humus complexes are highly correlated with the development of monomeric cationic forms of Fe and Al and that the trivalent species of both these elements have a greater capacity for forming organic complexes than all other valencies. The relatively high pH of the soils of Nisyros result in a low solubility of Al and Fe which, in

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Table 3 Chemical characteristics of the soils studied Horizon

Depth (cm)

Organic matter (%)

Exchangeable Ca+ 2

Exchangeable Mg+ 2

Exchangeable Na+

Exchangeable K+

CEC

−1

cmol(+) kg

BS

ESP

%

1st sampling site (trachyandesitic conglomerates) A 0–20 0.94 1.62 C1 20–60 0.54 1.75 C2 N60 0.2 1.13

1.06 0.73 0.29

0.25 0.19 0.03

0.12 0.15 0.2

2.49 2.18 1.35

100 100 100

10 8.72 2.22

2nd sampling site (trachyandesitic conglomerates) A 0–30 2.75 4.38 C 30–60 1.14 3.25

1.56 1.44

0.11 0.09

0.42 0.46

7.05 6.6

92 79

1.56 1.36

3rd sampling site (andesites) A 0–30 1.01 C 30–60 0.4

2.63 3.13

1.06 1.48

0.09 0.17

0.2 0.23

4.55 5.33

87 94

1.98 3.19

4th sampling site (pumice) A 0–30 2.30 C 30–60 2.14

3.13 2.38

1.25 1.46

0.11 0.09

0.26 0.17

5.74 5.33

83 77

1.92 1.69

5th sampling site (volcanic ash) A 0–30 2.01 C 30–60 1.14

1.86 2.63

0.75 1.2

0.11 0.09

0.09 0.12

3.81 4.17

74 97

2.89 2.16

6th sampling site (volcanic ash) A 0–30 1.94 C1 30–60 1.34 C2 60–90 1.14

2.75 2.75 2.63

1.25 1.5 1.56

0.09 0.09 0.09

0.17 0.2 0.2

4.55 4.17 4.93

93 100 91

1.98 2.16 1.83

1.96 –

0.25 –

0.46 –

7.05 –

93 –

3.55 –

7th sampling site (trachyandesitic conglomerates) A 0–70 1.14 3.88 C N70 0.8 –

combination with their low organic matter content, results in the absence of Al and Fe–humus complexes. Values of Sio, Alo and Feo are less than 0.022%, 0.09% and 0.35% respectively, highlighting the lack of noncrystalline components. Since acid oxalate is a selective extractant for the solubilisation of allophane and imogolite, the ratio (Alo–Alp)/Sio is used for estimating the allophane Al/Si ratio (Parfitt and Wilson, 1985). The (Alo–Alp)/Sio in most samples ranges from 0.14 to 5.63 (Table 6), which is a characteristic value for allophane. Consequently, the very low values of Alo and Sio may be the result of allophane which exists in trace amounts in these soils. Shoji and Fujiwara (1984) reported that the formation of allophane and imogolite is determined to a great extent by soil acidity, with a favorable pH being from 5.0 to 7.0. The presence of high quantities of organic matter in soils with pH less than 5.0 has a negative effect on allophane formation due to the sorption of Al by organic matter. Allophanes are rarely found in soils under ustic, xeric or aridic moisture regimes due to the restricted leaching of silica (Parfitt and Kimble, 1989). The lack of noncrystalline mineral components in Nisyros soils is further borne out by the pH values measured in NaF,

ranging from 7.2–7.5 (Table 2). In most cases the pH was less than 9.5. Values of pHNaF greater than or equal to 9.5 point to the dominance of noncrystalline minerals, indicative of soils characterised by andic properties (FAO, 1998). It should be noted that pHNaF is not a determinant for the presence of noncrystalline minerals in the clay fraction of volcanic soils, but rather is an indicator, since the anion exchange taking place is not selective for noncrystalline materials alone. The ratio (Fed–Feo)100/Fed expresses the degree of crystallisation of free iron oxides (Schwertman and Taylor, 1989). The ratios shown in Table 6 are quite high and indicate that Fe released during weathering forms mostly crystalline oxides. This is further supported by the local climatic conditions which facilitate water loss. Generally, Andisols are characterised by the presence of significant amounts of ferrihydrite (Childs et al., 1991) and a low degree of crystallisation, something not applicable in the case of the soils from the island of Nisyros. The results of the extraction of soils with hot 0.5 M NaOH are shown in Table 7. According to Wada (1989), hot sodium hydroxide extracts: 1) aluminium (AlNaOH) from organic complexes, hydrated oxides, gibbsite and also from 1:1 phyllosilicate minerals; 2) silica (SiNaOH) from opaline

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Table 4 Total elemental analysis of s and SiO2/Al2O3 molar ratio of the soils studied Horizon

Depth (cm)

SiO2

Al2O3

Fe2O3

Na2O

K2 O

CaO

MgO

MnO

SiO2/ Al2O3

1st sampling site (trachyandesitic conglomerates) A 0–20 54.52 16.93 C1 20–60 58.97 18.82 C2 N60 56.57 17.54

6.46 6.52 6.00

7.81 7.70 8.14

2.40 2.97 2.50

2.07 1.90 2.85

0.40 0.66 0.66

0.11 0.10 0.11

5.5 5.3 5.5

2nd sampling site (trachyandesitic conglomerates) A 0–30 51.01 18.14 C 30–60 55.20 19.20

6.58 6.86

7.15 7.70

2.78 2.59

2.18 1.96

0.86 0.80

0.12 0.11

4.8 4.9

3rd sampling site (andesites) A 0–30 C 30–60

61.71 60.08

17.54 15.65

5.89 4.46

7.15 6.73

3.06 3.44

1.46 1.06

0.60 0.40

0.08 0.05

6.0 6.5

4th sampling site (pumice) A 0–30 C 30–60

64.70 57.08

15.42 14.28

3.20 3.15

5.89 5.38

3.06 2.88

1.18 0.95

0.46 0.46

0.04 0.03

7.1 6.8

5th sampling site (volcanic ash) A 0–30 60.51 C 30–60 69.07

17.46 16.33

3.83 3.20

6.83 6.30

3.16 3.25

2.24 1.90

0.60 0.33

0.05 0.03

5.9 7.2

6th sampling site (volcanic ash) A 0–30 57.94 C1 30–60 57.43 C2 60–90 64.79

16.63 17.84 16.70

4.63 5.72 5.26

7.48 7.05 7.81

3.25 3.16 3.25

1.51 1.79 2.01

0.27 0.27 0.46

0.06 0.07 0.08

5.9 5.5 6.6

7th sampling site (trachyandesitic conglomerates) A 0–70 56.49 16.17 C N70 56.13 16.33

4.86 6.69

6.94 4.98

3.44 2.59

2.52 2.80

0.53 1.86

0.07 0.11

5.9 5.9

%

silica and to a lesser extent from crystalline silica. The values of AlNaOH range from 0.12%–1.13% and of SiNaOH from 0.83%–3.16%, being much higher than using the values determined using the other selective methods (Table 5). In all horizons, even surface ones, there is an excess of amorphous silica (SiO2) after the complexing of SiO2 with Al2O3 according to 2SiO2Al2O3. Bearing in mind that hot NaOH can extract these elements from all the above minerals, the quantities determined must largely derive from amorphous silica. The likelihood of Al being extracted from gibbsite is improbable as its formation in such young soils in which leaching of silica is not favored is very low. The molar ratio SiO2(NaOH)/Al2O3(NaOH) is greater than 2.5 (Table 7) indicating that in these soils colloid have been hydrolyzed in an alkaline environment (Stogiannis, 1971). The results of the extraction of soils with Tiron are shown in Table 5. The values of silica extracted with Tiron (SiT) range from 0.71% to 2.18% and are lower than those extracted with hot NaOH. This difference may be explained by the fact that Tiron is more effective in extracting noncrystalline components from aluminosilicates without affecting crystalline components (Kodama and Ross, 1991).

Tiron is as effective as oxalate in dissolving allophane, imogolite and anhydrous oxides as ferrihydrite. The difference between silica dissolved by Tiron (SiT) and that dissolved by oxalate (Sio) can be used as a measure for amorphous silica determination. Values of silica dissolved by oxalate are very low (0.022%). Consequently, all quantities of silica dissolved by Tiron were derived from amorphous silica. Values of alumina extracted with Tiron (AlT) range from 0.24% to 0.81% and on average is the same as that extracted by hot NaOH. Generally, for the formation of allophane and imogolite, conditions should prevail that ensure a high presence of Al and Si while minimizing factors which lead to a reduction in Al, such as the presence of high amounts of organic matter at low pH (Wada, 1989). The pH of the soils studied and the soil organic matter content are not prohibitive for the production of allophane and imogolite. The major reason for the lack of noncrystalline forms is the slow weathering of the parent material, resulting in the slow release of small quantities of Si and Al such that saturation of the soil solution with Al is not possible. In the case of the Nisyros soils, the SiO2-rich parent material, combined with low levels of rainfall, restricted leaching and an extended dry season, result in accumulation

S. Drouza et al. / Catena 70 (2007) 340–349

347

Table 5 Results of selective dissolution analysis of the soils studied Horizon

Depth (cm)

Fed

Feo

Fep

Ald

Alo

Alp

AlNaOH

Sio

SiNaOH

SiT

AlT

0.02 0.01 0.01

0.02 0.02 0.01

0.02 0.02 0.01

0.02 0.01 0.00

0.20 0.22 0.12

0.01 0.01 0.00

0.83 1.07 0.94

0.98 1.09 0.71

0.40 0.53 0.24

2nd sampling site (trachyandesitic conglomerates) A 0–30 0.44 0.35 0.02 C 30–60 0.48 0.34 0.02

0.04 0.04

0.06 0.06

0.04 0.04

0.56 0.62

0.01 0.01

2.33 2.71

2.18 1.83

0.70 0.81

3rd sampling site (andesites) A 0–30 0.55 C 30–60 0.63

0.23 0.17

0.01 0.01

0.03 0.05

0.03 0.05

0.02 0.01

0.34 0.62

0.00 0.01

1.75 2.76

1.16 1.50

0.42 0.47

4th sampling site (pumice) A 0–30 C 30–60

0.59 0.69

0.17 0.1

0.04 0.03

0.03 0.03

0.03 0.03

0.03 0.03

0.90 1.13

0.01 0.01

2.04 2.43

1.51 1.78

0.31 0.48

5th sampling site (volcanic ash) A 0–30 0.48 C 30–60 0.54

0.1 0.15

0.02 0.02

0.02 0.02

0.02 0.02

0.02 0.02

0.63 0.77

0.02 0.01

1.65 1.83

1.25 1.05

0.33 0.36

6th sampling site (volcanic ash) A 0–30 0.33 C1 30–60 0.33 C2 60–90 0.37

0.18 0.08 0.08

0.01 0.01 0.01

0.02 0.02 0.03

0.02 0.03 0.03

0.02 0.02 0.03

0.24 0.28 0.42

0.01 0.01 0.01

1.43 1.50 1.99

1.14 1.04 1.69

0.45 0.48 0.76

0.02 0.01

0.02 0.02

0.02 0.09

0.01 0.02

0.33 0.33

0.01 0.02

2.63 3.16

2.08 1.88

0.61 0.45

%

1st sampling site (trachyandesitic conglomerates) A 0–20 0.25 0.24 C1 20–60 0.22 0.18 C2 N60 0.15 0.01

7th sampling site (trachyandesitic conglomerates) A 0–70 0.44 0.05 C N70 0.78 0.06

of Si in the soil solution and its deposition in amorphous forms. 5. Soil properties and classification As reported in the introduction, Alo + 1/2Feo and Pretention values must be greater than 2% and 0.4% for andic, or 0.4% and 25% for vitric soil properties, respectively. For all samples tested, values of the Alo + 1/2Feo index are extremely low (b0.24%) (Table 6); in addition, the P-retention index values are b12.6%, much lower than the 25% required. Hence these soils do not exhibit either andic or vitric soil properties and cannot be classified as Andisols. It is well known that Andisols are not commonly found under xeric moisture regimes. Of the approximately 350 Andisol pedons identified by ICOMAND (International Committee on the Classification of Andisols) only 15 have a xeric soil moisture regimes and one-half of these are in northern California (Leamy, 1988). Previous studies shows that have been found soils with under xeric moisture regimes and with volcanic parent material that meeting the requirements to be classified as Andisols. Takahashi et al. (1993) found a transition of Andisols–Inceptisols–Alfisols soils in a xeric moisture regime in California with 1000 mm annual precipitation in

andesitic lava and andesitic basalt flows parent materials (late Miocene and early Pliocene age). Broquen et al. (2005), studying five profiles in a bioclimatic sequence of volcanic ash soils of Argentina, reported that being a drier climate drier and combined with changing vegetation changing has a resulted in differentiation in soil classification of the most drier profiles and the others classified as Humic Vitrixerand (Andisols) and the other as Vitrandic Haploxeroll (Mollisols). In most of the above cases, precipitation was about 1000 mm/year, that is much greater than this at occurring in Nisyros Island. The climatic condition, the semi-arid vegetation type and the relatively high pH values are not the favorable conditions for the formation of amorphous constituents (such as allophane) and Al–humus. Parfitt et al. (1984) examined the allophane formation in a rhyolitic volcanic ash bed (20.000 years age) under different leaching environments in New Zealand and their results showed that with decreasing of annual precipitation, and consequently the drainage, characteristically has as results in a characteristic decreasing in allophane content of the clay fraction. Jahn et al. (1987) studying a chronosequence in four landscapes (250 years old, subrecent, young pleistocene and late tertiary) of soils that are forming in on volcanic parent materials (basic to

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ultrabasic pyroclastic fall deposits and basalt) under a semiarid climatic type (140 mm average annual precipitation) found that X-ray amorphous formations like allophane exist for a limited period of time at the early stage of soil formation and in the older soils dominates smectite formation and illitization. On the basis of Soil Taxonomy, the soils of Nisyros belong to the Entisol order, psamments Suborder and xeropsamments great group. 6. Conclusions The soils of Nisyros have not yet formed noncrystalline minerals such as allophane, imogolite, ferrihydrite, or Al and Fe–humus complexes due to the very slow rate of weathering, restricted leaching, the longevity of the dry season and high pH. With such low rates of weathering, the Al and Fe, which are soluble, participate in the formation of phyllosilicate clay mineral structures, whilst silica accumulates and precipitates in amorphous forms. The soil forming factors which distinguish Nisyros soils from those developing on volcanic material worldwide would appear to be: a) the nature of the parent material, where great porosity prevents the retention of soil moisture long enough to allow chemical weathering and b) the local climate which, due to its short wet

Table 6 Index values of selective dissolution of the soils studied Horizon Depth (cm)

Alo + 1/ 2Feo

(Alo–Alp)/ Sio

(Fed–Feo)100/ Fed

Table 7 Al2O3 and SiO2 % values of the soils studied extracted with 0.5 N NaOH Horizon Depth (cm)

Al2O3NaOH SiO2NaOH 2SiO2Al2O3 Excess SiO2/ SiO2 Al2O3 %

1st sampling site (trachyandesitic conglomerates) A 0–20 0.38 1.77 0.83 C1 20–60 0.41 2.30 0.89 C2 N60 0.22 2.02 0.48

1.32 1.82 1.76

4.66 5.60 9.18

2nd sampling site (trachyandesitic conglomerates) A 0–30 1.06 4.99 2.30 C 30–60 1.17 5.80 2.54

3.75 4.43

4.71 4.96

3rd sampling site (andesites) A 0–30 0.64 C 30–60 1.17

3.75 5.90

1.39 2.55

3.00 4.52

5.86 5.04

4th sampling site (pumice) A 0–30 1.69 C 30–60 2.13

4.36 5.21

3.68 4.64

2.37 2.70

2.58 2.45

5th sampling site (volcanic ash) A 0–30 1.18 3.53 C 30–60 1.45 3.91

2.57 3.15

2.14 2.21

2.99 2.70

6th sampling site (volcanic ash) A 0–30 0.45 3.06 C1 30–60 0.53 3.20 C2 60–90 0.79 4.26

0.98 1.15 1.72

2.53 2.58 3.33

6.80 6.04 5.39

7th sampling site (trachyandesitic conglomerates) A 0–70 0.62 5.63 1.35 C N70 0.61 6.76 1.33

4.90 6.04

9.08 11.08

SiO2/ Al2O3

1st sampling site (trachyandesitic conglomerates) A 0–20 0.14 0.48 4.32 C1 20–60 0.11 0.32 16.51 C2 N60 0.02 3.30 93.04

4.66 5.60 9.18

2nd sampling site (trachyandesitic conglomerates) A 0–30 0.24 1.96 20.05 C 30–60 0.23 1.40 27.84

4.71 4.96

3rd sampling site (andesites) A 0–30 0.15 C 30–60 0.13

2.85 5.63

58.22 72.46

5.86 5.04

4th sampling site (pumice) A 0–30 0.11 C 30–60 0.08

0.60 0.26

71.02 85.18

2.58 2.45

5th sampling site (volcanic ash) A 0–30 0.07 C 30–60 0.09

0.14 0.28

79.56 72.75

2.99 2.70

6th sampling site (volcanic ash) A 0–30 0.11 C1 30–60 0.07 C2 60–90 0.07

0.19 0.35 0.65

46.00 75.18 78.11

6.80 6.04 5.39

7th sampling site (trachyandesitic conglomerates) A 0–70 0.05 0.77 88.77 C N70 0.12 3.30 91.69

9.08 11.08

season and extended dry season, contributes to the restricted action of soil moisture. This particular combination of parent material and climate prevents rapid soil genesis and thus the soil formation is very low. The soils of Nisyros, whilst forming on purely volcanic material, are not classified as Andisols, but rather as Entisols (Vitrandic Xeropsamment). The soils in Nisyros whilst forming for a greater period of time than those in Thera, by at least 5000 years, are not characterised by andic soil properties. Acknowledgements The authors would like to thank Susan Coward for her invaluable help in the preparation of this manuscript. Ph.D. student F. Georgoulias would like to thank also Greek Scholarship Foundation (I.K.Y.) for its economical support. References Aran, D., Zida, M., Jeanroy, E., Herbillon, A.J., 1998. Influence of parent material and climate on andic properties of soils in a temperate montane region (Vosges, France). Eur. J. Soil Sci. 49, 269–281. Bascomb, C.L., 1968. Distributions of pyrophosphate extractable iron and organic carbon in soils of various groups. J. Soil Sci. 19, 251–268.

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