The relationship between pedogenic and geomorphic processes in mountainous tropical forested area in Sierra Madre del Sur, Mexico

Catena 62 (2005) 14 – 44 www.elsevier.com/locate/catena

The relationship between pedogenic and geomorphic processes in mountainous tropical forested area in Sierra Madre del Sur, Mexico P.V. Krasilnikova,c,1,2, N.E. Garcı´a Caldero´na,T,1, S.N. Sedovb,3, E. Vallejo Go´mezb,3, R. Ramos Belloa,1 a

Lab. Edafologı´a Nicola´s Aguilera, Facultad de Ciencias, UNAM, DF 04510, Me´xico b Dep. Edafologı´a, Instituto de Geologı´a, UNAM, DF 04510, Me´xico c Laboratory of Soil Ecology and Soil Geography, Institute of Biology, Karelian Research Centre RAS, Petrozavodsk 185610, Russia Received 24 June 2003; received in revised form 14 February 2005; accepted 16 February 2005

Abstract Both mature and underdeveloped soils are present in tropical mountainous landscapes. The spatial arrangement of mountainous soils is ascribed mainly to geomorphologic processes. We studied two soil toposequences (one on a convex, and the other on a concave slope with a gradient 40–60%) at the coffee-growing farm La Caban˜a, situated in the mountains of the Sierra Madre del Sur, southern Mexico. Mature (Alisols) as well as moderately developed (Luvic Phaeozems) and underdeveloped soils (Fluvic and Skeletic Phaeozems) were detected in the study area. The sequence of sediments and soils is unusual for a classical soil catena. Buried clayey reddish soils are present on the shoulder of a slope; colluvial sediments with weakly developed soils form convex and concave footslopes. The peculiarity of the slopes and spatial distribution of soils in the studied toposequences were ascribed to intensive linear erosion (due to tectonic uplift), seismically induced landslides, and K-cycles of laminar erosion. The diversity of sediments leads to considerable variation in soil texture

T Corresponding author. E-mail address: [email protected] (N.E.G. Caldero´n). 1 Tel.: +52 55 56 22 49 22. 2 Tel.: +7 8142 76 98 10. 3 Tel.: +52 55 56 22 42 86. 0341-8162/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.catena.2005.02.003

P.V. Krasilnikov et al. / Catena 62 (2005) 14–44

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and mineralogical composition, and, thus, to the diversity of soil properties, which is considered to be a positive feature of a landscape. D 2005 Elsevier B.V. All rights reserved. Keywords: Humid tropics; Mountains; Pedogenesis; Catena; Landform stability

1. Introduction Soil formation in mountainous areas is strongly affected by active geomorphic processes (Birkeland, 1999). Where erosion dominates, the majority of soils are formed on fresh colluvial materials or on exposed erosional surfaces (Feldman et al., 1991). Thus, the bcentral imageQ of soils formed in mountains is a weakly developed profile formed on recent sediments (Graham et al., 1988). However, because the time of soil development depends particularly on the slope, aspect, and vegetation of the site, soils in mountainous areas are arranged in a kind of mosaic formed by a combination of slope processes and pedogenesis (Graham and Buol, 1990; Graham et al., 1990). The mosaic of soils at various stages of development is illustrative in tropical mountains where geomorphic processes and pedogenesis are both intensive (Embrechts and Sys, 1988). Mature soils as well as profiles at incipient stages of soil formation are present there, and the difference in morphology between these soils is striking (Rohdenburg, 1983; Schaefer et al., 2002). Although the slopes are severely affected by slumps and landslides in steeplands, well-developed deep soils occupy about 25% of slopes with gradients of 45–90% (Drees et al., 2003). Sometimes, mature soils in tropical mountainous areas are considered to be related to ancient stable surfaces, which were dissected by recent erosional processes. The change in the balance between erosion and weathering is ascribed to climatic change (Birkeland, 1999), land use change (Glade, 2003), or catastrophic events (Rohdenburg, 1983; Wilcke et al., 2003). From another point of view, the zero point for soil formation and geomorphic processes is the same, but the intensity of slope processes varies in space and changes over time. Periodical refreshment of parent material is localized in gullies, areas of landslides, and steep slopes. A slope is generally dynamic, but there are zones of stability where the rate of pedogenesis is higher than the rate of denudation or accumulation, and mature soils do form. We had several objectives in this study. The first one was to understand the origin of the slopes and sediments of some typical toposequences in the Sierra Madre del Sur mountains of southern Mexico. The other objective was to investigate the effect of geomorphic processes on the composition of parent material and pedogenesis. Finally, we wanted to study the specific features of mature and younger soils forming in dynamic conditions of mountain slopes in the study area.

2. Materials and methods The research was conducted at the coffee-growing farm La Caban˜a, situated in the Pluma Hidalgo municipality, Oaxaca state, Mexico (Fig. 1). The study area is located at

16

P.V. Krasilnikov et al. / Catena 62 (2005) 14–44

Mexico Pacific Ocean

Mexico City

1,500 km

Oaxaca state

La Cabaña 200 km

2

1

300 m

Fig. 1. Topographic map of the study area (coffee-growing farm La Caban˜a): lines 1 and 2 are for toposequences La Can˜eria and Palo Piedra, respectively.

P.V. Krasilnikov et al. / Catena 62 (2005) 14–44

17

15854V48UN and 96823V19UW. This region represents a typical landscape of the southwestern macroslope of the Sierra Madre del Sur mountains, the system formed by a tectonic uplift in the Miocene (Mora´n et al., 1996); minor uplifts also occurred in the Pliocene and even the Quaternary. The rocks are mainly gneiss and amphibolites formed during the Paleozoic epoch, and Cenozoic granites (Herna´ndez et al., 1996). The entire Pacific coast of Mexico is a seismically active zone (Rojas et al., 1987). Several major earthquakes in this area were reported in the 20th century, with the most recent being registered in 1999. The farm La Caban˜a is situated at an altitude of 650–1200 m above sea level (m asl). The mountain slopes are complex, with aspects varying from north-eastern to southwestern and with inclination averaging about 408. The surface of the slopes is dissected by stable and vegetated gullies, and the foot is a pediplain. The climate of the region is classified as warm humid isothermal with annual precipitation of 1800–2000 mm and a mean annual temperature of 21–21.9 8C (Garcı´a, 1973). The region has two main seasons—dry from December through May and wet from June through November. Vegetation in the area consists of coffee plantations (Coffea arabica var. tipica L.) under the canopy of residual natural vegetation: Bosque tropical subcaducifolio, Tropical Semideciduous Forest (Rzedowski, 1978). The most abundant natural plant species of the zone are: Brosimum alicastrum, Enterolobium cyclocarpus, Pterocarpus acapulcensis, Bursera simaruba, Caesalpinia coriacea, Ceiba pentandra, Cordia aliodora, and Ficus spp. (Lorence and Garcı´a, 1989; Flores and Manzanero, 1999). Coffee growing under the shade of natural vegetation is a common agroforestry practice in the study area; the quality of coffee grown under canopy is believed to be high, and the natural ecosystem is conserved due to relatively dense tree vegetation (Staver, 1998). A soil study of the site was published recently (Garcı´a et al., 2000); according to the data presented, the soils are mainly Acrisols (A/(E)/Bt/C), Luvisols (A/(E)/Bt/C), and Cambisols (A/Bw/C) (FAO-ISRIC-ISSS, 1998). The total area of the farm is 700 ha. Two soil toposequences on the altitudes ranging from 750 to 800 m asl were selected for study: one on a convex slope (La Can˜eria), and the other on a concave slope (Palo Piedra). Morphological description was done according to Schoeneberger et al. (2002); colours were identified using Munsell Soil Color Charts (2000); field observations were supported by mesomorphological studies using a stereoscopic microscope. Soils were classified according to the World Reference Base (FAO-ISRIC-ISSS, 1998). Samples for chemical analysis were collected from soil horizons, and analysed according to the routine methods of soil chemical and physical analysis (van Reeuwijk, 2002). In each sample, we determined: texture using the pipette method, pH with a glass electrode–calomel electrode pH meter method (H2O and 1 M KCl extraction with a soil:solution ratio 1:2.5), organic C by wet combustion, exchangeable bases by the NH4Ac method, cation exchange capacity as a sum of exchangeable bases and exchangeable acidity by the BaCl2-triethanolamine method, and iron extracted by dithionite–citrate–bicarbonate and acid oxalate buffer solutions. Thin sections were obtained from the horizons, where the microstructure seemed to be potentially informative, and were studied under a petrographic microscope. The clay fraction was separated and pre-treated using the method of Dixon and White (1999). X-ray difractograms were obtained on a diffractometer DRON-3 (SIE bBurevestnik,Q St. Petersburg, Russia, 1987), Cu-Ka radiation with graphite monochromer, 2h 2–458, U = 40 kV, I = 25 mA.

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P.V. Krasilnikov et al. / Catena 62 (2005) 14–44

3. Results 3.1. Toposequence 1 (La Can˜eria) The first toposequence consists of eight profiles on a slope underlain by a gneiss rock. The first profile was made on a slope summit at an altitude of 790 m asl (Fig. 2a). The second and third profiles were made on a convex slope at altitudes of 770 and 755 m asl, respectively. The fourth, fifth, and sixth profiles were made on a linear backslope at altitudes of 748, 730, and 717 m asl, respectively. The last two profiles were made on a convex footslope at altitudes of 702 and 692 m asl. Below, there is a small river flowing across the slope. 3.1.1. Soil morphology A brief morphological description of the studied profiles is presented in Table 1. Profile 1.1, classified as an Umbric Leptosol (A/R) (FAO-ISRIC-ISSS, 1998), is a darkcoloured shallow soil with a good granular structure. Fragmented anorthositic rock with humus-rich material in cracks underlies a thin layer of fine earth. Profiles 1.2 and 1.3, classified as Chromic Alisols (A/EB/Bt/C), are relatively deep red-coloured soils. The upper 65 cm of the Profile 1.3 consist of a subprofile with A, Bw, BC, and C horizons. Micromorphological study showed that the Bw horizon has few thin clay coatings (Fig. 3a) occasionally fragmented by biogenic turbation (Fig. 3b). In the BC and C horizons of this profile, the amount of clay coatings and biogenic aggregates decreases. At a greater depth, there are buried soils; in Profile 1.3, it is tentatively divided into 2EBb, 2Btb, and 2BCb horizons. Horizon 2EBb is mottled with red and reddish-yellow zones. The number of clay coatings increases and is much more frequent in the red zones compared to the bleached zones (Fig. 3c). Horizon 2Btb has a clay texture, well-developed clay coatings both in voids and on ped surfaces (Fig. 3d), and very few soft rock fragments. The structure is well-developed subangular blocky. Both 2EBb and 2Btb horizons of the buried profile are poor in weatherable minerals: the majority of these minerals are strongly altered. Horizon 2BCb is similar to 2Btb, but the content of clay coatings decreases, while the number of saprolite fragments increases. Profile 1.2 is similar to Profile 1.3, but less complex: the upper layer is subdivided into A and EB horizons, while the buried profile has only a 2Btb horizon. Profile 1.4, classified as Chromic Cambisol (Bw/C), is an eroded soil without any humus-enriched horizon. Rather, a reddish Bw horizon lies right at the surface under a shallow discontinuous layer of slightly decomposed litter. This horizon is similar in morphology to the 2Btb horizons of Profiles 1.2 and 1.3, but lacks overlying soil; thus, it is impossible to classify it as an argic horizon. Profiles 1.5 and 1.6, classified as Skeletic Phaeozems (A/Bw/C), are stony and poorly developed. The upper horizon of Profile 1.5 has a granular structure and relatively high biological activity. The soil contains unweathered rock fragments of various diameters; their content increases with depth from 30% to more than 90%. Profile 1.6 has a complex morphology. The upper horizons (A and AB) have a dark colour, well-developed granular structure, and high content of unweathered rock fragments (40–50%). Under this material,

P.V. Krasilnikov et al. / Catena 62 (2005) 14–44

a

A Bw BC C 2Ebb 2Btb 2BCb

- hard rock (gneiss and amphibolite) - sandy saprolite - coarse stony colluvial sediments

Bw BC C

- sandy colluvium - red clayey material A1 A2 AB Bw1 Bw2 BC C

1.8

A1 A2 Bw 2Ab 2Bwb 3Ab 3Bwb 3BCb 3Cb

A AB 2Bw 2BC

A AB Cr

19

A AE 2Btb

A1 A2 AR

1.1

1.2

1.3 1.4

1.5

1.6

1.7

b A 2Bwb 3Bwb 4Bwb 4BCb

A ABw R

2.5

A EB Bw R

A EB Bw Cr

A Bw 2Ab 2Bw1 2Bw2 2BCb

2.1

2.2

2.3 2.4

Fig. 2. Schematic profiles of the physiography and lithology of the toposequences La Can˜eria (a) and Palo Piedra (b). Dotted line is for a probable equilibrium slope.

there are two horizons: 2Bw and 2BC. These horizons are yellowish, angular, and subangular blocky structure, and the content of rock fragments is much lower than in the overlying horizons.

20

Table 1 Morphological description of soil profiles of the toposequence La Caban˜a, Pluma Hidalgo, Oaxaca State, Mexico Horizon, depth [cm]

Colour (moist)

Mottles (colour) Field Structureb a texture

Profile 1.1. Umbric Leptosol, summit, slope 0%, 790 m asl A1 0–12 10YR 3/3 dark brown SL 10YR 3/3 dark brown 10YR 3/3 dark brown

SL SL

Clay coatingsc

Rootsd

Soil fauna

Poresd

Bordere

GR

10

N

N

3vf,f,m

2vf

S

N N

N N

2f,m 1f

Worms and lichens Worms N

GR GR

30 b90

2vf 1f,m

I

3vf,f,m 2co Worms, lichens, 2vf insects, diplopoda 2f,m 1co Worms, lichens 3m,co 2f,vf 1m,co 2f,m

In On voids aggregates

Profile 1.2. Chromic Alisol, convex slope 60%, 770 m asl A 0–40 5YR 3/3 dark SL reddish brown EB 40–66 5YR 7/4 pink SL

GR

20

N

VF

SAB

30

N

C

2Btb 66–100

SC

ABK, PR

30 (increase C with depth)

M

Profile 1.3. Chromic Alisol, convex slope 50%, 755 m asl A 0–20 7.5YR 3/3 dark brown CL

GR, SAB

b10

F

VF

Bw 20–30

CL

SAB, PR

10

C

CL

SAB

10

SCL

SAB, PR

CL C SCL

BC 30–40 C 40–65 2EBb 65–85

2.5YR 3/6 dark red

5YR 3/4 dark reddish brown 5YR 4/4 reddish brown 5YR 5/6 yellowish red

5YR 7/8 reddish yellow 2Btb 85–165 2.5YR 5/6 red 2BCb 165–200 2.5YR 4/8 red

2.5YR5/8 red

5YR7/8 reddish yellow

W W

C

3vf,f,m 2co Worms, insects, diplopoda 2m,co 1vf,f Worms, insects

2f,m 1co W 2m 1co

I

F

VF

2m 1f

2m,co

I

20

VF

N

1m,f

PR, ABK

10

N

VF

PR PR, SAB

b10 10

M F

C VF

1m

Few worms, insects Few worms, insects Very few worms

2f,m

I

0 0

No No

1f 1f,m

W

3m,co 2f W

P.V. Krasilnikov et al. / Catena 62 (2005) 14–44

A2 12–20 AR 20–70 (in cracks)

Rock fragments [vol.%]

Profile 1.4. Chromic Cambisol, concave slope 70%, 748 m asl Bw 0–50 7.5YR 6/6 reddish SL yellow BC 50–65 10YR 6/6 brownish SL yellow C 65–120 2.5Y 6/4 light 2.5Y 6/6 olive LS yellowish brown yellow

VF

N

3m 2f

ABK, SAB 20–30

N

N

2m 1f

SAB

50–60

N

N

1m,f

GR

30

N

N

2m,f

W

GR, SAB

80

N

N

1co,m 3f,vf Worms, biogenic tunnel 1m Very few worms

1m,vf

S

N90

N

N

2m 1f,vf W

b10

Profile 1.6. Skeletic Phaeozem, backlope 50%, 717 m asl A 0–35 10YR 5/3 brown SCL

GR

40

N

N

2m 3f,vf

AB 35–78

SL

GR, ABK

50

N

N

SL

ABK, SAB 20

N

SCL

SAB

b10

GR GR, ABK

2Bw 78–100

10YR 5/4 yellowish brown 2.5Y 8/3 pale yellow

2BC 100–145

2.5Y 8/3 pale yellow

2.5Y 5/4 pale yellow

2Bbw 80–93

10YR 5/8 yellowish brown 10YR 7/4 very pale brown

7.5YR 5/8 strong brown

3g,m,f

W

3g,m 2f

W

1m

2fvf

Larvae, ants, few worms Very few worms

3m,f,vf

W

N

1m,f

No

2m,f,vf

S

N

N

1f

No

2f,vf

b5 5–10

N N

N N

2m 3f,vf 1m 2f

1m 3f,vf W 2m 3f,mf W

SAB

b5

N

N

1co,f 2m

SL

SAB

0

N

N

1m,f

Worms, larvae Larvae, worms, biogenic tunnel Few worms, larvae Very few worms

SL

SAB

0

N

N

1m,f

No

1m 2f,vf S

Profile 1.7. Fluvic Phaeozem, convex slope 50%, 705 m asl A1 0–13 10YR 4/3 brown SL A2 13–50 10YR 5/4 yellowish SL brown Bw 50–75 10YR 7/6 yellow SL 2Ab 75–80

Few insects, worms Ants, very few worms No

P.V. Krasilnikov et al. / Catena 62 (2005) 14–44

Profile 1.5. Skeletic Phaeozem, backslope 50%, 730 m asl A 0–30 10YR 5/4 yellowish 7.5YR 6/4 SL brown light brown ABw 30–70 7.5YR 6/4 light 7.5YR 6/6 SL brown reddish yellow CR 70+ 7.5YR 6/6 reddish yellow

SAB

1m 2f,vf S 1m 2f,vf W

21

(continued on next page)

22

Table 1 (continued) Horizon, depth [cm]

Colour (moist)

3Ab 93–97

Rock fragments [vol.%]

Clay coatingsc

Rootsd

Soil fauna

Poresd

Bordere

In On voids aggregates

brownish

SL

SAB

b5

N

N

1m,f

No

1m 2f,vf W

very pale

SL

SAB

5–10

N

N

1m,f

No

2f,vf

W

very pale

SL

SAB

10–20

N

N

1m

No

2f,vf

W

very pale

SL

SAB

20

N

N

0

No

1f,vf

GR

0

N

N

1co 3m,f 2vf 1co 2m 3f,vf

Worms, larvae

3m,f,vf

Larvae, worms, biogenic tunnel

2m 3f,vf I

2m 3f,vf S

Profile 1.8. Haplic Phaeozem, convex slope 70%, 692 m asl A1 0–13 2.5Y 4/2 dark greyish LS brown SL A2 13–25 10YR 5/4 yellowish 2.5YR 3/4 brown dark reddish brown AB 25–52 10YR 7/6 yellow SL

GR, ABK

b5

N

N

ABK, GR

b5

N

N

2m,f

Bw1 52–108

SL

ABK

10–20

F

N

1m,f

Few worms, larvae Very few worms

brownish

SL

ABK, SAB 5–10

N

N

1m,f

No

1co 2m,f,vf 1m,f,vf

yellow yellow

SL LS

SAB, ABK 10–20 SAB b5

N N

N N

1m,f 1f

No No

1co,m,vf S 2co,m,vf

Bw2 108–142 BC 142–169 C 169–194 a b c d e

10YR 6/6 yellow 10YR 6/6 yellow 10YR 7/6 10YR 7/6

brownish

7.5YR 4/6 strong brown

LS–loamy sand; SL—sandy loam; SCL—sandy clay loam; CL—clay loam; C—clay. GR—granular; SAB—subangular blocky; ABK—angular blocky; PR—prismatic. N—no clay films; VF—very few; F—few; C—common; M—many. 0—no roots or pores; 1—few; 2—common; 3—many; vf—very fine; f—fine; m—medium; co—coarse. S—smooth; W—wavy; I—irregular,; B—broken.

W

W W

P.V. Krasilnikov et al. / Catena 62 (2005) 14–44

10YR 6/6 yellow 3Bbw 97–140 10YR 7/4 brown 3BCb 140–160 10YR 7/3 brown 3Cb 160–180 10YR 8/2 brown

Mottles (colour) Field Structureb texturea

P.V. Krasilnikov et al. / Catena 62 (2005) 14–44

a

23

b CC

CC

CC

CC 400 µm

c

400 µm

CC

d

400 µm

400 µm

Fig. 3. Soil micromorphology of Profile 1.3 (Chromic Alisol): (a) thin clay coatings (CC) in the horizon Bw; (b) biogenically fragmented clay coatings in the same horizon; (c) bleached compact zone of the EB horizon; (d) clay infillings in pores in the 2Btb horizon.

Profile 1.7 is classified as Fluvic Phaeozem (A/Bw/C) and has three distinct sequences of horizons with two buried profiles (Ab/Bwb). The whole profile has well-sorted silt loam material with no or few stones or gravel; only in the deeper sub-profile, starting from 93 cm, does the content of rock fragments increase with depth. Soil material is porous throughout the profile and has a weak structure with no silt or clay coatings. Profile 1.8 is classified as Haplic Phaeozem (A/Bw/C) and is similar in morphology to Profile 1.7, but lacks distinct buried soils. Except for the upper humus-enriched horizon, the whole profile is poorly differentiated into horizons. Content of rock fragments is low, with some accumulation in the middle of the profile. 3.1.2. Soil chemical properties The chemical properties of the soils are summarized in Table 2. In Profile 1.1 (Umbric Leptosol), the upper A subhorizon is acidic with base saturation slightly below 50%. The deeper subhorizon is less acidic and has a base saturation of almost 70%. Profiles 1.2 and 1.3 (Chromic Alisols) are slightly acidic in the surface horizon but pH values (both in water and KCl extracts) drop with depth. Content of exchangeable bases is relatively high in the A horizon and decreases with depth. Ca is the main

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Table 2 Chemical and physical properties of soil profiles of the toposequence La Caban˜a, Pluma Hidalgo, Oaxaca State, Mexico pHH2O

pHKCl

Clay [g kg

Ca2+ 1

Mg2+

K+

Na+

EAa

CEC

] cmolc kg

CEC kg clay 1b

BS [%]

1

C g kg

Fed

Feo

Feo/Fed

1

Profile 1.1. Umbric Leptosol, summit, slope 0%, 790 m asl A1 0–12 4.9 3.8 152.0 5.6 A2 12–20 5.3 4.1 295.8 15.1

2.8 4.4

1.3 0.2

0.2 0.2

10.2 10.2

20.1 33.7

132.2 113.9

49.3 69.7

16.8 33.0

10.7 6.7

3.0 2.4

0.28 0.36

Profile 1.2. Chromic Alisol, convex slope 60%, 770 m asl A 0–40 5.2 4.6 339.8 8.9 EB 40–66 4.7 3.8 372.0 2.4 2Btb 66–100 4.6 3.7 399.8 1.1

3.8 3.1 1.6

0.2 0.1 tr.

0.2 0.3 0.1

6.3 5.2 8.7

19.4 11.1 11.5

57.1 29.8 28.8

67.5 53.1 24.4

18.4 3.2 2.8

27.1 22.5 44.7

3.3 2.6 3.5

0.12 0.12 0.08

Profile 1.3. Chromic Alisol, convex slope 50%, 755 m asl A 0–20 5.1 4.5 371.8 10.4 Bw 20–30 4.9 4.1 367.8 6.9 BC 30–40 4.9 4.0 379.8 7.3 C 40–65 4.5 3.4 279.8 5.6 2EBb 65–85 4.5 3.7 339.8 4.5 2Btb 85–165 4.6 3.9 400.0 1.8 2BCb 165–200 4.4 3.7 439.8 0.7

3.9 3.8 4.0 4.2 4.2 2.8 2.7

0.2 0.1 0.1 tr. tr. tr. tr.

0.3 0.2 0.4 0.4 0.3 0.2 0.3

7.2 5.8 6.8 7.5 6.8 8.7 10.0

22.0 16.8 18.6 17.7 15.8 13.5 13.7

59.2 45.7 49.0 63.3 46.5 33.8 31.2

67.3 65.5 63.4 57.6 57.0 35.6 27.0

16.8 5.2 6.0 2.4 0.8 0.8 0.8

23.2 24.8 19.8 10.2 31.1 64.8 64.4

3.0 1.4 1.2 0.9 1.1 3.3 3.3

0.13 0.05 0.06 0.09 0.04 0.05 0.05

slope 70%, 748 m asl 127.7 9.9 2.7 131.7 22.4 2.8 197.0 7.4 2.7

0.1 0.1 tr.

0.1 0.3 0.6

9.1 9.1 5.6

21.9 34.7 16.3

171.5 263.5 82.7

58.4 73.8 65.6

8.9 2.4 0.6

24.7 25.0 16.6

6.4 5.2 4.1

0.26 0.21 0.25

Profile 1.4. Chromic Cambisol, concave Bw 0–50 5.4 3.8 BC 50–65 5.3 3.2 C 65–120 5.5 3.1

P.V. Krasilnikov et al. / Catena 62 (2005) 14–44

Horizon, depth [cm]

Profile 1.5. Skeletic Phaeozem, backlope 50%, 730 m asl A 0–30 5.7 4.5 131.7 15.7 ABw 30–70 5.0 3.0 83.7 23.0

3.5 3.3

0.2 0.1

0.2 0.6

8.9 7.9

28.5 34.9

216.4 –

68.8 77.4

17.6 12.1

24.2 21.7

4.8 4.1

0.20 0.19

Profile 1.6. Skeletic Phaeozem, backlope A 0–35 5.5 4.8 AB 35–78 5.2 3.7 2Bw 78–100 5.3 3.3

50%, 717 m asl nd 13.3 119.7 17.0 123.7 10.1

2.6 3.0 1.3

0.2 0.1 tr.

0.1 0.3 0.3

6.1 8.3 5.4

22.3 28.7 17.1

nd 239.8 138.2

72.6 71.1 68.4

23.4 11.0 4.2

25.0 17.0 11.0

5.1 4.8 3.3

0.20 0.28 0.30

Phaeozem, 6.2 6.0 5.8 5.7 5.6 5.5 5.3

convex slope 50%, 705 m asl 5.5 191.7 14.0 4.2 175.7 11.5 3.5 167.7 7.2 3.2 127.7 6.7 3.0 87.7 7.4 3.0 144.0 3.8 2.5 103.7 7.4

1.7 1.9 2.2 2.1 1.2 1.5 2.6

0.2 0.1 tr. tr. tr. tr. tr.

0.2 0.3 0.3 0.2 0.2 0.3 0.3

6.5 6.4 4.5 2.6 2.9 3.0 5.9

22.6 20.2 14.2 11.6 11.7 8.6 16.2

117.9 115.0 84.7 90.8 – 59.7 156.2

71.2 68.3 68.3 77.6 75.2 65.1 63.6

31.4 10.6 3.0 2.8 2.2 1.3 1.2

16.1 16.6 17.0 17.9 17.3 19.0 17.0

4.5 4.6 4.3 4.1 3.1 3.6 2.9

0.29 0.28 0.25 0.23 0.18 0.19 0.17

Profile 1.8. Haplic A1 0–13 A2 13–25 AB 25–52 Bw1 52–108 BC 142–169

Phaeozem, 6.3 6.2 5.7 5.7 5.5

covex slope 70%, 692 m asl 5.6 81.6 20.9 5.3 88.0 14.2 3.6 91.7 21.0 3.6 135.7 18.7 2.8 147.7 6.0

2.6 2.4 3.5 3.4 2.6

0.3 0.2 0.1 0.1 tr.

0.2 0.3 0.3 0.3 0.6

6.2 5.7 5.2 4.2 2.0

30.2 22.8 30.1 26.7 11.2

– – – – –

79.5 75.0 82.7 84.3 82.1

46.2 32.9 3.9 1.9 1.1

17.5 22.3 39.2 12.6 13.0

4.7 4.4 5.2 2.3 2.7

0.27 0.20 0.13 0.18 0.21

a b

Extractable acidity, BaCl2–triethanolamine method (van Reeuwijk, 2002). The value is not calculated for sandy soils.

P.V. Krasilnikov et al. / Catena 62 (2005) 14–44

Profile 1.7. Fluvic A1 0–13 A2 13–50 Bw 50–75 2Bwb 80–93 3Ab 93–97 3Bbw 97–140 3BCb 140–160

25

26

P.V. Krasilnikov et al. / Catena 62 (2005) 14–44

exchangeable base in the A horizon, while in the deeper ones, Mg is present in a higher concentration. Base saturation is over 50% in the upper horizons and much less in the buried profiles. Clay content is relatively high, with an increase from the surface to 2Btb. The content of Fed is high in all the horizons, with maximum values in 2Btb horizons. The content of Feo is much lower, about one-tenth of Fed content; in the three deeper horizons of Profile 1.3, it is 1–20 or less. Its distribution is the same as Fed. Bulk chemical composition of the soils illustrates the difference in the composition of the upper subprofile and the buried soil: silica content is lower, and iron content is higher in the buried soil (Table 3). The horizons of the Profile 1.4 are moderately acidic and base-saturated. Feo content is higher, and Fed content is lower than in the buried Bt horizons of Profiles 1.2 and 1.3. The whole profile is poor in organic C. Skeletic Phaeozems (Profiles 1.5 and 1.6) are characterized by moderate acidity and high base saturation; pH values in KCl extract are surprisingly low (DpH 2). Both profiles have Fed and Feo contents and their ratio is similar to those found in the surface horizons of the upper members of the toposequence. Organic C content decreases with depth. Fluvic and Haplic Phaeozems (Profiles 1.7 and 1.8, respectively) have pH values close to neutral in the surface horizons, and moderately acidic in the deeper ones; DpH values are high. Base saturation is high; organic C content gradually decreases with depth. 3.1.3. Clay and sand mineralogy Composition of the clay fraction from horizons of Profile 1.1 (Umbric Leptosol) is shown in Fig. 4. The three horizons have similar mineralogical composition. The main components are kaolin minerals giving basal signals at 0.72 and 0.365 nm. The kaolin group is presented by both kaolinite sensu stricto and dehydrated halloysite, which is Table 3 Bulk chemical composition of the fine earth of selected soils of the toposequence La Can˜eria, g kg (recalculated to 1 kg) Horizon, depth [cm]

SiO2

TiO2

Al2O3

Fe2O3

FeO

MnO

1

MgO

CaO

Na2O

K2 O

P2O5

Profile 1.1. Umbric Leptosol, summit, slope 0%, 790 m asl A1 0–24 728.6 3.1 207.9 35.8 5.9 A2 24–44 717.8 2.1 209.9 39.2 3.0

0.9 0.7

7.7 6.2

7.1 5.0

9.6 12.9

23.9 25.9

2.3 1.7

Profile 1.2. Chromic Alisol, convex slope 60%, 770 m asl A 0–40 680.1 7.2 222.5 70.7 7.2 EB 40–65 711.8 5.7 209.7 56.2 3.7 2Btb 65–100 618.9 6.9 266.0 98.9 1.0

0.8 0.4 0.3

1.1 9.4 7.7

5.1 0.8 1.6

2.9 1.6 2.0

19.2 20.7 20.9

2.0 1.2 1.6

Profile 1.3. Chromic Alisol, convex slope 50%, 755 m asl A 0–20 700.6 12.3 210.4 72.2 5.9 Bw 20–30 685.1 7.7 236.9 65.6 3.4 BC 30–40 701.0 5.0 237.3 51.9 3.4 C 40–60 722.0 1.6 235.5 36.6 4.9 2EBb 65–85 662.9 5.9 257.6 77.4 0.8 2Bt1b 85–165 585.6 14.0 259.3 148.3 3.5 2Bt2b 165–200 534.3 18.6 283.2 174.3 0.4

0.8 0.5 0.3 0.2 0.2 0.5 0.8

10.6 7.3 7.2 3.6 4.7 8.3 13.2

5.1 5.0 3.4 3.3 5.0 2.6 0.8

1.7 0.8 0.7 0.7 0.6 0.9 0.7

14.3 15.9 16.1 15.6 11.1 3.1 2.9

1.8 1.4 1.1 0.8 1.2 2.1 2.9

P.V. Krasilnikov et al. / Catena 62 (2005) 14–44

27

Fig. 4. X-ray diffractograms of oriented clay fractions of the horizons of Profile 1.1 (Umbric Leptosol); grey line is for the sample saturated by glycerine.

detected by an intensive non-basal signal at 0.445 nm (Dixon, 1989); dehydrated halloysite seems to dominate in both A1 and A2 horizons. In the A horizon, the 2:1 minerals are represented by mica–vermiculite showing a peak of 1.38 nm when air-dried, and 1.01 after heating, and mixed-layered chlorite–vermiculite minerals which contract to 1.17 nm. In the A2 horizon, those minerals are disordered and, after heating, give a broad signal from 1.0 to 1.3 nm. In the AR horizon, all the minerals are better crystallised. After heating, they contract down to 1.01 nm with a bshoulderQ to smaller angles; thus, there are less vermiculate layers in the mineral structure. Some swelling (up to 1.6 nm) was detected after glycerine treatment, which is evidence for the presence of some smectite packages in

28

P.V. Krasilnikov et al. / Catena 62 (2005) 14–44

the structure of clay minerals. Generally, the 1:1 and 2:1 minerals are present in proportional amounts, with an increase in 2:1 components with depth. Fine sand mineralogy of Profile 1.1 is dominated by quartz and quartz–mica aggregates, with a lesser amount of K–Na feldspars (Table 4). Minerals of 1:1 structure, mainly kaolinite, dominate the horizons of Profile 1.2 (Chromic Alisol) (Fig. 5). The presence of halloysite is also detected, but the relation of the intensities of the signals 0.71 and 0.445 is much greater than in the horizons of Profile 1.1. Three minerals giving signals at 1.4, 1.2, and 1.0 nm in air-dried samples represent the 2:1 group. After heating, 1.2 and 1.0 peaks were detected. This combination was interpreted as the presence of mica, mica–vermiculite, and chlorite–vermiculite. The content of 2:1 minerals is relatively high in the A and EB horizons, while in the 2Btb horizon, the presence of these minerals is at the limit of detection. Clay mineralogy of Profile 1.3 (Chromic Alisol) is similar to that of Profile 1.2. The dominant mineral is kaolinite sensu stricto. The amount of halloysite is slightly higher in the 2EB horizon. The presence of 2:1 minerals was detected in all the horizons of the upper subprofile with maximum content in the A and BC horizons. Signals at 1.4, 1.2, and Table 4 Mineralogical composition (%) of fine sand fractions (0.1–0.25 mm) of selected soils horizons of toposequences La Can˜eria and Palo Piedra Horizon, depth [cm]

Quartz Quartz–mica K–Na Plagioclases Biotite Mica Amphiboles Pyroxenes Opaques aggregates feldspars aggregates

La Can˜erı´a Profile 1.1. Umbric Leptosol, summit, slope 0%, 790 m asl A1 0–24 27 47 11 1 2 Profile 1.3. Chromic Alisol, convex slope 50%, 755 m asl A 0–20 43 44 2 1 1 Bw 20–30 27 64 0.5 0.5 0.5 2EBb 16 61 – 1 – 65–85 Profile 1.5. Skeletic Phaeozem, backlope 50%, 730 m asl A 0–30 11 8.5 12 42 2 ABw 21 10 29 17 0.5 30–70 Profile 1.7. Fluvic Phaeozem, convex slope 50%, 705 m asl A2 13–50 30 13 28 33 1 2Ab 12 8 4 51 – 75–80 3Ab 14 10 7 45 – 93–97 Palo Piedra Profile 2.2. Fluvic Phaeozem, Backslope 40%, 789 m asl A 0–35 27.5 9 29 12 2 4BCb 33 10 31.5 19 – 120–150 Profile 2.3. Luvic Phaeozem, concave slope 50%, 777 m asl A 0–45 26 5 41 3 – Cr 145–170 2 1.5 41 – 7

9

–

–

4

3 3.5 13

– – –

– – –

8 4 9

8 8.5

11 8

0.5 –

6 6

4 1

6 16

– –

4 8

5

13

–

6

5 0.5

– –

8 3

5 15.5

– 11

10 8

6.5 3

9 14

P.V. Krasilnikov et al. / Catena 62 (2005) 14–44

29

Fig. 5. X-ray difractograms of oriented clay fractions of the horizons of Profile 1.2 (Chromic Alisol).

1.0 nm, similar to Profile 1.2, should correspond to mica, mica–vermiculite, and chlorite– vermiculite. In the deeper subprofile, the presence of 2:1 minerals is at the limit of detection. Fine sand mineralogy of Profile 1.3 (Chromic Alisol) shows a difference between the upper subprofile and the buried soil (Table 4). The upper horizons have equal amounts of clean quartz grains and quartz–mica aggregates, while in the buried red clayey soil, quartz–mica aggregates absolutely dominate. In Profile 1.5 (Skeletic Phaeozem), the mineralogical composition of sand is different from the upper profiles. The dominant minerals are plagioclases and K–Na feldspars

30

Table 5 Morphological description of soil profiles of the toposequence Palo Piedra, Pluma Hidalgo, Oaxaca State, Mexico Colour (moist)

Mottles

Field Structure texturea

Profile 2.1. Fluvic Phaeozem, convex shoulder, 810 m asl A 0–20 10YR 4/3 brown SL Bw 20–30 10YR 5/6 yellowish SL brown 2Ab 24–44 10YR 5/4 yellowish SL brown 2Bw1b 30–50 10YR 5/6 yellowish SL brown 2Bw2b 50–80 10YR 6/6 brownish SL yellow 2BCb 80–115 10YR 6/6 brownish SL yellow Profile 2.2. Fluvic Phaeozem, backslope 40%, 789 m asl A 0–35 10YR 4/4 dark yellowish brown 2Bwb 35–70 10YR 5/6 yellowish brown 3Bwb 70–95 10YR 5/6 yellowish brown 4Bwb 95–120 10YR 6/6 brownish yellow 4BCb 120–150 10YR 6/6 brownish yellow

SL SCL LS SL SL

Rock Clay films Roots fragments In On [vol.%] voids aggregates

Soil fauna

Pores

Border

GR ABK

0 10

N N

N N

2m 3f,vf 1m 2f

Worms, larvae Larvae, worms

1m 3f,vf 2m 3f,mf

W S

GR, ABK ABK, SAB ABK, SAB SAB

0

N

N

1co,f 2m

Few worms, larvae 1m 2f,vf

W

10–20

N

N

1m,f

Very few worms

1m 2f,vf

W

b10

N

N

1m,f

No

1m 2f,vf

W

10

N

N

1m,f

No

1m 2f,vf

GR, SAB ABK

20–30

N

N

2co,m 1f,vf Worms, larvae

2m,f 1vf

I

b10

N

N

1m,f

Larvae, worms

3co,m 2f

W

ABK, SAB ABK, SAB SAB

80

N

N

1m,f

Few worms, larvae 2co, 1m,f

W

20–30

N

N

1m

No

3co,m 1f,vf W

30

N

N

1m

No

2m,f

P.V. Krasilnikov et al. / Catena 62 (2005) 14–44

Horizon, depth [cm]

Profile 2.3. Luvic Phaeozem, concave slope 50%, 777 m asl A 0–45 10YR 3/3 dark brown SL EB 45–70 Bw 70–145 Cr 145–170

10YR 7/3 very pale brown 10YR 4/6 dark yellowish brown 7.5YR 5/6 strong brown

LS 10YR 3/4 dark SL yellowish brown SL

Profile 2.5. Leptic Phaeozem, concave slope 50%, 755 m asl A 0–30 10YR 3/4 dark SL yellowish brown ABw 30–65 10YR 4/4 dark SL yellowish brown R 65+ a

For explication, see Table 1.

10

N

N

3m,co

W

VF

3vf,f,m 2co Worms, insects, diplopoda 2vf,f 1m Single worms

SAB, PR PR

10

F

3m 2f,co

W

10

C

C

1m

No

1f,m

I

ABK

40

VF

VF

0

No

1f

GR

20

N

N

3m,co

W

SAB, PR PR, Sap

30

F

N

3vf,f,m 1co Worms, insects, diplopoda 2m 1vf,f Single worms

3m 2f

W

20

C

F

1m

No

1f

GR

30

N

N

3co,m,f

Few worms, larvae 1m 2f,vf

GR, ABK

60

N

N

3co,m 2f,vf No

2f,vf

W

P.V. Krasilnikov et al. / Catena 62 (2005) 14–44

Profile 2.4. Luvic Phaeozem, concave slope 40%, 765 m asl A 0–33 10YR 3/4 dark SL yellowish brown EB 33–70 10YR 5/4 yellowish LS brown Bw 70–110 10YR 4/6 dark 10YR 3/4 dark SL yellowish brown yellowish brown R 110+

GR

31

32

P.V. Krasilnikov et al. / Catena 62 (2005) 14–44

followed by quartz; amphiboles and mica aggregates are also present in significant amounts. The sand mineralogy of Profile 1.7 (Fluvic Phaeozem) is similar to those of Profile 1.5, with a higher percentage of quartz. The content of K–Na feldspars decreases, and amphiboles increase with depth in this profile. 3.2. Toposequence 2 (Palo Piedra) The second toposequence consists of five profiles, underlain with gneiss rocks and amphibolites. The first profile was made on the shoulder of a convex slope at an altitude of 810 m asl (Fig. 2b); the slope continues upwards until 1200 m asl. The second profile was made on a convex slope at an altitude of 789 m asl. The third, fourth, and fifth profiles were made on a concave backslope at altitudes of 777, 765, and 755 m asl, respectively. Below, there is a road cut and a river (at an altitude of 730 m). 3.2.1. Soil morphology A brief morphological description of the studied profiles is presented in Table 5. Profiles 2.1 and 2.2 were classified as Fluvic Phaeozems (A/Bw/C). In Profile 2.1, soil material is well sorted. A distinct buried A horizon was detected at the depth of 24–44 cm. At depths of 50 and 95 cm, two bstone linesQ were noted. The soil structure is weak, and no clay coatings are present throughout the profile. Profile 2.2 is generally similar in morphology to the previous profile. It has distinctly variable content of rock fragments in its horizons. The A horizon has 20–30% of unweathered rock fragments, mainly gravel and small stones. The underlying 2Bw horizon is poor in rock fragments. In contrast, the next 3Bw horizon has up to 80% of unweathered rock fragments, mainly big angular stones. The deeper horizons have fewer rock fragments, and most of those are weathered to the state of soft saprolite. Profiles 2.3 and 2.4, classified as Luvic Phaeozems (A/Bw(t)/C), are soils with a developed granular structure of biogenic origin of the A horizon (Fig. 6a). Deeper on, there is a porous EB horizon which is relatively compact and lacking any clay coatings on the ped surfaces. In the EB horizon, micromorphological study showed mainly skeletal microstructure with silt coatings partially oriented along the skeletal grains (Fig. 6b). The Bw horizon has a better developed angular blocky to prismatic structure—it is more compact; skeletal grains are few but large (size of coarse sand). Clay coatings on the surface of microaggregates are practically absent; however, they are present inside cracks in the rock fragments (Fig. 6c). The Cr horizon mostly retains the structure of parent rocks (amphibolite), although weathered to soft saprolite. Only a few pedofeatures were detected; among them are illuvial clay coatings and black Fe–Mn infillings in the cracks (Fig. 6d). In Profile 2.4, the Cr horizon is nearly absent, and hard fragmented rock lies just under the Bw horizon. Profile 2.5, classified as a Leptic Phaeozem, is a shallow soil with two horizons. The upper one is a well-structured humus-enriched horizon with abundant rock fragments. The ABw horizon is stony. Rock fragments occupy about one-half of the volume in the upper part, whereas, in the deeper part, fragmented rocks occupy up to 90% of the volume, and soil material can be found only in cracks.

P.V. Krasilnikov et al. / Catena 62 (2005) 14–44

a

33

b

400 µm

400 µm

c

d

CC

400 µm

CC

400 µm

Fig. 6. Soil micromorphology of Profile 2.3 (Luvic Phaeozem): (a) the structure of the A horizon; (b) the structure of the EB horizon; (c) clay coatings in weathering amphibole grain, Bw horizon; (d) Fe–Mn infillings, BC horizon.

3.2.2. Soil chemical properties The chemical properties of the components of the toposequence are presented in Table 6. The upper profile is slightly acidic (but with high DpH values), and base saturation is high (over 80% in some horizons). Clay content shows the difference between two sediments: the upper soil has a heavier texture. The most abundant cation is Ca, followed by Mg; K and Na are present in negligible quantities. Organic C content is high in the A horizon, much lower in the Bw horizon, and negligible in the deeper soil. The contents of Fed and Feo are significant in all the profile, with a Feo/Fed ratio close to 0.5. Bulk chemical composition is relatively uniform for all elements except Ca (its content increases with depth), and Na and K (showing irregular profile distribution). Profile 2.2 is also characterized by moderate acidity and high DpH values. Exchangeable base content and base saturation vary with depth according to sediment sequence. The content of Fed is high, while the content of Feo is much lower compared to Profile 2.1. Profiles 2.3 and 2.4 (Luvic Phaeozems) are slightly acidic. The DpH is high due to high values of CEC, reaching 40 cmolc kg 1 in the BC horizon. The lowest CEC was detected in the EB horizon. The dominant cation in the exchangeable complex is Ca; its content is four to five times higher than that of Mg; Na and K are present in negligible amounts. Base saturation is high in all the horizons. Clay content is relatively uniform across the profile;

34

P.V. Krasilnikov et al. / Catena 62 (2005) 14–44

Table 6 Chemical and physical properties of soil profiles of the toposequence Pala Piedra, Pluma Hidalgo, Oaxaca State, Mexico Horizon, depth [cm]

pHH2O pHKCl Clay [g kg

1

]

Ca2+ Mg2+ K+ Na+ EAa CEC CEC kg BS C clay 1 [%] cmolc kg

1

g kg m asl 0.2 0.4 tr. 0.1 tr. 0.2 tr. 0.4

Fed Feo Feo/ Fed 1

Profile 2.1. Fluvic Phaeozem, A 0–20 6.1 5.1 Bw 20–30 5.5 3.5 2Bw2b 50–80 5.8 3.3 2BCb 80–115 5.7 3.2

convex shoulder, 810 259.8 30.1 3.4 239.8 11.8 3.2 79.8 16.0 3.5 79.8 24.0 3.9

42.4 163.2 21.3 88.8 25.7 – 34.7 –

80.4 32.0 12.9 6.8 70.9 6.8 10.4 6.6 76.7 0.4 7.6 3.5 81.6 0.4 9.0 3.2

0.53 0.63 0.46 0.35

Profile 2.2. Fluvic Phaeozem, A 0–35 5.7 4.4 2Bwb 35–70 5.1 3.2 3Bwb 70–95 5.3 3.4 4Bwb 95–120 5.3 3.4 4BCb 5.4 3.6 120–150

backslope 40%, 789 m asl 143.7 10.9 3.3 0.2 0.3 9.1 23.8 165.6 175.7 4.5 1.9 0.1 0.3 8.3 15.1 85.9 132.0 3.7 0.7 0.1 0.3 4.3 9.1 68.9 27.0 1.7 0.9 tr. 0.2 3.2 6.0 – 43.0 3.5 0.9 tr. 0.3 1.5 6.2 –

61.8 18.9 20.2 2.5 45.0 2.8 14.8 2.7 52.7 2.8 8.4 1.8 46.7 0.9 24.0 5.4 75.8 0.9 24.9 5.6

0.12 0.18 0.21 0.23 0.22

8.3 6.2 6.0 6.4

Profile 2.3. Luvic Phaeozem, A 0–45 5.3 4.4. EB 45–70 5.9 4.0 Bw 70–145 5.8 3.5 Cr 145–170 5.8 3.8

concave slope 50%, 777 m 259.8 21.1 4.6 tr. 223.8 16.2 3.8 tr. 123.8 23.7 3.9 tr. 199.8 25.7 6.7 tr.

asl 0.3 0.2 0.6 0.4

128.2 126.0 294.8 200.2

78.1 10.0 14.7 6.5 0.44 71.6 4.4 15.8 8.7 0.55 77.3 0.8 22.9 16.7 0.73 82.0 0.4 31.2 20.7 0.66

Profile 2.4. Luvic Phaeozem, A 0–33 5.9 4.6 EB 33–70 6.0 3.9 Bw 70–110 6.0 3.6

concave slope 40%, 765 m 199.8 18.2 6.0 0.1 239.8 24.2 4.4 tr. 211.8 23.4 4.9 tr.

asl 0.2 7.2 31.7 158.7 0.2 7.4 36.2 151.0 0.3 6.7 35.3 166.7

77.3 20.0 21.8 7.3 0.27 79.6 4.8 37.2 12.6 0.34 81.0 2.8 21.9 12.6 0.57

7.3 8.0 8.3 7.2

33.3 28.2 36.5 40.0

Profile 2.5. Leptic Phaeozem, concave slope 50%, 755 m asl A 0–30 6.2 5.5 123.7 15.4 2.8 0.3 0.3 6.9 25.7 207.8 ABw 30–65 6.2 4.9 199.7 12.8 3.2 0.1 0.4 7.3 23.8 119.2 a

73.2 49.5 nd 69.3 21.4 nd

nd nd

nd nd

Extractable acidity.

the Bw horizon, where clay coatings were detected morphologically, has an even lower clay content compared to the overlying horizons. Organic C content is only 10 g kg 1 in the upper horizon and decreases with depth. Fed content is high and increases with depth; Feo content is about one-half of the Fed. The bulk chemical composition of Profile 2.3 shows an evident difference between the topsoil and the Bw and Cr horizons, where Ca and Mg content is much higher, and K and Na contents are lower than in the surface horizons. The difference is not so evident in Profile 2.4. Profile 2.5 has a reaction close to neutral, a high base saturation, and a high organic C content. 3.2.3. Clay and sand mineralogy Composition of the clay fraction from the horizons of Profile 2.1 (Fluvic Phaeozem) is shown in Fig. 7. The detected clay minerals are kaolinite and a series of minerals giving

P.V. Krasilnikov et al. / Catena 62 (2005) 14–44

Fig. 7. X-ray difractograms of oriented clay fractions of the horizons of Profile 2.1 (Luvic Phaeozem).

35

36

P.V. Krasilnikov et al. / Catena 62 (2005) 14–44

Fig. 8. X-ray difractograms of oriented clay fractions of the horizons of Profile 2.3 (Luvic Phaeozem).

P.V. Krasilnikov et al. / Catena 62 (2005) 14–44

37

signals from 1.0 to 1.42 nm. The interpretation was that mica (mainly trioctahedral, as no second-order basal peak was found) and mixed-layered chlorite–vermiculite were present in the clay fraction. The mixed-layered mineral seems more likely to be a product of chlorite degradation rather than Al-interlayered vermiculite because chlorite is an abundant component of local rocks. The diffractogram of the Bw horizon shows strong reflections at 0.72 and 1.42 nm. We interpreted it as the presence of vermiculite and mixed-layered chlorite—vermiculite; the 0.72 nm peak also includes a kaolin mineral signal. The situation in the buried 2Ab humus horizon was different from the overlaying horizons. The kaolinite signal at 0.72 was much more intensive than the signals at 1.2 and 1.4 nm. After heating, the peak was diffuse and localised at 1.0–1.2 nm. The interpretation was that mixed-layered mica–vermiculite and chlorite–vermiculite were present. The diffractogram of the 2Bw2 horizon was similar to that of the Bw horizon. The presence of chlorite and chlorite–vermiculite was detected. Fine sand mineralogy of Profile 2.2 (Table 4) is characterized by dominance of quartz and K–Na-feldspars, followed by quartz–mica aggregates and plagioclases. No significant difference between the surface horizon and buried 4Bwb horizon was noted. In the A horizon of Profile 2.3 (Luvic Phaeozem), the dominant mineral is kaolinite (Fig. 8). The other minerals are present in significant amounts giving signals in air-dried state at 1.4, 1.2, and 1.0 nm (in the sequence of their intensity). After heating, these minerals give signals at 1.2 and 1.0 nm; the intensity of 1.0 nm peak is higher than that of 1.2 nm peak. We interpreted it as simultaneous presence of vermiculite (dominant), chlorite–vermiculite (less abundant), and mica (the less important mineral). The presence of a signal at 0.776 nm in the diffractogram of the heated sample, interpreted as a secondorder signal of mixed-layered chlorite–vermiculite, suggests some order in the structure of this mineral. In the EB horizon, the ratio of kaolinite to other components was higher than that in the overlying horizon. The other components were chlorite–vermiculite and vermiculite. In the Bw horizon, the content of kaolinite was relatively lower compared to the EB. The other minerals were also vermiculite and mixed-layered chlorite–vermiculite. In the Cr horizon, the ratio of kaolinite is also relatively low. The other components were represented by mixed-layered chlorite–vermiculite, which gave a signal at 1.42 in air-dried state and at 1.2 after heating. The fine sand fraction (Table 4) of the A horizon of Profile 2.3 is dominated by quartz and K–Na-feldspars as in the upper part of Profile 2.2. In the Cr horizon of the same profile, the content of quartz is much lower, while the content of amphiboles, pyroxenes, biotite, and mica aggregates is considerably higher compared to the surface horizon. Mineralogical composition of the clay fraction of Profile 2.4 (Luvic Phaeozem) was found to be similar to that of Profile 2.3.

4. Discussion 4.1. Odd sequences of sediments Both relief and sediments of the studied toposequences are unusual. A classic catena consists of certain parts (summit, shoulder, backslope, and footslope) naturally formed by

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gravitation and the action of water (Birkeland, 1999). Moreover, every part of a catena is characterized by its own slope processes: the shoulder and backslope are eroded, and the footslope is an accumulative position (Gerrard, 1992). The first toposequence (La Can˜eria) is different from the common concept of a catena. Colluvium accumulation occurs at the shoulder of the slope; fine sorted material containing several buried soils forms a convex slope instead of a concave footslope in the lower part of the toposequence. The second toposequence (Palo Piedra) has unusual sorted sediments in the upper part (shoulder and backslope) containing buried soil profiles, although the shape of the slope resembles that of a typical catena. These odd features need an explanation. 4.1.1. The origin of colluvial accumulations (La Can˜eria) We believe that the sediments forming the upper 65 cm of the slope shoulder are of colluvial origin (Fig. 2b), evidenced by the presence of angular rock fragments partially weathered to various degrees. The underlying material is distinctly different from these sediments: it is redder, more clayey, unsaturated in bases, and richer in DCB-extractable Fe; the bulk content of Si and K is lower, and of Fe and Mg is higher compared to the overlying material. It seems that an older soil was eroded down to the Bt (in Profile 1.2— to the lower part of the EB) horizon, and then this soil was buried by colluvium (except for the eroded Profile 1.4). Recent soil formation has resulted in a shallow A/Bw/C soil within the upper part of Profile 1.3. The mystery is: Why did these sediments accumulate on a steep slope shoulder? Generally, colluvial sediments are transported down to the footslope (Gerrard, 1992). Our hypothesis is that such an accumulation resulted from a landslide. In the colluvial layer, rock fragments increase in content, the texture becomes more sandy, and the content of Si and Fe(II) increases with depth in the C horizon of Profile 1.3; the distribution of properties in Profile 1.2 is similar. This indirectly supports the landslide hypothesis: the sequence seems to be a complete weathering profile transported along the slope. However, it could also be ascribed to soil weathering after colluvial deposition. The landslide hypothesis is also attractive because several recent landslides were detected by the authors in the region (unpublished data). These landslides are believed to be seismically induced; Rojas et al. (1987) reported several major earthquakes in the 20th century for this area and also noted mass movement on the slopes caused by those earthquakes. Thus, we hypothesise that the presence of soil buried under colluvium in an unusual shoulder position should be due to a seismically induced landslide. The presence of a shallow Profile 1.1 at the summit of the slope may be explained by the same event, when the soil mass was transported from the summit to the shoulder. The similarity of fine sand composition in this profile and in the colluvial material of Profile 1.3 (Table 4) also supports this hypothesis. 4.1.2. The origin of sorted material accumulations The presence of relatively well-sorted material accumulations in both toposequences is also unusual. In the first toposequence (La Can˜eria), it is found in the footslope (where it should be, if we suggest slope deposits), but the convex form of the footslope is unusual (Fig. 2a). The two soils formed in these sediments (Profiles 1.7 and 1.8) bear evidences of sediment discontinuity. Profile 1.7 has two buried profiles; they are clearly indicated by

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39

shallow buried A horizons and by the variation in clay content (although minor) between the superficial and buried soils (Tables 1 and 2). Differences between the sediments are well illustrated by the composition of the fine sand fraction, especially by the distribution of K-feldspars and quartz (Table 4). Profile 1.8 has no distinct buried soils, but clay content does increase with depth (with no morphological evidences of clay illuviation), and organic carbon content decreases gradually with depth, as it was noted for cumulative soil profiles (Birkeland, 1999). A similar situation is observed in the second toposequence (Palo Piedra). In Palo Piedra, accumulated sorted material, also with buried soil profiles, exists on a slope shoulder, which is an odd place for such sediments. Three main sources of accumulated material may be hypothesized: eolian deposits, alluvial sediments, or sorted colluvium, periodically transported by overland flow of water, according to the Kcycle model (Butler, 1982). 4.1.2.1. Eolian deposits. The hypothesis seems to be improbable due to the lack of a source of wind-transported particles in the region. The wind comes from the Pacific ocean; all the territories from the coast to the highest peaks of the Sierra Sur de Oaxaca mountains are a forested area. The sand beaches on the Pacific coast occupy a negligible area. In the Pleistocene epoch, when the sea level was considerably lower, vast areas of littoral sediments could be transported by the wind, and eolian sediment could form in the region. However, this does not seem to be the case for the sediments accumulated on the studied slopes. Pleistocene-aged soils should be much more developed, especially in wet tropical conditions (Eswaran, 1972). 4.1.2.2. Alluvial sediments. The hypothesis of alluvial sedimentation is attractive for several reasons. The landforms of an unknown accumulated material resemble accumulative river terraces (Fig. 2a and b). There are rivers flowing just below the lowest component of both toposequences. The presence of the buried soils can be ascribed to fluctuations in the water regime of the rivers. However, the difference in absolute altitude between the unknown sediments in the toposequence Palo Piedra and the actual level of the river is 75 m. A river needs several centuries to deepen by 75 m, even in mountains. Soil formation in the tropics for several hundreds of years should lead to the formation of much more developed profiles than Profiles 2.1 and 2.2 (Table 5). The alluvial model cannot explain the presence of angular rock fragments, which are numerous in certain layers of Profile 2.2 (Table 5). The mineralogical composition of the accumulated material in the two toposequences is not very similar (Table 4); therefore, it cannot be alluvial material from the same river system. 4.1.2.3. K-cycles model. The K-cycles model developed by Butler (1982) includes periods of slope stability and instability. When the upper part of the slope is unstable, erosion occurs, leading to sediment accumulation in the footslope position. This model explains the accumulation of sorted fine material transported by overland water flow as well as angular rock fragments transported by gravitation. Backslopes, with fine sand mineralogy similar to the accumulated material, seem to be the source of the material (Table 4). The period of instability can be ascribed to various reasons: seismically induced mass movement, tree fall due to hurricanes, or forest fires.

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It is not clear why the slope sediments are poor in organic C, which should be transported and accumulated by water erosion of the surface horizons of soils in upper positions. One possible explanation is that the events causing erosion were catastrophic, and surface A horizons were severely eroded and transported outside the toposequence (e.g., by the neighbouring river). After the event, the material poor in organic matter was accumulated by overland water flow. When organic matter had formed in the soils of the toposequence, they were already fixed by root systems of plants. 4.1.3. The form of the slopes The form of the footslope of the toposequence La Can˜eria is unusual for catenas, described in the literature (Graham et al., 1990; Feldman et al., 1991; Birkeland, 1999). However, a convex footslope has been described in geomorphological literature (Penck, 1924; Rohdenburg, 1983) as a result of intensive linear erosion at the foot of a slope. It is a case of an bopen catenaQ (Birkeland, 1999) where the material is removed from the slope. In this case, a gradual footslope and toeslope are cut by perpendicular water flow (see dotted line at Fig. 2a), a natural process for an uplifting mountainous system (Mora´n et al., 1996). The form of the slope on Palo Piedra site is much more like a classical one, but the colluvium accumulation on the shoulder should be explained. We hypothesise that the actual slope is a secondary one worked out by erosion (see dotted line at Fig. 2b). The initial slope was partially formed by colluvial sediments. Still parent material of the profiles at the backslope and footslope of the toposequence is discontinuous (e.g., Profile 2.3 has bulk chemical composition and sand mineralogy distinctly different from the underlying regolith) (Tables 4 and 7). The secondary slope was formed in Palo Piedra (in contrast to La Can˜eria) because of a lower basis of erosion, which corresponds to the level of the river. Table 7 Bulk chemical composition of the fine earth of selected soils of the toposequence Palo Piedra, g kg (recalculated to 1 kg) Horizon, depth [cm]

SiO2

TiO2

Al2O3

Fe2O3

FeO

1

MnO

MgO

CaO

Na2O

K2 O

P2O5

Profile 2.1. Fluvic Phaeozem, convex shoulder, 810 m asl A 0–20 591.4 10.4 213.6 96.7 16.3 Bw 20–30 575.2 8.7 213.6 98.0 14.7 2Bw2b 70–80 549.9 6.3 236.5 92.5 12.9 2BCb 100–115 554.8 5.8 219.4 87.3 14.3

1.6 1.4 1.3 1.3

47.5 43.2 44.4 43.4

28.8 36.0 45.6 54.1

23.9 20.4 26.8 30.3

13.9 12.7 9.5 10.7

3.1 2.6 4.2 5.3

Profile 2.3. Luvic A 0–45 EB 45–75 Bw 75–145 Cr 145–170

Phaeozem, concave slope 570.8 15.5 205.5 569.9 13.7 219.5 500.2 34.9 171.2 518.6 10.8 168.0

50%, 777 103.9 102.5 155.1 145.8

m asl 18.1 16.3 34.1 24.5

1.8 1.5 2.3 2.1

31.3 31.0 19.5 87.4

36.9 30.1 59.7 73.4

27.3 25.0 19.4 5.4

9.9 10.6 8.3 7.2

5.0 3.1 25.5 7.2

Profile 2.4. Luvic A 0–33 EB 33–70 Bw 70–140

Phaeozem, concave slope 539.4 16.8 217.2 468.9 21.6 218.8 465.8 25.0 196.0

40%, 765 112.9 133.7 137.7

m asl 27.7 43.3 46.5

1.7 1.9 2.0

31.7 36.0 35.9

31.9 47.1 61.0

23.4 14.8 8.2

13.4 7.1 6.2

11.8 37.1 45.9

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4.2. The origin of red clayey soils Red clayey soils occupy minor territories in the study area; in the studied toposequences, they are presented as buried profiles. They are more clayey, less saturated in bases, and contain more Fed compared to the other soils of the study area; their clay fraction is almost completely kaolinitic. From the point of view of soil-forming processes (weathering, clay illuviation, and lixiviation), they are much more developed than the other profiles. These soils may be expected to be residues of ancient stages of soil formation (paleosols). Still, the hypothesis is not very strong. A paleosol is generally considered to be a soil formed under environmental conditions different from the present ones. There is no proof of that being the case in the area of our study. Unfortunately, there are no detailed paleoclimatic records for the studied area. However, a general consideration is that the area affected mainly by winds from the Pacific ocean did not go through significant climatic changes from the time of the Miocene uplift (Herna´ndez et al., 1996). Analysis of the actual soil-forming processes supports the idea that the red-coloured clayey soils are normal components of a landscape. The soil-forming processes in the younger soils are: humus accumulation (evidenced by the presence of developed A horizons), clay illuviation (evidenced by morphological and micromorphological observations), and actual soil weathering (evidenced by the presence of advanced clay transformation products such as kaolinite, and relatively high Feo contents). The redcoloured profiles have more pronounced clay illuviation, and clays completely transformed to kaolin minerals. Non-silicate iron is more crystallized in these soils. We should regard the red-coloured clayey soils as mature products of the same soil-forming processes that are active in younger soils. Development of these soils is synchronized with development of the entire mountainous system; their minor occurrence should be ascribed to high activity of slope processes in uplifting mountains (Mora´n et al., 1996). 4.3. The diversity and spatial organisation of soils The diversity of soils in the study area is regulated by diversity of soil-forming materials and by the activity of recent slope processes. The parent material originated from gneiss, amphibolites, and products of their weathering later transported by water and gravitation. The diversity of soil-forming materials is illustrated by composition of fine sand fractions of the soils (Table 4). However, the composition of parent materials does not completely change the pedogenesis; it just modifies soil properties such as texture, cation exchange capacity, the ratio of exchangeable cations, etc. The studied slopes originated from the action of rivers and/or temporal streams. Only small residual zones resisted the intensive water erosion. It is one of the reasons for an odd shape of the slope: it is not a classical, well-balanced profile of a catena but a steep slope of a canyon affected by recent mass movement. The residual soils are red-coloured, clayey, kaolinitic Alisols (Profiles 1.2 and 1.3); these soils are partially transported and buried by landslides, mostly seismically induced. The soils of the backslopes and footslopes (Profiles 1.5, 1.6, 2.3, and 2.4) have deep, well-structured A horizons; they are characterized by slightly acidic reaction and high base

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saturation. Clays contain significant amounts of 2:1 minerals together with kaolin minerals. In soils formed on concave slopes (Luvic Phaeozems, Profiles 2.3 and 2.4), there are also morphological evidences of clay and iron illuviation. These soils, although young, are developing under minor effects of slope processes. Some of the chemical properties of these soils are unusual (e.g., high DpH values). The phenomena were ascribed to possible non-exchangeable K fixation by highly charged mica–vermiculite minerals. The soils formed on recent colluvial sediments are Fluvic Cambisols (Profiles 1.7, 1.8, 2.1, and 2.2) that are similar in morphology. All these soils are characterised by incipient soil formation. The whole profiles have weakly developed subangular structure, and even the change of colour is insignificant in the Bw horizons with respect to the parent material. The X-ray diffraction data show only slight differences between horizons in clay mineral composition. Although these profiles show an accumulation of organic matter (reflected both by A horizon depth and high organic C content), the properties of underlying horizons reflect the difference in sediments composition, rather than actual pedogenesis. What practical implication can be expected from the obtained results? The presence of various stability zones results in diversity of soils in the region. McBratney (1992) proposed the term pedodiversity to emphasize the significance of variation of soil properties for agriculture. He pointed out that various soils have different responses to environmental conditions. For example, some soils are more productive in a wet year, while others—in a dry year. Thus, pedodiversity is the basis for environmental sustainability. The presence of highly weathered Alisol profiles in the region is regarded by the local population as a positive factor for coffee growing: the local name for these soils is tierra colorada hu´meda (red humid earth) due to their high capacity for water retention. The younger soils also have properties favourable for higher plants, such as high cation exchange capacity, high organic matter content, and perfect drainage. The diversity of soils is also responsible for diversity of surface plant species (Krasilnikov, 2001), as it was shown for temperate forest ecosystems. That is why we regard the effect of geomorphic processes on pedogenesis in tropical mountains as a positive phenomenon.

5. Conclusions 1. The mountainous system Sierra Madre del Sur, southern Mexico, is the zone of a tectonic uplift leading to the development of river canyons and gulleys. It is still a seismically active zone; seismic activity causes landslides. 2. Steep slopes of the canyons are still underdeveloped; recent mass movement results in odd sequences of sediments and unusual shape of the slopes. Complex relief and sediments result in high diversity of soils in the study area. 3. Weakly developed soils formed on eroded surfaces and on recent colluvium are present in the tropical forested mountainous landscape of Sierra Madre del Sur, southern Mexico, along with mature profiles of Chromic Alisols and Luvic Phaeozems. 4. Chromic Alisols exist on upper positions, not yet affected by linear erosion; they are not paleosols, but the products of actual pedogenesis in geomorphologically stable conditions. The weathering profile of these soils is inverse due to fresher material accumulated on the surface.

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5. Less developed soils (Luvic and Skeletic Phaeozems) form on backslopes and footslopes. The weakest pedogenesis was detected for soils formed on recent colluvium.

Acknowledgements The authors would like to thank E. Fuentes Romero and M. del S. Galicia Palacias (Lab. Edafologia, Facultad de Ciencias, UNAM) for making some important chemical analyses, Dr. A.N. Safronov (Institute of Geology, KarRC RAS), for providing X-ray diffraction data, the analytical laboratory team of the Institute of Geology KarRC RAS for making bulk chemical composition analyses of the soil samples, and M.S. Emily McClang for correcting the language. The authors are also grateful to Professors P.W. Birkeland and H. Eswaran for a fruitful revision of the text. The study was done with a financial assistance of the projects UNAM-CONACyT 5-28227-B and SEP-CONACyT 43702.

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