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Published in Soil Sci. Soc. Am. J. 67:1496-1506 (2003).
© 2003 Soil Science Society of America
677 S. Segoe Rd., Madison, WI 53711 USA

DIVISION S-5—PEDOLOGY

Pedogenesis in Lutitic Cr Horizons of Gypsiferous Soils

O. Artieda and J. Herrero*

Dep. of Soils and Irrigation, Laboratorio asociado de Agronomía y Medio Ambiente (DGA-CSIC), P.O. Box 727, 50080 Zaragoza, Spain

* Corresponding author (jhi{at}aragob.es).


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
The weathering of lutites in aridic or near-aridic environments leading to soil horizons is not well known. Lutites are a common soil parent rock composed of silt and clay, that may be massive or may have their sedimentary origin marked by layers of alternating color or granulometry, or by fissility along planes. Lutites underlying soils are considered Cr horizons and show different alteration stages recognizable in the field by the degree of the layering disturbance or by the loss of coherence. In the study area, neither the dry climate nor the kind of clay minerals, mainly illite, can explain the weathering of lutites. This study was conducted to investigate the initial weathering reactions as encountered in Cr horizons, and to depict the processes responsible for pedogenesis from lutites in gypsiferous soils of the Ebro Valley (Spain). We described 12 soils in the field under two different soil moisture regimes, with further characterization through chemical analyses and micromorphology by thin section and scanning electron microscope (SEM). The microscopic study revealed two distinctive pedofeatures in the Cr horizons: (i) lenticular gypsum crystals; and (ii) "queras," sub-millimetric biotic features produced by calcification-decalcification processes. Growth of gypsum crystals in these horizons resulted in an "isles fabric," that is, isles of fine materials in a mass of gypsum crystals, with much greater porosity than the parent lutite. The growth of gypsum crystals and the development of queras result in a particle-size distribution change and an increased porosity of the Cr horizons.

Abbreviations: BSE, backscattered electron • CCE, calcium carbonate equivalent • EDS, energy dispersive spectrometer • GE, gypsum equivalent • MAP, mean annual precipitation • MAT, mean annual temperature • OC, organic C • RET, reference evapotranspiration • SEM, scanning electron microscope


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 
SOILS DEVELOPED on lutites in environments rich in carbonates and gypsum are common in dry regions, such as the central Ebro Valley (northeastern Spain). Most studies relating to the weathering of lutites have focused on the clay minerals, for example Ducloux et al. (1995), or El-Kammar and El-Kammar (1996). Other studies have looked at the weathering of lutites under wetting-drying or freezing-thawing cycles (Pardini et al., 1996; Cantón et al., 2001). However, little attention has been paid to other factors of the comminution of the lutite, which can be considered as the inception of soil formation. Studies of the processes of gypsification or of calcification and decalcification have been limited to the solum itself or other unconsolidated materials (Chen, 1997; Toomanian et al., 2001). In this study we stress the action of these processes on lutitic Cr horizons. These processes reorganize the soil components, either with or without loss or gain of gypsum or carbonates at the horizon scale.

Several authors (Stoops and Ilaiwi [1981], Allen [1985], or Jafarzadeh and Burnham [1992]) have described the sizes and morphology of pedogenic gypsum crystals. However, to properly understand the genesis and the properties of the soil horizons in areas with ubiquitous gypsum, one needs to take into account the generalized formation of gypsic pedofeatures, that can reach the isles fabric stage (Herrero et al., 1992). Soil carbonates have been studied either from a genetic point of view (Goudie, 1996; Kaemmerer and Revel, 1996) or from a micromorphological one (Monger et al., 1991a). However, little attention has been paid to queras, pedofeatures of 1 to 2 mm wide and <2 cm long in thin section, including calcification and decalcification traits found by Herrero et al. (1992) in the sola of hypergypsic soils; queras appear in the present study in lutitic Cr horizons.

Queras (Herrero et al., 1992) are a complex pedofeature composed by both calcification and decalcification features. The calcification feature is made by carbonatic grains, whose size, shape, and internal characteristics are associated to root pseudomorphs in many studies. References to features somewhat similar to queras have been found in Barzanji (1973), Barzanji and Stoops (1974), Bal (1975), Fang et al. (1994), and Khokhlova et al. (2001). Klappa (1980) observed calcite crystals replacing cells of the root parenchyma, but these crystals (20 µm) were smaller than the quesparite grains studied here (from 80–120 µm). Jaillard (1984)(1987) and Jaillard et al. (1991) reported rhizomorphic, carbonated structures in soils resulting from marl alteration. These works consider these structures, which are similar to queras, as calcified root residues. These calcified cells are homometric, with diameter of about 80 µm, similar to our quesparite grains. Similar morphologies in Quaternary loess paleosols of several sites in Europe were interpreted by Becze-Deák et al. (1997) as root pseudomorphs. The same authors report that these morphologies occur not only in soils with a carbonatic matrix, but also in non-calcareous matrices. The decalcification feature described by Herrero et al. (1992) is similar to the decalcified crown reported by Jaillard (1984) and by Jaillard and Callot (1987) on both sides of the rhizomorphous structure.

In this article we focus on the genesis of soil horizons from in situ lutites in areas with abundant gypsum and dry climate. Our objectives are (i) to describe the two most abundant pedofeatures found in the lutitic Cr horizons, (ii) to show that the development of both these pedofeatures have parallel effects leading to a soil material coarser and looser than the parent lutites, and (iii) to propose a genetic model explaining the production of soil horizons from lutite.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 REFERENCES
 
The Tertiary Basin of the Ebro Valley (N.E. Spain) contains detritical rocks, limestones, and evaporitic rocks. The authors (Herrero, 1991; Artieda, 1996) surveyed two areas of gypsiferous soils in the Ebro Valley (Fig. 1) . Area A is on the nucleus of the Barbastro-Balaguer anticline, close to the northeastern border of the Ebro Basin. The outcropping parent materials are gyprock with intercalated lutites. Area B is on horizontal strata in the center of the Ebro Basin. The parent materials are lutites with gyprock intercalations, except in the higher elevations of the western end of the area where the gyprock is predominant. Gypsum and lutites are ubiquitous materials in Areas A and B. Lutites are composed of silt and clay and can either have a laminated structure or be massive. Some of our lutites contain centimetric nodules of gypsum of diagenetic origin after the petrographical study. The abundance of geological gypsum and its redistribution by chemical and mechanical processes explain the ubiquity of gypsum in the landscape.



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  		Fig. 1. Location of the two study Areas A and B, and weather stations, in the Tertiary Basin of the Ebro Valley. Sketches (not to scale) of the distribution of the pedons on a cross-section of the landscape of Areas A and B are also shown.

 
The climate data and calculations of the evapotranspiration can be found in Martínez-Cob et al. (1998). The climate of Area A is represented by the weather station of Tamarite-Melusa (Fig. 1), with mean annual temperature (MAT) 14.0°C, mean annual precipitation (MAP) 406 mm, and reference evapotranspiration (RET) 1210 mm. The climate of Area B is represented by the weather station of Zaragoza Airport (Fig. 1), with MAT 14.7°C, MAP 319 mm, and RET 1272 mm. The soil temperature regime is thermic in both areas, and the soil moisture regime is xeric in Area A and aridic in Area B (Soil Survey Staff, 1999). The Area A is occupied by rainfed barley (Hordeum vulgare L.) and almond (Prunus amygdalus Batsch) trees with patches of the natural forest whereas in area B only sparse shrubs can grow, and marginal barley is cultivated, except in the sprinkler-irrigated fields (Table 1).


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  		Table 1. Site characteristics and classification of pedons in Areas A and B.

 
For the present study, we selected the Cr horizons of 12 representative pedons overlying lutites (Table 1, Fig. 1). Pedons A1 to A3 correspond to Pedons IB-134, PE-29, and PE-34, respectively, from Herrero (1991). Pedons B1 to B9 correspond to the pedons QT-514, BE-17, QE-163, BE-11, BE-3, QE-157, BE-6, QE-186, and QE-162, respectively, where pedons marked "QE" are from Artieda (1996). These pedons occur on one of the following landscape positions: mesa on gyprock, hilltop, slope, structural step, fluvial terrace, and depression.

The soils were described in the field according to the criteria of the Spanish Ministry of Agriculture (Comisión del Banco de Datos de Suelos y Aguas, 1983) and classified to the subgroup level according to Soil Taxonomy (Soil Survey Staff, 1999) (Table 1). The nomenclature for genetic horizons follows Soil Survey Staff (1999). The only exception is the uppercase Y, which is used in the sense of Artieda (1996) for horizons with high content of pedogenic gypsum (>50% in volume). Moreover, we use the lowercase w with k or with y to remark the occurrence of pedological structure.

The chemical analyses reported in this paper are: 1:2.5 soil/water pH, 1:5 soil/water electrical conductivity, calcium carbonate equivalent (CCE), gypsum equivalent (GE), and organic C (OC). These analyses were performed using the classical methods described in Porta et al. (1986); gypsum equivalent was determined with the method of Nelson et al. (1978) modified by Artieda (1993). Thin sections were manufactured following the procedure of Guilloré (1985), and several of them were stained according to the method of Dickson (1965) while employing certain modifications to the concentration of reagents and the attack times as proposed by Lindholm and Fikelman (1972). The thin sections were described with the system of Bullock et al. (1985), with minor adaptations (Herrero and Porta, 1987; Herrero, 1991). The study of the thin sections under the polarizing microscope was complemented by SEM observations. Uncovered thin sections were carbon-coated and then observed with a DSM 960A Zeiss microscope (Carl Zeiss, Jena, Germany) equipped with a four-diode, semiconductor backscattered electron (BSE) detector and a Link ISIS (Oxford Instruments, Oxford, UK) microanalytical energy dispersive spectrometer (EDS) system. Backscattered electron and EDS examinations of the samples were simultaneously performed. The microscope operating conditions were as follows: 25 kV acceleration potential, 10-8 A electric current, 8-mm distance for image acquisition, and 25 mm for microanalyses.

To avoid gypsum transformations, 40°C was not exceeded, either in drying the samples for analysis or during the preparation of the thin sections.


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
REFERENCES
 
All the studied soils are underlain by lutites, but some of them have alluvial or colluvial materials in the sola. Table 2 displays morphological and chemical data of the pedons under study. The field criteria for Cr, Cry, and By designation are as follows: (i) Cr is a soil horizon composed of lutites with not evident or slight alteration; (ii) we use Cry for lutites that retain their original structure in >50% of their volume and at the same time register >1% in volume of gypsum accumulation; and lastly (iii) we label as By those horizons that have >1% in volume of gypsum accumulation and <50% in volume of the lutite structure, that is, lutite if recognizable, occurs only as pararock.


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  		Table 2. Morphological and chemical properties of the pedons.{dagger}

 
The CCE in the studied Cr horizons ranged from 80 to 450 g kg-1 and GE ranged from 60 to 740 g kg-1. The OC content is <6 g kg-1. The pH was slightly alkaline, with an average of 7.8. Clays are mainly illitic in both Areas A and B (Herrero, 1991; Artieda, 1996). The differences in composition between the samples are more related to the degree of alteration of the lutite than to the area where they come from.

In the field, pedogenesis from lutites is characterized by abundant accumulations of lenticular gypsum, either as single crystals or in rosettes of about 1 cm in diameter. If the accumulation of gypsum is dense, the material acquires a sandy field texture. In some cases the accumulation of gypsum surpasses 40% in volume, but the material can be recognized as formed from a rock by features like fissility and alternating color beds.

Micromorphological studies (Table 3) revealed two features related to the pedogenic transformation of lutites: (i) the interstitial growth of lenticular and xenotopic gypsum crystals resulting, in the advanced stages, in masses of nongypsic material embedded in a mass of gypsum crystals, the so-called isles fabric of Herrero et al. (1992) (Fig. 2) ; and (ii) the occurrence of queras, a calcification-decalcification pedofeature described by Herrero and Porta (1987) (Fig. 3) .


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  		Table 3. Micromorphological data of the studied Cr horizons.

 


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  		Fig. 2. (a) Incomplete infilling of the voids (V) between lutitic fragments (L) by lenticular gypsum crystals (G). Thin section of Pedon B8 at the 75- to 90-cm depth (Cry2 horizon) under cross-polarizers (length of bar: 1 mm). (b) The increase in concentration of gypsum crystals (G) in the voids (V) leads to isles fabric, that is, isles of lutitic material (L) separated by gypsum. Thin section of pedon B3 at the 115- to 200-cm depth (2Cry horizon) under cross-polarizers (length of bar: 1 mm). (c) An advanced stage in isles fabric shows lutitic isles (L) on a groundmass of lenticular gypsum (G) Thin section of the pedon B7 at the 35- to 65-cm depth (By2/Cry horizon) under cross polarizers (length of bar: 1 mm).

 


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  		Fig. 3. This thin section is of the Pedon A2 at the 65- to 95-cm depth (Cry horizon). Several coalescent queras are shown (a) under plane-polarized light, and (b) under cross-polarizers (length of bar: 1 mm). The upper half of micrograph shows a transverse cross-section of queras, whereas the lower half shows a longitudinal cross-section of a quera. (c) Sketch of upper left corner of the above photographs illustrating the components of the queras. (d) Detail of another field in the same thin section showing a transverse section of the quera with the quesparite grains, or quemosaic (QM), infilling the central void, and the decalcified area, or quedecal (QD) (length of bar: 400 µm).

 
The original structure of the lutites in the 12 pedons ranges from alternating layers of few centimeters thickness of several distinct colors, to massive lutite strata of several meters thickness. Unweathered lutite, or Cr horizon, appears in the Pedons B2 and B5, whereas in the other pedons the unweathered lutite occurs as pararock. Several stages of lutite alteration were observed under the microscope (Fig. 2), beginning with the interstitial growth of lenticular gypsum along planes of weakness (Fig. 2a). The internal characteristics of the resulting pararocks were identical to those of the fresh lutite. The gypsum growth continues in the voids (Fig. 2b), and is recognizable in the field as vermiform gypsum. In an advanced stage (Fig. 2c), the masses of nongypsic fine material are embedded in a continuous crystalline pedofeature, a mass of lenticular gypsum.

The kind of soil fabric shown in Fig. 2 can be named as isles fabric. The isles are made up of silt and clay, and range in size from millimeters to centimeters. Angular shapes are common, often with a fissure structure, or are more rounded in the smaller sizes. Isles fabric was interpreted as a result of the displacing growth of gypsum, keeping in mind that the nucleation and growth of gypsum can produce pressures of about 20 MPa (200 atm) at 25°C at 150% oversaturation (Winkler and Singer, 1972). This continuous growth produces smaller and smaller parts (isles) of carbonatic-aluminosilicatic material that are dispersed in the gypsic mass, where the gypsum crystals range in size from <20 µm to several millimeters. The process ends in horizons with a generalized accumulation of gypsum, designated Y horizons, as is the case of Pedon B7 (Fig. 2c). Based on experiments with pyritic minespoil materials packed in lysimeters, Doolittle et al. (1993) suggested that gypsum precipitation in conductive vughs and channels could lead to a gypsic or petrogypsic horizon which would impede the vertical movement of water and oxygen. Other workers (Delmas et al., 1985; Herrero et al., 1992; Poch and Verplancke, 1997), however, have observed that progressive gypsum accumulation increases porosity through displacement and disruption, a result that corresponds to our own findings presented here.

The other observed pedofeatures are related to calcification-decalcification processes affecting the rock. The most common are queras (Fig. 3), described by Herrero and Porta (1987) as complex pedofeatures produced by the occurrence of calcification and decalcification phenomena in soil materials. Under the polarizing microscope, a complete typical quera contains three parts (Fig. 3c). First, a central void or central channel hosting plant residues, probably root residues, sometimes with a high degree of decomposition. Second, a complete or incomplete infilling of the central channel, made by a mosaic of grains of sparite (quesparite), often occupying the cells of the plant residue. The size of the quesparite grains observed in this work ranges from 80 to 120 µm. The quesparite grains are more limpid in Area A than in Area B.

There was also a decalcified area of the groundmass conterminous with the central channel. This area, or quedecal, has been observed only on one side of the central channel. The quedecal often contains elongated and parallel voids, or quevoids, starting in and perpendicular to the central channel. Quevoids seem to be produced by root hairs because they are undulated, with a circular cross-section (diam. <35 µm), and often uniform in length. Some birefringent domains in the quedecal seem to be inherited from the parent material, while others are associated with the quevoids or with lenticular gypsum crystals.

Some quesparite grains studied under the SEM showed an irregular void in the center of the grains (Fig. 4a) that suggests growth from several peripheral points. This hypothesis agrees with the relationships between quesparite grains and the plant residue in the quera where the enveloping of quesparite grains by the remaining cell walls is often seen (Fig. 5) . These spatial relationships, together with the uniformity in size and shape of the quesparite grains, strongly support the idea that each quesparite grain is generated from several points on the interior face of the wall of a root cell.



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  		Fig. 4. Backscattered electron microscope micrograph of the Cry1 horizon of Pedon B1 at the 10- to 25-cm depth (length of bar: 50 µm) shows a quemosaic fragment containing a quesparite grain with a central void (CV). The EDS spectrum shows the predominance of Ca, O, and Mg in the composition of quesparite grains.

 


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  		Fig. 5. This organic residue (OR) contains quesparite crystals (Q) within residues of cell walls (CW). This thin section is of Pedon A1 at the 37- to 45-cm depth (Bwy horizon) (a) under cross-polarizers, and (b) under polarizers at 45° (length of bar: 100 µm).

 
The study of stained thin sections under optical microscope was not conclusive about the Ca/Mg ratio in the carbonate, showing irregularities similar to those reported by Klappa (1978) in similar crystals found in Pleistocene petrocalcic horizons. This uncertainty was overcome by compositional analysis by EDS microprobe (Fig. 4b) showing that the quesparite grains have dominantly a CaCO3 composition, with <5% Mg. The accumulation of CaCO3 inside plant cells was reported by Pobeguin (1951), both in vivo and in vitro. She concluded that amorphous carbonate accumulates in living cells and becomes crystalline when the cell dies.

The incomplete crystalline growths from cell walls of the remaining tissues (Fig. 5) agree with the observations of Herrero (1991). A pseudomorphosis of the root tissue can be invoked, even though the size of the quesparite grains does not agree with the common size of the root cells. Some samples in this study clearly show the quesparite grains inside the plant residues, but these tissues could possibly be only a support, and later a mold, for CaCO3 precipitation produced by microorganisms. The microbial precipitation of pedogenic calcite in Ca-rich medium has been demonstrated by Monger et al. (1991b).

The observations of Jaillard (1984) and Jaillard and Callot (1987) of a decalcified crown of between 50 and 200 µm do not exactly agree with our observations, which indicate that quedecal appears only at one side of the central channel, and without preferential polarity. In the soils of our Area A, a quedecal can attain a diameter of several millimeters whereas in Area B the maximum observed size is 500 µm. In some cases, quemosaics appear in continuity with lutitic material, without a decalcified zone. Quedecal can be in contact with gypsum lenticular crystals, showing birefringence domains associated with the gypsum crystal wall. The analysis of quedecal areas by EDS microprobe confirmed the decalcification, as shown in Fig. 6 . This process of carbonate loss increases the porosity, as shown in Fig. 6b.



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  		Fig. 6. Electron micrographs of Pedon B1 at the 10- to 25-cm depth (Cry1 horizon). (a) EDS dot maps showing spatial distribution of Si, Al, Mg, S, and Ca in the contact between a lutitic fragment (left) and the residue of a quemosaic (right). The length of each frame is 750 µm. (b) The diagonal marked by the arrows is the contact between the unweathered lutitic material (lower half) and the quedecal (upper half). The BSE image shows that the quedecal is much more porous than lutite, because of the decalcification process (length of bar: 100 µm).

 
Queras can be attributed to the dissolution of the carbonatic fraction of the parent lutite and further reprecipitation of CaCO3, which in turn produces quesparite grains. This hypothesis is supported by the studies of Williams and Coleman (1950), Graham and Baker (1951), and Weisenseel et al. (1979) on the cationic exchange between roots and soil, with Ca2+, Mg2+, and K+ entering into the root, and H+ going into the soil solution. This process acidifies the soil surrounding the root and allows the dissolution of carbonates. Jaillard (1987) states that the entrance of Ca2+ into the cell vacuoles, plus the accumulation of CO2, produces CaCO3. He stresses that the process is restricted to calcicolous plants or to plant species indifferent to Ca and that accumulate Ca2+ in their vacuoles, but not oxalic acid. In that case calcium oxalate should appear because its solubility product is lower than CaCO3. The formation of calcium oxalate (CaC2O4· nH2O) and CaCO3 in terrestrial and aquatic plants has been reviewed by Franceschi and Horner (1980), Borowitzka (1984), and Libert and Franceschi (1987). Analysis with EDS could not distinguish between these substances because of their similar stoichiometry. Franceschi and Horner (1980) showed a collection of scanning electron micrographs of calcium oxalate crystals from many plant species, but the shapes and sizes of these crystals differed from those we encountered in quesparite.

On the other hand, the presence of quedecal at only one side of the main channel of the quera, the lack of quedecal in some cases, and the abundance of gypsum as a potential source of calcium suggest that more than one process is needed to produce queras. Other factors, abiotic or biotic (bacteria, fungi, or other microorganisms) cannot be discarded as inductors of CaCO3 production within the plant residues, either dead or alive.

In two of the horizons studied (4Cry1 in B9, on a fluvial terrace, and 2Cry in B3 on a slope), queras and quesparite grains are minor components. Micritic and microsparitic carbonates appear as incomplete dense infillings of the fissures in the carbonatic-silicatic isles (Fig. 7) , that is, in the fragments of lutite. These kind of carbonates also appear as coatings on the isles. These carbonatic masses (coatings or infillings) show an internal structure, with areas having carbonatic rings of 15 to 20 µm in width and external diameters of 40 to 70 µm (Fig. 7 and 8) ; these rings are often coalescent. The whole feature has a crystallitic b-fabric (carbonatic) following Bullock et al. (1985). The observation of the whole thin section shows gypsum lentils between the carbonatic rings (Fig. 8b). We can surmise that a gypsic fabric is being or has been developed, and that even in these areas the pedogenesis is controlled by gypsum.



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  		Fig. 7. Photomicrographs of the 2Cry horizon of Pedon B3 (115-200 cm) under crossed polarizers. (a) Carbonate rings (CR) forming an incomplete infilling in lutitic material with a decalcified area (D), (length of bar: 200 µm). (b) Detail of (a), (length of bar: 100 µm).

 


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  		Fig. 8. Photomicrographs of carbonate rings. (a) Fragments of the carbonate rings (CR) dispersed in both the gypsic and the carbonatic-silicatic groundmass of the 2Cry horizon of Pedon B3 (115–200 cm) under crossed polarizers (length of bar: 200 µm). (b) Stained thin section under plane-polarized light of the 4Cry1 horizon of pedon B9 at 160-190 cm, showing carbonate rings (CR) dispersed in gypsic (G) groundmass (length of bar: 200 µm).

 
The horizons studied show the gypsum translocation process proposed by Herrero (1991) resulting in soils with isles fabric. Of course, all combinations of precipitation and dissolution stages may exist, as well as the production and decay of queras presented in the above paragraphs. The residues of the queras are incorporated into the soil as single quesparite grains dispersed in the groundmass or as fragments of the quemosaic. In some cases the incomplete quemosaics are still in contact with a fragment of the surrounding quedecal, which is often highly preserved as shown by the remaining quevoids, indicating active formation or high stability of the queras.

Genetic Model
The two described pedofeatures, lenticular gypsum and queras, are typical of the pedogenesis in the studied Cr horizons, and they produce converging effects (Fig. 9) . The growth of lenticular gypsum crystals in pores or in weak planes of the rock (layering, fissility) produces fissures acting as paths for the penetration of water and roots (Fig. 9a). Ion exchange around roots leads to lutite decalcification (Fig. 9b) with consequent increase in porosity. The quemosaics, irrespective of their exact origin, are new paths for the penetration of water (Fig. 9c). Water, in the studied soilscapes with ubiquitous gypsum, is able to precipitate gypsum, which then destroys the initial structure of the quera (Fig. 9d). The continuous saturation in SO4 and Ca ions facilitates structural stability, making these preferential paths for water circulation durable, ensuring the continuity of the process. The decay of queras by gypsum growth or by bioturbation disperses the coarse sand sized quesparite grains in the soil, modifying the initial texture of the soil in the more advanced stages of pedogenesis. This stage is best developed in soils of Area A, which has higher precipitation and better plant cover. The low water retention capacity together with the aridic conditions makes the development of vegetation difficult on soils of Area B. However, the key pedofeatures—lenticular gypsum and queras—are present, indicating that similar soil formation processes occurred. It is likely that this genetic model can explain genesis of lutitic Cr horizons in other parts of the world.



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  		Fig. 9. Sketch of the proposed pedogenic model, illustrated in four stages from (a) to (d). The development of gypsum crystals and queras are the main weathering agents, transforming the lutite in a looser and coarser material, which can become a B horizon.

 
Translocation of gypsum and carbonates are the main pedogenic processes affecting the lutites in the soils studied. The precipitation of gypsum within the host lutitic material, and its further dissolution/precipitation episodes cause a transformation from a material that cannot be penetrated by roots to a material with an increasing porosity and a decreasing consistence. The production of queras seems related to a CaCO3 precipitation process inside the cells of root residues belonging to certain unidentified plants. The development of queras and the mechanical effects of gypsum growth converge to favor root penetration in the resulting soil. Some of the obvious effects like the change of the water holding capacity, or the increase of infiltration ratio and rooting capacity have environmental interest and should be evaluated in the field.


   ACKNOWLEDGMENTS
 
The first author was funded by the Government of Aragón from October 1991 to September 1995. We gratefully acknowledge the help of Dr. J. Wierzchos and Prof. R. Rodríguez with the electron microscope of the University of Lleida. The work of the referees on the manuscript is highly appreciated, in particular John Jacob who also helped with the English version. Some preliminary results of this work were presented at the 6th International Meeting on Soils with Mediterranean Type of Climate held in 1999 in Barcelona.

Received for publication November 10, 2000.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 REFERENCES
 





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The SCI Journals Agronomy Journal Crop Science
Journal of Natural Resources
and Life Sciences Education		 Vadose Zone Journal
Journal of Plant Registrations Journal of
Environmental Quality		 The Plant Genome