Agua–suelo–clima

 

Pedogenesis and characteristics of the terra rossas developed on different physiographic position and their classification

 

Pedogenésis y características de las terra rossas desarrolladas en posición fisiográfica distinta y su clasificación

 

Cumhur Aydinalp¹* and Ewart Adsil FitzPatrick²

 

¹ Uludag University, Faculty of Agriculture, Department of Soil Science, 16059 Bursa, Turkey. * Author for correspondence: (cumhur@uludag.edu.tr)

² Department of Plant and Soil Science, University of Aberdeen, Aberdeen AB24 3UU, Scotland, UK.

 

Received: November, 2007.
Aproved: December, 2008.

 

Abstract

Terra Rossas occur in the eastern part of the Bursa plain, Turkey. The region is covered by large forests growing on Terra Rossa soils. This area has ecological importance for these forests. The purpose of this study was to characterize five Terra Rossa profiles and to relate their properties to the pedogenic processes responsible for their formation under these important forests. Five soil profiles were selected on five different elevations ranging between 200 to 350 m under natural vegetation. Soils are dominated by the influence of climate and lithology. A special classification problem has occurred for two out of five profiles (World Reference Base for Soil Resources). These soils did not meet the clay increase requested by the WRB (2006) classification system for Luvisols. But they were classified due to the high contents of clay, clay coating and infilling in the B horizons and based on field characteristics. The soil samples were examined and classified according to the systems of USDA Soil Taxonomy (2003) and WRB (2006) as Calcic Haploxeralfs and Calcic Luvisols.

Key words: Bursa plain, Mediterranean climate, soil genesis, Terra Rossa.

 

Resumen

Las Terra Rossas se dan en la parte oriental de la llanura de Bursa, Turquía. La región está cubierta por extensos bosques que crecen en suelos de Terra Rossa. Esta zona tiene importancia ecológica para estos bosques. El propósito de este estudio fue caracterizar cinco perfiles de Terra Rossa y relacionar sus propiedades con los procesos pedogénicos responsables de su formación bajo estos importantes bosques. Se seleccionaron cinco perfiles de suelo en cinco elevaciones distintas que variaron de 200 a 350 m con vegetación natural. Los suelos están dominados por la influencia del clima y la litología. En dos de los cinco perfiles hubo un problema de clasificación específico. Estos suelos no cumplieron con el aumento de arcilla establecido por el sistema de clasificación para Luvisoles de la WRB (World Reference Base for Soil Resources, (2006). Sin embargo, se clasificaron por su alto contenido de arcilla, revestimiento y relleno de arcilla iluvial en los horizontes B y con base en características de campo. Las muestras de suelo se examinaron y clasificaron conforme a los sistemas de la USDA Soil Taxonomy (2003) y la WRB (2006) como Haploxeralfs cálcicos y Luvisoles cálcicos.

Palabras clave: Llanura de Bursa, clima mediterráneo, génesis del suelo, Terra Rossa.

 

INTRODUCTION

Terra rossa is reddish clayey to silty–clayey soil especially widespread in the Mediterranean region, which covers limestone and dolomite in the form of discontinuous layer ranging in thickness from a few centimeters to several meters. Red colour (5YR to 10R Munsell hues) is a classical diagnostic feature of Terra Rossa and is the result of rubification, i.e., formation of hematite (Guerra, 1972). In Soil Taxonomy (USDA, 2003), Terra Rossa is classified as Alfisols and according to WRB (2006) Terra Rossa is recognized as Luvisols.

The nature and relationship of Terra Rossa to underlying carbonates is a long–standing problem and there are different opinions with respect to its parent material and origin. The most widely accepted theory is that Terra Rossa has developed from insoluble residue of carbonate rocks (Kubiena, 1953; Bronger et al., 1983; Moresi y Mongelli, 1988). However, other authors have emphasized that Terra Rossa could not have been formed exclusively from insoluble residue of carbonate rocks. Soil geomorphic studies made by Olson et al. (1980) in southern Indiana, USA, indicate that the Terra Rossa is mainly debris, derived from erosion of clastic sedimentary rocks, transported on pediments cut into lower lying limestone. Aydinalp (1996, 2001a, 2001b) stated that Terra Rossas in northwestern Turkey developed on limestone and dolomitic limestone; besides, the Terra Rossa of southern Europe might be wind–borne material from Africa (Rapp, 1984). Eolian contributions have been recognized due to the similarities in clay mineralogy (Aydinalp, 1996; Balagh and Runge, 1970), similarities in heavy mineral fraction (Durn et al., 1992; Durn and Aljinovic, 1995), particle size distribution (Aydinalp, 1996 and McLeod, 1980) and the divergence of oxygen isotopic ratios of associated fine quartz (Jackson et al., 1982). The cryptogamic imprint of the rocks can be detected on the rock faces under present soil surface. Danin et al. (1983) concluded that eolian source is the principal contributor to the formation of the upper soil layer of Terra Rossa in Israel.

Previous statements imply a polygenetic nature of Terra Rossa. In some isolated karst terrain it may have formed exclusively from insoluble residue of limestone and dolomite but, much more often it comprises a diversity of parent materials which derived on carbonate terrain via different transport mechanisms. For example, Yaloon (1997) concluded that practically all soils in the Mediterranean region were influenced by the addition of eolian dust from North Africa. Erosion and deposition processes, which are superimposed on karst terrains and induced by both climatic changes and tectonic movements, might be responsible for thick colluvial Terra Rossa accumulations in uvala and dolina type of karst depressions. In this case, we can consider Terra Rossa as a pedo–sedimentary complex.

As long as the general pedoenvironment remains essentially suitable for the formation of Terra Rossa it is of a little relevance for the process of rubification whether the primary Fe sources are autochthonous or allochthonous (Boero and Schwertmann, 1989).

Merino et al. (2006a) concluded that Terra Rossa soils have long been thought to result from residual dissolution of limestone or to form by accumulation on preexisting limestone karst of detrital mud, ash, or especially dust. Conclusive new field and petrographic evidence for the Terra Rossa in southern Indiana, USA, shows that this soil is formed by replacement of limestone by authigenic red clay at a moving metasomatic front, with the clay's major elements, Fe, Al and Si, coming from dissolved dust, as suggested by strontium isotope ratios. Strikingly, the clay–for–limestone replacement triggers a reactive–infiltration instability, that causes the front to become 'fingered' and 'funneled' on a cascade of scales–precisely the characteristic morphology of karst. That is, the replacement of limestone by clay turns out also to carve the repeated karst funnels and sinks that contain the Terra Rossa itself. This is why Terra Rossa and karst are associated, and how the karst morphology arises. Terra Rossa is thus a metasomatic 'claystone' plus its lateritic or pedogenetic modifications, all hosted in a simultaneously karstified limestone. Karst limestone weathering is driven ultimately by eolian dust supply. The partial validity of both the residual and detrital origins has been a smoke screen that for decades has kept investigators from even suspecting that the true origin of Terra Rossa could be different, or that the way to find it should be petrographic, not chemical.

According to Merino et al. (2006b), Terra Rossas in the Bahamas, the Antilles, Yucatán, Florida, Texas, Kentucky, southern Europe, Israel, southern Australia, and elsewhere, probably have the same origin as the Bloomington Terra Rossa, but a replacement origin, which can be detected only with an optical polarizing microscope, has never been suspected or sought. The iron, aluminum, and silicon needed for clay growth are provided probably by dust; this can be established by matching of strontium and neodymium isotope ratios. Saharan dust is certainly abundant in southern Europe and the Caribbean, both of which are abundant in karst and Terra Rossa too. After settling, the dust dissolves at the surface, the solutes leak in, reach the reaction front at several meters of depth, and drive the precipitation of the red clay crystals that replace the limestone. Terra Rossa is thus a unique laterite – one none of whose major chemical elements comes from its parent limestone

Terra Rossas were derived from hard and unconsolidated limestones that occur widely in the east side of the Bursa region, Turkey. Therefore, the objective of this research was to study genesis of the Terra Rossas formed on five different elevations and to classify them according to the Soil Taxonomy (USDA, 2003) and WRB (2006) systems with their morphological, physical, and chemical properties.

MATERIAL AND METHODS

The soil profiles are located on: 1) 40° 10' 26" N–28° 19' 42" E; 2) 40° 10' 32" N–29° 19' 42" E; 3) 40° 10' 38" N–29° 19' 18" E; 4) 40° 10' 44" N–29° 19' 08" E; 5) 40° 10' 55" N–29° 19' 01" E; in the eastern side of the Bursa plain, Turkey (Figure 1). Profiles represented five Terra Rossas ranging in elevation between 200 m to 350 m and they were derived from unconsolidated limestone; present vegetation is shrubland biome.

The research area was formed during the upper Pleistocene and modified in the Holocene period (Penck, 1953). The soils were formed under Mediterranean climate characterized by hot summers and mild winters. The mean annual precipitation and temperature are 713.1 mm and 14.4 °C. The soil temperature and moisture regimes are thermic and xeric.

The soil profiles in each site were dug down to C horizon (120 cm) and described according to Soil Survey Manual (1993). In each profile, bulk samples of the horizons were taken for laboratory analysis. Particle–size distribution was determined by the hydrometer method (Gee and Bauder, 1982), and pH in a 1:2 soil:water ratio (McLean, 1982). Other variables were organic carbon (Nelson and Sommers, 1982), total nitrogen (Bremner and Mulvaney, 1982) and calcium carbonate (Nelson, 1982), CEC (Rhoades, 1982). Exchangeable cations were extracted from the soil by 1 M NH₄OAc (Thomas, 1982). EC (Rhoades, 1996) was determined by placing 25 g of air dry soil in a 125 mL Erlenmeyer flask, adding 100 mL of sterile deionized water, shaking for 30 min at 250 rpm, allowing the solution to settle for 30 minutes; then gravity filtering the solution through a Whatman 2 filter paper. The clear extract solution received two drops of a 0.1 % sodium hexametaphosphate solution prior to EC measurement. A Hach CO150 conductivity meter was used to determine EC, which was reported as dS m^-1. Iron and manganese oxides were extracted by citrate–bicarbonate–dithionite (Mehra and Jackson, 1960) and analyzed by atomic absorption spectrophotometer. These soils were classified according to the systems of Soil Taxonomy (USDA, 2003) and WRB (2006).

 

RESULTS AND DISCUSSION

The morphological properties of Terra Rossas are described in Table 1. The color of all studied profiles has Munsell color of 2.5 YR hue with value 4 to 5 and chroma 3 to 8 when moist. The morphological properties of soils are quite similar and A horizons are usually quite well preserved, being 10–25 cm thick and with moderate medium subangular blocky structure. The profile thickness varied from 90 to 115 cm and topography changed from convex to concave in the study area. All profiles are well drained.

The physical and chemical properties of the soils are presented in Table 2. Texture ranged between clay and clay loam and clay content (32.9 to 57.1 %) increased appreciably with depth.

These soil profiles developed on unconsolidated limestone and the clay fraction showed higher values than the sand and silt fractions throughout the profiles. The geomorphic position of profiles and their locations affected distribution of the clay content which was significant for profiles 3, 4 and 5 had. Clay content increased sharply with depth for profiles 3, 4, and 5. The 1.2 ratio of total clay in the argillic horizon to that of an overlying eluvial horizon satisfies the requirements of Soil Taxonomy (USDA, 2003). These profiles had higher clay values than the other profiles, because they were developed on the lowest part of the research area. The high clay accumulation in the Bt horizons was due to the high degree of clay illuviation at the lower elevation, their topography and a better water movement throughout these profiles. Therefore, the profiles 1 and 2 did not meet the clay increase requested by the WRB (2006); however, they showed high contents of clay, clay coating and infilling in the B horizons, which are characteristics of argillic horizon. Hopkins and Frazer. (2003) have stated the similar soil forming processes for similar soil profiles; they studied, on the eastern North Dakota till plains since the inception of the Holocene, soils with argillic horizon which were recognized on a very limited basis. The examined profiles showed evidence of genetic argillic horizons based on soil structure, hand texturing, and cutan presence.

The soil pH varied from 7.7 to 8.2 and values increased with depth. The cation exchange capacity (CEC) of these soils ranges between 30.4 to 50.8 cmol (+) kg^–1 and organic matter conditions were low. This means that CEC was largely due to clay presence. Exchangeable calcium and magnesium are dominant cations followed by sodium and potassium. The use of ammonium acetate pH 7 could lead to overestimation of exchangeable calcium. The values of CaCO₃ (1.2 to 22.8 %) were high and increased with depth due to presence of the alkaline parent material, which is unconsolidated limestone. This trend shows that there is a calcification in these soils. The organic C, total N, and C/N values are generally very low; the low carbon values are due to the high rate of decomposition of organic matter, occurring in most soils of the region (Aydinalp, 1996). The electrical conductivity values decreased with depth and the results indicated that the soils are not saline under these conditions.

The profiles 3, 4, and 5 had higher citrate–bicarbonate–dithionite extractable Fe values than the other two profiles. The citrate–bicarbonate–dithionite extractable Mn values decreased in the all soils with depth. The results of the present research indicated that the soils of the study area exhibit a similar degree of development. But, local microtopograpghy have caused some differences in the chemical and physical properties of the soils, especially the clay illuviation and the citrate–bicarbonate–dithionite extractable Fe values. The differences in soil properties were due to different location of the profiles in relation with the geomorphological position of the study area at the profiles 3, 4, and 5. Sgouras et al. (2007) studied some soils from Greece and classified them as Haploxeralfs and Rhodoxeralfs. They found similar distribution of the citrate–bicarbonate–dithionite extractable Fe and Mn and stated that the Mediterranean climate affects many soil properties in this region.

The studied soils were classified according to their type and sequence of horizons. The cambic, argillic and calcic horizons were defined in for these soils. The middle horizons were identified as cambic B (Bw) and argillic B (Bt) horizons. The lowest horizons were identified as calcic (Ck) horizon due to accumulation of carbonates. These soils were classified according to the Soil Taxonomy (USDA, 2003) and WRB (2006) systems as Calcic Haploxeralfs, and Calcic Luvisols.

 

CONCLUSIONS

The formation and evolution of Terra Rossa soils under a xeric climate in Turkey, similar to other that prevails in most areas around the Mediterranean basin, are affected by the role and the relative importance of soil genesis factors, such as parent material, time, topography, and climate. The profile developments when all these factors have an optimal effect correspond with the Alfisols order. These soils have slightly to moderate alkaline reactions, low organic matter, and high CaCO₃ content in the Ck horizons. The red color is related to amounts of iron oxides in the profiles. A special problem was encountered in classifying two out of five profiles. These soil profiles did not meet the clay increase requested by the WRB (2006) classification system to be classifies as Luvisols, but were classified as Calcic Luvisols due to the high contents of clay, clay coating and infilling in the B horizons and based on field characteristics. The soils were examined and classified according to the Soil Taxonomy (USDA, 2003) system as Calcic Haploxeralfs. These soils are similar in their physical, chemical and morphological properties. Soils are dominated by the influence of climate and lithology.

 

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