Characterization and Classification of Soils in the Coastline of Abu Dhabi Emirate

Transcription

1 Characterization and Classification of Soils in the Coastline of Abu Dhabi Emirate Mahmoud A. Abdelfattah 1 and Shabbir A. Shahid 2 (1) Environment Agency, ABU DHABI, P.O. Box 45553, Abu Dhabi, UAE, mabdelfattah@ead.ae (2) International Center for Biosaline Agriculture, P.O. Box 14660, Dubai, UAE. s.shahid@biosaline.org.ae Abstract Soil classification of over 400 km coastline of Abu Dhabi Emirate was completed using the Soil Taxonomy of the United States Department of Agriculture. Landsat-7 ETM (Enhanced Thematic Mapper) image acquired in 2002 as well as GIS technique were used to develop soil map. Soil investigation revealed that the uniform looking landscape presents a diversity of subsurface features that enabled soil categorization into thirteen families under two soil orders (Aridisols and Entisols). In general the soils are highly saline, seasonally wet and flooded, have salinc horizon and present water table within the upper 100 cm depth (Typic Aquisalids), and below 100 cm (Typic Haplosalids), highly saline and gypsiferous with water table in the upper 100 cm (Gypsic Aquisalids), others show dominantly coastal shells (Typic Torriorthents), aeolian sand (Typic Torripsamments), gypsum accumulation starting within upper 18 cm (Leptic Haplogypsids) and calcareous hardpan in the upper 100 cm (Typic Petrocalcids). Factors and processes of soil formation are also described. Areas of halite (NaCl), anhydrite (CaSO 4 ) and Gypsum (CaSO 4 2H 2 O) occurrence are highlighted. A total of 220 soil polygons covering an area of 377,045 hectares were identified and soil map published at scale of 1:50,000. Key-words: Soil Classification, Abu Dhabi Emirate Coastline, Sabkha, USDA Soil Taxonomy. Introduction Arid and semi-arid environments occupy around 37% of the land on earth. Soils in these environments differ from those in humid zones due mostly to scant moisture conditions (Cooke et al., 1993 cited from OALS, 2005). In arid and semi-arid regions, most of the USDA soil orders can be found except Gelisols, Histosols and Spodosols; however, the common orders are Aridisols, Entisols, Vertisols and to a lesser extent Mollisols, Inceptisols and Alfisols (OALS, 2005). Soil classification allows transfer of knowledge regionally and internationally without conducting long-term experimental trials. Abu Dhabi Emirate is one of the seven Emirates of the United Arab Emirate (UAE). The UAE is an arid country and occupies an area of about 83,300 km 2, of which Abu Dhabi Emirate occupies an area of about 68,000 km 2. Detailed soil information and soil maps are non-existent for the country. However, other Gulf Cooperative Council (GCC) countries such as Kingdom of Saudi Arabia, Sultanate of Oman, State of Kuwait, and State of Qatar have already completed soil inventory for land use planning. Coastlines very often present distinctive geomorphological and geo-ecological phenomena as a result of specific morphodynamics with certain hydrologic

2 conditions. Therefore, the coastline of Abu Dhabi which stretches over a distance of 400km was selected as a pilot area to complete the present study. Recently an Emirate wide soil inventory has commenced on April 2006 to be concluded by October In this paper results from pilot study are presented and relation with the arid conditions of the Arabian Peninsula and arid & semi-arid soils compared. More information can be obtained from Shahid et al. (2004). The study area The UAE is located between 22 o 50' and 26 o north latitude and 51 o and 56 o 25' east longitude at the southeastern tip of the Arabian Peninsula (Fig. 1). The country is composed of seven Emirates namely, Abu Dhabi, Dubai, Sharjah, Ajman, Fujairah, Ras Al Khaimah, and Umm Al Quwain (Boer, 1998). Abu Dhabi Emirate is the largest among them constituting about 84% of the total area (Fig. 2). Abu Dhabi's major ecosystems comprise the coast, numerous islands, mountainous areas, gravel plains and sand desert (UAE Yearbook, 2003). The present pilot study is focused only to the coastline excluding the offshore islands. Fig. 1. Location of UAE Fig. 2. Location of Abu Dhabi and the other Emirates Materials and Methods Remotely Sensed data, Landsat-7 ETM image (Enhanced Thematic Mapper) acquired in 2002 was used to develop a preliminary soil map through visual interpretation that was used in the field as base map. Representative soil samples have been collected from typical profiles, processed and analyzed for physical and chemical parameters. Standard USDA laboratory procedures were used (Soil Survey Staff, 1996 and 2004) to analyze soils for saturation extract analyses (cations and anions), coarse fragments, texture, gypsum, organic matter, phosphorous, nitrate, saturation percentage, soil reaction, sodium adsorption ratio, carbonates equivalents, total pretreatment loss, available K, cation exchange capacity, exchangeable sodium percentage. The whole soil mineralogy and clay mineralogy of the control sections of typical soil profiles were established using X-ray diffraction technique using Cu K alpha radiation, Ni filter and variable divergent, receiving/scattering slits.

3 Fieldwork and production of soil map Owing to its wider scope, comprehensiveness and relevance to other regions, the United States Department of Agriculture soil classification system (USDA Soil Taxonomy) has been used to classify the soils of the study area (Soil Survey Staff, 1993; 1996; 1998a; 1998b; 1999; 2003 and 2004). The USDA Soil Taxonomy is used in more than 75 countries including neighbouring gulf countries, e.g. Kingdom of Saudi Arabia (1985), Sultanate of Oman (1990), State of Kuwait (1999) and State of Qatar (2005). To prepare an accurate soil map, the boundaries between the map units were developed using an integrated approach that is combining the visual interpretation of remotely sensed data with the information obtained from field observations (ground truthing). The compositions of map units (association, consociation and complex) are determined using random transects across the landscape with generally hand augers, pits, mini-pits and typical profiles. A total of 775 observations were made. Soil information recorded at each site includes; landform element and pattern, relief/modal slope class, microrelief, surface condition, land use/ vegetation cover, coarse fragments, slope, erosion, moisture condition, drainage, permeability class, root restriction depth and water table depth. In addition, a complete morphological description for the soil pits, auger-holes and typical profiles was completed including color, texture, size/shape of aggregates, fragments, consistence, plasticity, cementation, roots, pores, segregation, mottling, concentration, etc. and all other features that enable to identify soils. Results and Discussion Formation of soils During soil mapping the pedologists explore about soil genesis including factors and processes of soil formation which continue to affect soils even on stable landscapes. A brief description of these factors and processes dominant in the study area is given below. Formation of soils Factors Parent material: The soils of Abu Dhabi coastline are formed from different kinds of parent materials such as coastal shells, marine and aeolian sand, silts, clays and evaporites of Na +, K +, Ca 2+, Mg 2+ with Cl -, SO4 2- and HCO - 3. In wadis or depressional landscapes, these materials are laid down by water. The soils surrounding the highlands have an influence from its composition (colluvium); however, in places soils around highlands are formed by combination of alluvium and colluvium. The soils also have an influence from shallow groundwater and are very strongly saline (locally known as sabkha, which in Arabic means salt flat). The coastal sabkha of Abu Dhabi are hyper-saline where seawater flooding is the dominant process by which sabkha are recharged (Sanford, 2001). The seasonal fluctuation of water table determines the kind of Salids (Haplosalids or Aquisalids) and influences the depth and development of subsoil. Climate: The UAE has harsh climatic conditions. High temperature in association with high salt concentration affects the growth of vegetation, activity of microorganisms, chemical reactions and weathering of parent material. As a result of such conditions and also due to the occurrence of lithologic

4 formations such as gypsiferous marls, shales and limestone, climofunctional and lithofunctional soils occur extensively. Evaporation results saline surface, major salt minerals are halite, anhydrite and gypsum. Wind redistributes sand and other particles. Rainfall is very low and infrequent; however, it affects the extent of salt leaching and movement in the depressional areas. Topography (Relief): Topography in the coastline of Abu Dhabi determines to a large extent the type of soils. In the depressions, water table is near the surface (mostly within the upper 1 meter) which is a conducive environment for forming Aquisalids. In nearly level to undulating areas, water table is deeper than 1 meter hence forming Haplosalids. Adjoining to coastline sandy desert soils (Psamments) are dominant. Biological factors: Vegetation has insignificant role in soil formation in the saline areas due to high salts that prevent vegetation growth. However, it has role in the formation of Orthents, Psamments, Gypsids and Calcids; particularly in soil binding and nabkha development as habitats for living organisms. Time: In arid regions and due to less precipitation, development of soil profile is very slow compared with humid areas. The coastal soils (tidal flat areas) receive regular sediments from sea and some wind blown deposits and with time soil development proceeds. The inland sandy soils (Psamments) are prone to wind erosion and their development is very slow rather they are young and require long periods of special management for development. Formation of soils Processes Salinization: Salinization is the most dominant process recognized in the coastline. The surface salt crust was found in different forms; polygonal and hexagonal patterns, sealed surfaces, and upturned salt flakes, associated with high water table with EC reaches 200 ds/m. The halite is the dominant salt mineral accompanied by gypsum, anhydrite and calcite in different quantities. Calcification and decalcification: In the coastline the most likely source of carbonates in addition to parent material is coastal shells. In these soils, carbonates were disseminated throughout the soil matrix such that specific carbonate features were not visible. In some profiles, carbonates are precipitated or cemented to a hardpan (Petrocalcids), which is common in the eastern coast of the area. The calcium carbonate accumulation is coupled with its removal from overlying horizons (decalcification). The process where carbonate accumulated is called calcification and where dissolved is called decalcification. Gypsification: Gypsum is the second frequently occurring mineral and identified in different morphological forms (flakes, acicular, lenticular, rosettes etc). The gypsic soils are mapped as Gypsic Haplosalids, Gypsic Aquisalids, Leptic Haplogypsids and Typic Haplogypsids. Gypsum was also mapped as gypsum outcropping. Distribution of soils in Abu Dhabi Coastline The distribution of soils in the coastline of Abu Dhabi Emirate is systematically related to local geomorphology, pedogenic accumulations, salt concentrations, calcium carbonate content, and mineralogical compositions. The uniformly looking landscapes present a diversity of subsurface features that enables identification of thirteen soil classes at family level. Soil towards the coast is highest in salt contents (Typic Aquisalids and Typic Haplosalids), high salts and rich in gypsum contents (Gypsic Haplosalids and Gypsic Aquisalids), and calcareous hardpan (Typic

5 Petrocalcids). Far from the coast (within sand dunes), deep sandy soils occur (Typic Torripsamments). Pedogenic accumulations varied from one area to another; i.e., rich in gypsum contents (Leptic Haplogypsids), and in calcareous coastal shells (Typic Torriorthents). Almost all soils of Abu Dhabi are calcareous in varying degrees (Table 1). Table 1. Comparison between using USDA Soil Taxonomy FAO/UNESCO Soil Classification Soil No. Soil Types USDA Classification at family level FAO Classification 1 sandy, carbonatic, hyperthermic Typic Aquisalids Calcic Solonchacks 2 sandy, mixed, hyperthermic Typic Aquisalids Haplic Solonchacks 3 fine-clayey, anhydritic, hyperthermic Gypsic Aquisalids Gleyic Solonchacks 4 sandy, gypsic, hyperthermic Gypsic Aquisalids Gypsic Solonchacks 5 sandy, carbonatic, hyperthermic Typic Haplosalids Calcic Solonchacks 6 sandy, mixed, hyperthermic Typic Haplosalids Haplic Solonchacks 7 sandy, gypsic, hyperthermic Gypsic Haplosalids Gypsic Solonchacks 8 sandy, gypsic, hyperthermic Leptic Haplogypsids Haplic Gypsisols 9 sandy-skeletal, mixed, hyperthermic Leptic Haplogypsids Haplic Gypsisols 10 sandy, carbonatic, hyperthermic Typic Petrocalcids Petric Calcisols 11 sandy, carbonatic, hyperthermic Typic Torriorthents Calcaric Arenosols 12 carbonatic, hyperthermic Typic Torripsamments-inland Calcaric Arenosols 13 carbonatic, hyperthermic Typic Torripsamments-coastal Calcaric Arenosols Classification of soils using World Reference Base for Soil Resources (FAO/UNESCO) To allow better understanding and comparison of the study area soils, the World Reference Base (WRB) for Soil Resources (FAO/UNESCO Classification System) is used, which has 28 Major Soil Groupings at the highest level of generalization, subdivided into a total of 153 Soil Units at the second level. In the revised legend of 1988, a third hierarchical level that of the Soil Subunits was introduced. Using the WRB, the major soil groupings presented within the dry zones where most of them have been identified in the study area are: soils having accumulated soluble salts (Solonchaks), gypsum (Gypsisols), carbonates (Calcisols), a high percentage of adsorbed sodium (Solonetz), coarser to sandy loam textured (Arenosols), and having non developed diagnostic horizons (Regosols) (FAO/UNESCO; 1988 & FAO-ISSS-ISRIC, 1998; and Van Baren, 1988). Table 1 illustrates a comparison between classifying the study area soils using USDA Soil Taxonomy with the WRB for Soil Resources. When we classified the soils of the study area using WRB (FAO classification system), certain soils were classified as having salic diagnostic horizon is Solonchacks, gleyic properties within the first 50 cm as Gleyic Solonchacks. However, this classification doesn t take into consideration the presence of carbonates, gypsum, mineralogical characteristics etc, of the soils.

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