Assessing geomorphological and pedological processes in the genesis of pre-desert soils from southern Tunisia
Introduction
Soils affected by water deficiency and consequent salt accumulation cover ≈12% of the global ice-free land area (Sombroek, 1987). Such soils are particularly prevalent in the Sahara desert, sub-Saharan area, Turkestan desert, Gobi and Taklamakan deserts, central Australia, southern Argentina, south western Africa, south western USA, Mexico, Pakistan, and many countries of the Middle East. In many places, pedogenesis of drought-affected soils has been studied since the beginning of the last century with investigations based on soil survey, photo-interpretation, remote sensing, and laboratory analysis. For example, Khresat et al. (2004) investigated soils of arid environments from north eastern Jordan to assess how geomorphic features interact with soil forming processes. Yamnova and Golovanov (2010) studied how geomorphology influenced the formation of gypsum in Aridisols from Uzbekistan’s Turkestan desert. In the Saharan and pre-Saharan areas, geomorphological and pedological studies are of great concern because of the connection with themes of planetary interest such as desertification, global warming, famine, and human migrations. Many studies performed on soils from arid environments concern formation processes as an understanding of the phenomena involved in soil genesis may contribute to protecting them from degradation (Mtimet, 1983). As such, soil study in northern Africa has been fairly extensive. Morocco (e.g., Hofmann et al., 2000, Linstädter and Kehl, 2012), Algeria (e.g., Renac and Assassi, 2009), Tunisia (e.g., Kamoun et al., 1999, Zaaboub et al., 2005, Henchiri, 2007, Mannaï-Tayech, 2009, Gallala et al., 2010), Libya (e.g., Hunt et al., 2010, Zerboni et al., 2011), and Egypt (e.g., Wanas, 2002) all feature considerable studies of arid soils. Soil formation in such arid areas has mostly been led by a combination of climatic and geomorphic features, with the main processes represented by wind erosion/accumulation, evapotranspiration, and salt precipitation. Valley floors also feature alluvial sedimentation due to ancient flooding events. However, an understanding of the interrelationships among climatic changes, geomorphic processes, and pedogenesis in arid environment remains scarce.
As such, this study focuses on pre-Saharan areas of the southern part of Tunisia, where previous soil studies have provided information on both general and site-specific soil forming processes (e.g., Bureau and Roederer, 1961, Pouget, 1963, Mtimet, 1983, White et al., 1996, Fromm et al., 2005, Bismuth et al., 2009, Hannachi et al., 2015). However, details on the interconnection between climatic changes, development of geoforms, and pedogenesis remain rare. Especially lacking are considerations of the influence of the groundwater at shallow depths, ancient mudflows and aeolian transports, and palaeosols on pedogenetic processes like the development of horizons or mineral alteration. Knowledge of these aspects is essential for reconstructing soil developmental factors such as hydrology and historical climate, the results of which will help to protect the current soil from degradation. As such, this research seeks to further understand the processes involved in soil formation in the pre-Saharan area of southern Tunisia, considering both coastal oasis and inland environments.
Section snippets
Study area
The Jeffara Plain (JP) is a coastal plain of north Africa with a rough semicircular shape, extending from the low plains located south of the Gafsa Mountains (Tunisia) to Tripoli (Libya). The area is ≈37,000 km2 (Fig. 1) and geologically belongs to the northern sector of the Saharan platform, characterized by a sedimentary sequence associated with Lower Triassic to Upper Cretaceous material (Rigane and Gourmelen, 2011). The JP is separated from the Sahara by a series of cuestas running from
Chenini Nahel
At the soil surface, diffused saline efflorescences of gypsum and halite were observed (Table 1). The soil was mostly dominated by gypsum and showed a pale brown to light yellowish brown color, a poorly developed and friable structure (with the exception of the Ayy horizon), and a very low diffusion of the finer roots. Below 60–70 cm, redoximorphic features (e.g., Mn concretions, mottling) were present due to alternating periods of reduction and oxidation. No grains with paramagnetic behavior
Chenini Nahel
The development of weak soil structure was ascribed to the low diffusion of the fine roots (e.g., Cocco et al., 2013, Cocco et al., 2015, Fageria and Stone, 2006, Walley et al., 2005). However, scarce aggregation and poor root diffusion were rather expected as they are generally limited by the high salinity/sodicity of the soil (Al-Barrak and Rowell, 2006, Horneck et al., 2007, Khresat and Qudah, 2006, Seelig, 2000). Clues about possible sodicity of this soil were represented by the abundance
Conclusions
The dominant process of soil formation in the Jeffara Plain (southern Tunisia) is a combination of additions and losses controlled by climate changes over the last 10 K years. Some difference was noted among the three study sites, which all exist today in a similar aridity index. The soils at Chenini Nahel mainly formed from the accumulation of wind-blown sediments coming from a close area dominated by gypsum-bearing rocks. Further, the formation of gypsum-enriched horizons was also due to the
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We are grateful to the Arid Regions Institute (IRA) of Gabès (Ministry of Agriculture, Hydraulic Resources and Fisheries, Tunisia), and in particular with Rachid Boukchina and Naceur Haj Ahmed for their help during soil surveying. We are indebted with Naceur and Nader Hannachi for their assistance during field operations and Carlos Villamil for laboratory assistance. We also thank Alessandra Negri and Luigi Gobbi for their help in microscopy observations. We acknowledge the financial support of
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