Assessing geomorphological and pedological processes in the genesis of pre-desert soils from southern Tunisia

Arid environments are fragile and the associated soils are subject to serious threats like water deficiency, erosion, salt accumulation, and loss of fertility. In this context, understanding the processes involved in soil genesis may contribute toward protecting land from degradation. This study highlights the interconnection between geomorphic and pedogenetic processes in soil formation of the Jeffara Plain, a pre-Saharan area of southern Tunisia. To reach this goal, one coastal oasis (Chenini Nahel) and two inland environments (Matmata Nouvelle and Menzel Habib) were studied. After geomorphological and pedological surveys, the soils were sampled by genetic horizons and characterized by physical, mineralogical, and chemical analyses, and by microscope observation. Field observations and laboratory data suggest that soil formation in the Jeffara Plain was a combination of additions and losses controlled by climate changes. At Chenini Nahel, the soils developed by accumulation of wind-blown sediments coming from a close area dominated by gypsum-bearing rocks. At Matmata Nouvelle, the soils mainly formed from sedimentation of repeated mudflows during a rainy period between 9000 and 5000 years before present, followed by drought periods. Finally, the soils of Menzel Habib developed from an early gypsum formation in the presence of a salt-rich water table and repeated cycles of sedimentation/deflation of wind-blown materials. The different genesis of these pre-desert soils produced characteristic B horizons: Byy horizons with poorly developed soil structure at Chenini Nahel, Bw horizons with a hard rupture-resistance at Matmata Nouvelle, and Bk horizons at depth due to intense sedimentation with CA + BC horizons at the surface due to the accretion of wind-blown materials at Menzel Habib.