Evaluating the Impact of Tree Plantations on Soil Rehabilitation in Semi-Arid Phosphate Mining Lands of Morocco

Mining activities significantly disrupt land morphology, soil structure and ecosystem functioning, making effective rehabilitation strategies crucial for ecosystem restoration. In Morocco's phosphate mining regions, understanding the impact of rehabilitation strategies is essential for sustainable land management. This study evaluates the effectiveness of tree plantations in rehabilitating phosphate mining lands in Morocco by examining their impact on key soil properties. We compared soil properties between mined and unmined sites using a sampling design that considered five tree species (Eucalyptus sp., Ceratonia siliqua, Olea europaea, Argania spinosa, and Schinus molle), three proximities to the tree stems (0.5 m, 1.5 m, the intertree zone), and three soil depths (0–10 cm, 10–30 cm, 30–60 cm). Soil samples were also collected from non-planted mined areas and unmined sites as controls.

Our results revealed significant differences between mined and unmined soils, with tree plantations markedly increasing soil organic carbon (SOC) and cation exchange capacity (CEC) compared to controls. SOC displayed a spatial gradient, decreasing with distance from tree stems and increasing in the topsoil compared to deeper layers. Among tree species, olive trees (O. europaea) demonstrated the highest SOC enhancement in disturbed soils, with SOC levels near tree stems of 12.1 g/kg, and of 11.1 g/kg in the intertree zone, representing increases of 83%, and 68%, respectively, compared to control (6.6 g/kg). Similarly, under false pepper plantations (S. molle), SOC in the topsoil layer (0–10 cm: 10.7 g/kg) was significantly higher than deeper layers (8.0-8.2 g/kg). CEC in rehabilitated mined soils improved modestly, with the highest increase of 1.66 meq/100g observed in young false pepper stands compared to controls. Soil pH remained relatively stable, with minor decreases of up to 0.28 units, particularly under mature eucalyptus (Eucalyptus. sp.) and carob (C. siliqua) plantations. However, bulk density remained higher in mined soils (1.91 g/cm³) compared to unmined soils (1.35 g/cm³), reflecting incomplete soil structure recovery. In the other hand, natural soils outperformed rehabilitated mined soils in chemical properties, exhibiting higher SOC levels (12.4 ± 3.0 g/kg) and CEC (10.79 ± 6.40 meq/100g) compared to mined soils (8.5 ± 4.8 g/kg SOC and 4.99 ± 2.11 meq/100g CEC). The study underscores the importance of species selection in mine rehabilitation programs, particularly in semi-arid regions. By demonstrating the superior performance of certain tree species in enhancing soil properties, our results provide evidence-based recommendations for optimizing restoration strategies in post-mining landscapes in Morocco and similar semi-arid environments.

Keywords: Soil rehabilitation, Phosphate mining lands, Tree plantations, Semi-arid regions, Soil organic carbon (SOC)