Soil health impairment in response to long-term exposure to heavy metal-loaded effluents from a phosphate fertilizer industry in southwestern Tunisia

Heavy metal (HM) contamination of soils surrounding industrial areas represents a major threat to soil ecosystem functioning and agricultural sustainability. This study investigated the long-term impact of metal-laden effluents from a phosphate fertilizer industry on soil health in Mdhilla (Gafsa, southwestern Tunisia) using an integrated physicochemical and biological assessment. Total and EDTA-extractable (operationally defined bioavailable) concentrations of twelve HMs, together with microbial abundance and six soil enzymatic activities, were evaluated at 13 sites distributed along a gradient from the industrial source to adjacent agricultural fields. Substantial metal accumulation was detected, with total concentrations ranging from cadmium (Cd: 0.1 ± 0.01–17.9 ± 0.6 mg/kg), chromium (Cr: 3.2 ± 0.4–431 ± 5.1 mg/kg), arsenic (As: 0.9 ± 0.05–10.3 ± 0.4 mg/kg), cobalt (Co: 0.2 ± 0.004–11.1 ± 0.3 mg/kg), thallium (Tl: 0.02 ± 0.01–1.2 ± 0.05 mg/kg), molybdenum (Mo: 0.3 ± 0.1–8.5 ± 0.2 mg/kg), selenium (Se: 0.6 ± 0.2–20.1 ± 2 mg/kg), manganese (Mn: 7.4 ± 0.7–389 ± 10.7 mg/kg), nickel (Ni: 2.2 ± 0.7–34.4 ± 0.5 mg/kg), zinc (Zn: 9.5 ± 1.6–259 ± 4.5 mg/kg), and copper (Cu: 1.3 ± 0.2–14.04 ± 0.3 mg/kg). Elevated EDTA-extractable fractions of Cd (up to 12 ± 0.4 mg/kg), Cr (10 ± 0.2 mg/kg), and Tl (0.2 ± 0.01 mg/kg) were associated with high ecological risk levels, particularly near the industrial zone. Contaminated soils were characterized by reduced microflora and marked inhibition of enzymatic activities, with strong negative correlations between metal concentrations and enzyme responses. Activity Change Ratio analysis indicated severe inhibition, with β-glucosidase and fluorescein diacetate being the most sensitive indicators. Beyond the local context, this integrated framework combining soil physicochemical properties, total metal concentrations, EDTA-extractable metal fractions, microbial characterization, and soil enzyme activities provides a transferable approach for assessing long-term ecological risks and functional soil degradation in arid and semi-arid regions impacted by long-term discharges from phosphate fertilizer industries.