Sustainable restoration of endangered Limonium daveaui Erben: overcoming mining waste and saline soil challenges with soil organic and inorganic amendments

This study evaluates the growth and adaptation of the endemic and endangered halophyte Limonium daveaui Erben, native to the Tagus estuary (Portugal), within severely degraded soils from two distinct environments: gossan wastes contaminated with potentially hazardous elements (PHE) from the São Domingos mine, and saline soils from a degraded estuarine area. A microcosm assay was conducted with four substrate treatments: (i) gossan waste, (ii) amended gossan composite soil, (iii) Salic Fluvisol, and (iv) amended Salic Fluvisol. The amendments comprised organic and inorganic waste materials. Results revealed that both untreated gossan waste and saline soil exhibited extreme physicochemical challenges for plant growth, including high salinity, low organic matter, and elevated concentrations of PHE. However, the application of amendments significantly improved soil properties, mitigating these adverse conditions. Enhanced enzymatic activity was observed in amended soils, with enzymatic responses varying by substrate type. In the amended soils, plant biomass (both shoots and roots) doubled, flower production increased significantly, and nutrient cycling was optimized through salt gland activity, which facilitated the excretion of Na without accumulating PHE in salts. Despite these improvements, plants grown in amended gossan soils exhibited levels of As and Pb concentrations in their shoots that exceeded toxic thresholds for plants. These findings highlight the physiological adaptations of halophytes, particularly the role of salt glands in regulating ionic balance and enabling survival in saline and metal(loid)-contaminated environments. The study emphasizes the potential of combining halophyte cultivation with targeted soil amendments as a sustainable strategy for ecological restoration of degraded lands and the conservation of endangered species.