Metal accumulation and excluder behaviour of five indigenous herbaceous species at the abandoned Enyigba Pb–Zn mine, southeastern Nigeria

Artisanal lead-zinc mining at the Enyigba Pb–Zn mine, Ebonyi State, has left tailings soils heavily contaminated with lead and cadmium (soil Pb 1114.5–1125.4 mg kg⁻¹; soil Cd 125.7–135.7 mg kg⁻¹, approximately 13-fold and 157-fold above WHO agricultural-soil limits), but the metal-accumulation behaviour of the herbaceous vegetation naturally colonising the site has not previously been characterised. In this study, heavy metal accumulation in five dominant indigenous species (Gloriosa superba, Aspilia africana, Crotalaria retusa, Tacca leontopetaloides and Amorphophallus johnsonii) along a 100 m by 100 m sampling plot was assessed, analysing rhizosphere soil and root, stem and leaf tissue for Pb, Cd, Mn, Co and Cu by atomic absorption spectrophotometry. Bioconcentration and bioaccumulation values were below 1 for every species-metal combination (BCFroot 0.0002–0.0152; BACshoot 0.0000–0.0226), indicating a predominantly excluder-type accumulation pattern rather than strong accumulation relative to soil, while translocation factors exceeded 1 for Cd in all species (1.212–1.969) and for Pb and Cu in most species. A. johnsonii exhibited the lowest Pb concentrations in root, stem and leaf tissue while showing the highest shoot Cu bioaccumulation coefficient; C. retusa exhibited the highest BACshoot values for Pb and Cd among the species examined; T. leontopetaloides sequestered the most root Cu. Interspecific differences were statistically significant for Pb, Cd, Mn and Cu (p