Neutral and slightly alkaline arable soils from the vicinity of the former and the biggest metallurgical plant in southeastern Europe were analyzed for the status of the water soluble pool of heavy metals in 1–20% w/w biochar (BC)-amended contaminated soils. Heavy metal solubility was monitored over a 6-month period. The metals (Fe, Mn, Cu, Zn, Cd, Pb and Ba) exhibited significant relationships between each other and exceeded the maximum permissible concentrations (MPC) in surface waters for domestic and drinking purposes. In most of the investigated sites and BC treatments, metal concentrations decreased with time due to the transfer to more resistant soil pools. Cation exchange capacity, exchangeable Ca and pH increased after BC application, while electrical conductivity decreased. BC amendment led to the prevalence of humic acids (HAs) over fulvic acids (FAs) and increased the fraction of refractory organic carbon. The share of metal–organic complexes increased for the metals Zn, Cd, Mn and Ba in the BC-amended soils, and the share of free Me²⁺ species decreased. This trend was especially pronounced in the soils with the lowest pH of 6.4–6.9. In addition to improving soil physicochemical and ecochemical properties, biochar application contributed to metal species in solutions that were less mobile and bioavailable.

Six soil profiles with metamorphic and weakly developed structure, formed on old and young Quaternary proluvial and colluvial deposits, were studied. A comprehensive dataset was obtained for over 50 elements (including more than two-thirds of the CRMs), across 20 genetic horizons of various types - metamorphic, gleyed, and contaminated with agglomeration dust, coke, and other materials. Due to the growing interest and knowledge of these raw materials, this study has derived statistical relationships between many of them in the context of increased values of most. The soils are geochemically and technogenically enriched, reflecting both natural and anthropogenic influences. Three genetic and one technogenic geochemical associations were identified through XRD, LA-ICP-MS, statistical and geochemical analyses (PCA, Pearson’s correlation, Concentration coefficients - CC). The plutonogenic association Hf–P–W–Nb–Ce/LREE–Sr–Bi–Ge+Ta–Th–Rb–Cs–Au, derived from the potassic-alkaline Buhovo–Seslavtsi pluton, shows strong correlations (r (P) = 0.46–0.86; r (W) = 0.50–0.98; r (Th) = 0.55–0.98). The ore-related association As–Mo–Co–Ni–Ga–Mn–Sb–Cu–Zn–U–V (r = 0.45–0.94) reflects proluvial material enriched with sulfide and uranyl-phosphate minerals. The lithophillyc association Ga–Y/HREE–Ce/LREE–Al–Ti–Sc–Si (r(Sc) = 0.63–0.88) represents a stable Al–Si–oxide-controlled system, while the technogenic Ba–Mn–Pb–Ag–Sr–Sb–Cu association (r(Ba) = 0.51–0.75; r(Pb) = 0.41–0.91) records up to 11-fold Pb and 4-fold Ba enrichment in surface horizons due to persistent metallurgical impact.