Metals in Soils and Plants of Ultramafic Sites of South Bulgaria

The Balkan Peninsula hosts some of the largest continuous ultramafic and serpentine areas in Europe, with extensive outcrops in southern Bulgaria, particularly in the Central and Eastern Rhodopes. These environments are characterized by high concentrations of potentially toxic metals, nutrient imbalances, and distinctive ecological conditions that strongly constrain plant establishment and ecosystem functioning. This study aimed to (i) characterize the geochemical and mineralogical properties of ultramafic soils from southern Bulgaria, (ii) assess the distribution and bioavailability of nickel (Ni) as a key ecological factor, and (iii) evaluate plant responses, with emphasis on phytostabilization and hyperaccumulation strategies. Soil and plant samples were collected from representative ultramafic sites. Soil physicochemical properties, total and extractable metal concentrations, iron oxide fractions, and mineralogical composition were determined using standardized analytical methods. Plant samples were analyzed for Ni concentrations in aboveground biomass. The soils exhibited neutral to slightly alkaline pH, low to moderate cation exchange capacity, and high total Ni concentrations (912–2397 mg kg⁻¹). Mineralogy was dominated by serpentine-group phases, with variable proportions of secondary carbonates and clay minerals. Marked site-specific differences in iron oxide fractions reflected variability in pedogenetic development and exerted a primary control on Ni partitioning and bioavailability. Most plant species showed low shoot Ni concentrations, indicating exclusion or tolerance strategies. In contrast, Odontarrhena muralis and Noccaea caerulescens exceeded 1000 mg kg⁻¹ Ni, confirming their hyperaccumulating capacity. Overall, the study provides an integrated soil–plant perspective on temperate ultramafic ecosystems, with implications for serpentine ecology, conservation, and sustainable land management.