Integrated Assessment of Mercury Cycling in Pinus pinea: From Soil Re-emission and Nocturnal Atmospheric Accumulation to Foliar Immobilization

The Mining District of Almadén (Spain) provides a valuable natural setting for studying the biogeochemical behavior of mercury (Hg) under conditions shaped by long-term, legacy contamination. This study presents an evaluation of the Hg cycle through the simultaneous characterization of soils, atmospheric dynamics and Pinus pinea tissues (leaves and bark). The results reveal that, although soils present extreme concentrations (range: 2.9 – 123 mg kg^-1 dominated by stable species (α-cinnabar), there is a critical labile fraction associated with organic matter and metacinnabar that acts as a continuous source of re-emission. Mobile monitoring demonstrated a drastic dichotomy in total gaseous mercury (TGM) levels, identifying nighttime atmospheric stability as the main forcing mechanism for Hg accumulation in the tree canopy. Pyrolytic speciation in vegetation revealed a functional divergence between tissues: the cortex acts as a passive physical trap for cinnabar and Hg2+ while the needles function as active physiological sinks of gaseous Hg0, validating an intracellular immobilization mechanism. Lastly, data integration indicates that the pine tree functions as a "biological pump" that recirculates atmospheric mercury into the soil; however, this cycle is disrupted in urban settings by street cleaning and management, which attenuates the soil burden in comparison to natural systems; and by wind driven atmospheric dilution.