Legacy mercury emissions and vegetation-mediated Hg cycling in the world's largest Hg mining district.

The Almadén district (Spain), the world's largest historical mercury (Hg) mining area, remains a major legacy source of atmospheric Hg driven by soil re-emissions and nocturnal boundary layer stability. This study evaluates the role of Stone pine (Pinus pinea L.) as both a biomonitor and an active interface in the Hg cycle by integrating spatial modelling, mobile total gaseous mercury (TGM) measurements, and pyrolytic speciation across soil, and bark matrices in managed and unmanaged environments. A strong spatial coupling (ρ = 0.69, p < 0.01) was identified between nocturnal atmospheric TGM concentrations and the spatial distribution of soil Hg. Under these conditions, Hg⁰ is likely taken up by vegetation through combined cuticular and residual stomatal pathways. Once internalized, the pyrolytic signatures are compatible with the transformation of Hg⁰ into more stable Hg species, including Hg-OM and β-HgS. Our results suggest that P. pinea may function as an active biogeochemical interface, potentially promoting Hg transformation, contributing to Hg stabilization. These results highlight vegetation as a potential key driver of Hg transformation and attenuation in legacy mining environments and identify urban biomass management as a possible, previously overlooked control on Hg cycling.