Climate zone governs mercury fate along forest hydrological pathways

Forest hydrology critically influences mercury (Hg) cycling. We hypothesize canopy-soil-climate interactions govern Hg fate along waterflow pathways across climatic zones. Thus, this study monitored Hg transport for 3 -4 years via bulk precipitation, throughfall, and runoff in tropical rainforest, subtropical evergreen broadleaf, and alpine coniferous forests. We found that canopy processes consistently enhanced throughfall Hg flux in contrast to bulk precipitation Hg flux in all forests. The alpine coniferous forest showed the greatest amplification (∼4.9-fold) in throughfall Hg flux (37.84 ± 5.38 µg m^-2 yr^-1), driven by foliar adsorption and cold-trapping of atmospheric Hg. The subtropical forest exhibited a ∼4.2-fold increase in throughfall Hg flux (30.41 ± 7.58 µg m^-2 yr^-1) due to epiphytic humus enrichment. In contrast, rapid humus decomposition in the tropical rainforest limited the throughfall Hg flux (16.04 ± 3.19 µg m^-2 yr^-1) increase to just ∼1.2-fold. Surface soils retained > 90 % of throughfall Hg in all forests, but runoff patterns diverged. Alpine coniferous runoff had minimal Hg (2.22 ± 0.35 μg m^-2 yr^-1), dominated by dissolved Hg (DHg), due to high soil organic matter and epiphytic cover. Subtropical soils produced little runoff (0.24 ± 0.08 µg m^-2 yr^-1) but had the highest DHg concentrations. Tropical runoff (1.25 ± 0.69 µg m^-2 yr^-1) was particulate Hg-rich, reflecting lower soil retention. These inter-forest differences in hydrology, canopy and soil properties ultimately arise from climatic variation among the study sites. Overall, we highlight that canopy-soil-climate interactions drive forest-specific Hg cycling through waterflow, thus creating unique Hg ecological risks under different climates.