Divergent mercury sequestration dynamics in tropical dry and moist broadleaf forests

Abstract. Forests are major sinks for atmospheric mercury (Hg) due to the efficiency of stomatal uptake and litter deposition. Tropical forests, highly productive ecosystems, remain understudied despite their pronounced climatic and phenological variability. We investigated whether seasonal rainfall regimes and associated tree adaptations regulate the Hg dynamics – its uptake by leaves, deposition through litter, and storage in soils– in secondary tropical moist broadleaf (TMBF) and tropical dry broadleaf (TDBF) forests from Costa Rica. Seasonality strongly controlled Hg sequestration in TDBF. Deciduous trees showed 4.7 times higher foliar Hg concentration in the wet season, when leaves were mature, compared to the dry season, when newly flushed leaves emerged after leaf shedding. Evergreen trees in TDBF demonstrated 2.3 times lower foliar Hg concentrations in the dry season than in the wet season (23 vs. 53 µg kg^-1), likely due to various associated physiological processes (e.g., leaf flushing). However, the primary mechanism remains unclear given the complex and unexplored Hg dynamics in TDBF. TMBF showed no clear seasonal variation in foliar Hg in either deciduous (dry: 48; wet: 54 µg kg^-1) or evergreen trees (dry: 57; wet: 53 µg kg^-1), likely due to longer leaf lifespan sustaining year-round transpiration and stomatal Hg uptake under high humidity. Atmospheric Hg concentrations in TDBF were two times higher than in TMBF (1.2 vs. 0.6 ng m^-3) across both seasons, likely reflecting greater Hg capture per unit area in TMBF due to denser vegetation and enhanced wet deposition via rainfall. Foliar Hg was not correlated with stomatal density or specific leaf area in either forest type. Soil Hg concentrations, however, were correlated with litter-derived inputs, supporting litter as the dominant Hg transfer pathway. Higher seasonally averaged Hg inputs via litter (34 μg m^-2) in TMBF than in the TDBF (19 μg m^-2) resulted in 3.4 times higher soil Hg concentrations (0–30 cm) in TMBF (115 µg kg^-1) than in TDBF (34 µg kg^-1). Soil Hg stocks were 2.6 times lower at a previously deforested TMBF site, indicating persistent disturbance effects despite almost three decades of reforestation. Overall, seasonality regulated Hg sequestration in TDBF, yet both TDBF and TMBF serve as important global Hg sinks with contrasting dynamics that are potentially sensitive to climate change.