Vegetation uptake of mercury and impacts on global cycling

Mercury (Hg) is a global pollutant that emits in large quantities to the atmosphere (>6,000-8,000Mg Hg per year) through anthropogenic activities, biomass burning, geogenic degassing and legacy emissions from land and oceans. Up to two-thirds of terrestrial Hg emissions are deposited back onto land, predominantly through vegetation uptake of Hg. In this Review, we assemble a global database of over 35,000 Hg measurements taken across 440 sites and synthesize the sources, distributions and sinks of Hg in foliage and vegetated ecosystems. Lichen and mosses show higher Hg concentrations than vascular plants, and, whereas Hg in above-ground biomass is largely from atmospheric uptake, root Hg is from combined soil and atmospheric uptake. Vegetation Hg uptake from the atmosphere and transfer to soils is the major Hg source in all biomes, globally accounting for 60-90% of terrestrial Hg deposition and decreasing the global atmospheric Hg pool by approximately 660Mg. Moreover, it reduces the Hg deposition to global oceans, which, in the absence of vegetation, might receive an additional Hg deposition of 960Mg per year. Vegetation uptake mechanisms need to be better constrained to understand vegetation cycling, and model representation of vegetation Hg cycling should be improved to quantify global vegetation impacts. Mercury, a semi-volatile and globally abundant pollutant, is transported through the atmosphere and taken up by vegetation. This Review discusses the mechanisms of vegetation mercury uptake and the role of vegetation in the mercury cycle, highlighting its importance for redistribution in the terrestrial environment and influence on atmospheric mercury concentrations and deposition to oceans. Key pointsIn forest ecosystems, 60-90% of mercury (Hg) originates from vegetation uptake of atmospheric gaseous elemental mercury (Hg(0)), providing 1,180-1,410Mg per year of terrestrial Hg deposition.Vegetation uptake of atmospheric Hg(0) lowers the global atmospheric Hg burden by 660Mg and reduces deposition to global oceans, which would receive an additional Hg deposition of 960Mg per year without vegetation.Lichen and mosses show higher Hg concentrations than vascular plants, and, whereas Hg in above-ground biomass is largely from atmospheric uptake, root Hg is from combined soil and atmospheric uptake.The seasonality of atmospheric Hg(0) concentrations in the Northern Hemisphere is controlled by vegetation uptake. Simulations without vegetation show weak seasonal cycles and cannot reproduce observations.Large knowledge gaps exist in understanding physiological and environmental controls of vegetation Hg uptake and transport within plants, limiting our mechanistic and molecular-level understanding of vegetation Hg uptake.Improved model parameterizations and harmonized observational data of vegetation Hg uptake, along with whole-ecosystem Hg(0) exchange measurements, are needed to improve the assessment of vegetation impacts on global Hg cycling.