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Vegetation uptake of mercury and impacts on global cycling

Abstract

Mercury (Hg) is a global pollutant that emits in large quantities to the atmosphere (>6,000–8,000 Mg 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 660 Mg. Moreover, it reduces the Hg deposition to global oceans, which, in the absence of vegetation, might receive an additional Hg deposition of 960 Mg 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.

Key points

  • In forest ecosystems, 60–90% of mercury (Hg) originates from vegetation uptake of atmospheric gaseous elemental mercury (Hg(0)), providing 1,180–1,410 Mg per year of terrestrial Hg deposition.

  • Vegetation uptake of atmospheric Hg(0) lowers the global atmospheric Hg burden by 660 Mg and reduces deposition to global oceans, which would receive an additional Hg deposition of 960 Mg 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.

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Fig. 1: Global distribution of foliar Hg samples.
Fig. 2: Pathways of plant Hg uptake.
Fig. 3: Hg stable isotopes in foliage.
Fig. 4: Global Hg cycle.
Fig. 5: Global surface air concentrations and annual deposition of Hg.
Fig. 6: Surface air Hg(0) concentrations.

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Acknowledgements

We would like to thank X. Wang and C.-J. Lin for providing us with Hg litterfall deposition fluxes and biome geospatial boundary masks that allowed us to compare model results with litterfall deposition for various biomes of the world. We thank the three anonymous reviewers for their constructive comments on an earlier version of this manuscript. Funding was provided by the US National Science Foundation (AGS award no. 1848212 and DEB award no. 2027038). M.J. acknowledges funding from the Swiss National Science Foundation grant PZ00P2_174101. We thank James Gray for editorial comments on the manuscript.

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All authors contributed to the writing, editing and overall conceptualization of this review manuscript. J.Z. built and analyzed the database, and led the writing of the manuscript and overall design of graphics and tables. D.O. initiated and coordinated the project and co-led manuscript writing and editing. A.D. led the model approach, analysis and associated sections. A.R. built the modelling set-up and conducted simulations, analysis and associated graphics. M.J. led the sections on stable Hg isotope patterns, data collection and analysis, and associated graphics.

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Correspondence to Jun Zhou or Daniel Obrist.

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Glossary

Minamata Convention on Mercury

An international treaty named after the city of Minamata in Japan that experienced devastating Hg contamination in the 1950s.

Legacy emissions

Re-volatilization of past atmospheric deposition from anthropogenic and geogenic sources stored in surface reservoirs, such as soils and water.

Vascular plants

Group of plants with specialized tissues that include coniferous and flowering plants.

Stomata

Apertures in leaves that control gas exchange (such as carbon dioxide and water vapour) between plants and the atmosphere.

Cuticles

Outer protective layers on epidermal cells of leaves, often consisting of waxy, water-repellent substances.

Non-vascular vegetation

Plants that do not have specialized vascular tissues, which include algae, mosses, livermorts and hornworts; lichen are often grouped into this category, although they are symbiotic partnerships between a fungus and an alga.

Physiology

The study of plant function and behaviour, including growth, metabolism, reproduction, defence and communication.

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Zhou, J., Obrist, D., Dastoor, A. et al. Vegetation uptake of mercury and impacts on global cycling. Nat Rev Earth Environ 2, 269–284 (2021). https://doi.org/10.1038/s43017-021-00146-y

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