We initialize the simulated Hg to a depth of three meters using observed values 1 and ran the model for five thousand years to reach steady state where Hg losses balance gains. RCP45 represents a scenario close to the global target of 2 ☌ warming above pre-industrial levels while RCP85 represents a high emissions scenario of unconstrained burning of fossil fuels. We ran simulations from 1901 to 2299 using Representative Concentration Pathways 4.5 and 8.5 (hereafter RCP45 and RCP85). The model includes soil evasion and leaching Hg release pathways, but not stomatal transpiration and fire. The model accounts for all uptake pathways of Hg except leaf stomatal uptake. We added Hg to the Simple Biosphere/Carnegie-Ames-Stanford Approach (SiBCASA) terrestrial biogeochemistry model 17 (Fig. The biological and physical processes that control the carbon cycle also control the Hg cycle 2, 9, 16. Once leached into water, bound to Dissolved Organic Carbon (DOC) and Particulate Organic Carbon (POC), Hg can methylate, entering the food chain and accumulating in various species, particularly fish 8. Fire consumes soil organic matter, emitting carbon dioxide and Hg into the atmosphere 11. Whether leaves represent an Hg source or sink depends on the concentration gradient between the stomata and the atmosphere 14. Microbial decay frees Hg from organic matter, but plants and soil organic matter reabsorb 11 most of this liberated Hg. Hg has four release pathways: (1) evasion into the atmosphere after microbial decay 11, (2) leaf stomata transpiration 14, (3) fire 11, and (4) leaching into groundwater followed by eventual export by rivers into the oceans 16. The deposition of dead leaves, roots, and stems transfers additional Hg to the soil 11, 15. Once absorbed by plants, translocation by phloem assimilates Hg into leaves and wood 12, 13. Hg has three uptake pathways: (1) bonding to soil organic matter 8, 10, 11, (2) stomatal leaf uptake 12, 13, and (3) root absorption 12, 14. Because Hg bonds to organic matter, the terrestrial carbon cycle modulates the terrestrial Hg cycle. When permafrost thaws, microbial decay of the stored organic matter will resume and release Hg, but how much, where, and when remain unclear.Ītmospheric deposition is the dominant source of Hg to the terrestrial biosphere 8, 9. Model projections estimate a 30–99% reduction in northern hemisphere permafrost extent by 2100 6, 7. Observations indicate accelerated permafrost thaw over the past 30–40 years 4, 5. Based on soil measurements, permafrost regions store an estimated 1656 ± 962 Gg Hg in the top three meters of soil, of which 793 ± 461 Gg Hg are frozen in permafrost 1. Once frozen, microbial decay effectively ceases, locking the accumulated Hg into the permafrost. Sedimentation in permafrost regions has buried vegetation over thousands of years, freezing organic matter at the bottom of the active layer into permafrost 3. Permafrost is soil at or below 0 ☌ for at least two consecutive years and the active layer is the surface layer of soil above permafrost that thaws in summer and refreezes in winter. Microbial decay eventually consumes the organic matter 2, releasing the Hg. Naturally occurring and anthropogenic Hg deposits on land from the atmosphere and bonds to receptor sites in plant organic matter 1.
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