Doctor of Philosophy
Brian A. Branfireun
Northern peatlands (a type of wetland with large carbon storage) are sinks for atmospheric mercury (Hg) and “hotspots” of methylmercury (MeHg) production in catchments. The effects of climate change (especially warming) are predicted to affect northern latitudes most strongly. There is concern that this warming will affect Hg cycling in northern peatlands by changing both the deposition and accumulation of inorganic Hg, as well as enhancing the production of MeHg. To better understand the influences of warming on Hg cycling in northern peatlands, I conducted field-based investigations and laboratory-based incubations on the role of vegetation, soil moisture, and soil temperature in regulating Hg cycling in two types of northern peatlands (moss-dominated fen and sedge-dominated fen). Findings of my doctoral research include plant species and leaf age affected foliar Hg uptake from the atmosphere. Foliar Hg concentrations decreased from June to July due to the dilution effect of leaf growth for all plant species (sedges and sweet gale (a shrub)). Foliar Hg concentrations then steadily increased after July until October, with a strong positive correlation with leaf age. Sedge leaves accumulated less Hg from the atmosphere over the growing season than sweet gale leaves. Leaching of Hg from litterfall differed between plant species with sedges leaching more Hg from their litterfall than sweet gale. Given the fact that climate change will increase vegetation biomass and favor sedges and shrubs abundance at the expense of Sphagnum mosses, our results provide more information to predict changes in Hg cycling in northern peatlands. Changes in soil moisture affected the cycling of Hg, sulfate (SO42-), and dissolved organic matter (DOM) in northern peatlands with dry peat soils releasing more Hg, especially MeHg, and SO42-, than wet and moist peat soils, and both wet and dry peat soils released more DOM than the moist peat soils during the re-wetting sampling events. There was an equilibrium between inorganic Hg concentrations in the solid phase of soils and the liquid phase of leachate. SO42- concentrations peaked in the leachate of the initial re-wetting sampling event. As SO42- concentrations decreased, MeHg concentrations increased and peaked in the second re-wetting event. SO42- in peat soils with different soil moistures (wet, moist, and dry peat soils) depleted rapidly in the static system (no external sulfur deposition or input). These results imply that drought under global warming will increase Hg transport and affect Hg methylation by increasing the release of Hg, DOM, and SO42- during the re-wetting events in northern peatlands. Elevated soil temperature (+4.5 °C at 25 cm depth of soil) increased both inorganic Hg and MeHg concentrations in pore waters in the moss-dominated fen. However, elevated soil temperature (+3.8 °C at 25 cm depth of soil) slightly decreased inorganic Hg concentrations but significantly increased MeHg concentrations in pore waters in the sedge-dominated fen, which was due to the increase in conversion of inorganic Hg to MeHg under elevated temperature. Increases in pore water MeHg concentrations under elevated temperature were overall attributed to the enhancement in both net MeHg production and release of MeHg from soils in both fens. These findings imply that vegetation community shift and biomass increase, soil moisture decrease, and soil temperature increase under global warming can affect Hg transport and methylation in northern peatlands.
Summary for Lay Audience
Mercury (Hg) is a naturally occurring metalloid that uniquely exists as a solid, liquid, and gas at ambient temperatures, and exists everywhere on Earth. Gaseous Hg is released to the atmosphere by natural and anthropogenic processes, transported globally, and can then be assimilated by plant leaves, and finally enters soils after plant leaves senesce. Inorganic Hg can be converted into methylmercury (MeHg), a neurotoxin, by bacteria in oxygen-free soils and sediments. Northern peatlands are a place where these conditions exist, and often are “hotspots” of MeHg production. It is established that climate change is disproportionally affecting northern latitudes where most northern peatlands are found. Changes in Earth surface temperature and moisture will influence soil moisture, soil temperature, and plant species composition, which can all impact aspects of the Hg cycle. The overall goal of my thesis was to study the influence of soil warming on Hg cycling in two types of northern peatlands, a Sphagnum spp. mosses-dominated peatland and a sedge-dominated peatland. My thesis first investigated the Hg accumulation by leaves of dominant plant species, including sedges and a shrub (sweet gale), in the sedge-dominant peatland, and Hg release from these dead or senesced leaves. I found that Hg concentrations in leaves differed between plant species; shrub leaves had higher Hg concentrations than sedge leaves. Mercury concentrations in all leaves generally increased with time. Dead shrub leaves released less soluble Hg than dead sedge leaves. Future plant species composition changes under climate change will affect Hg input from plant leaves to northern peatlands. My second objective was to study how drying soils under climate change will influence Hg release from soils and net MeHg production. My results showed that drier soils released more Hg, especially MeHg during re-wetting events, such as rain, and wet soils favored MeHg production. Drier peat soils also released more sulfate (SO42-) and bioaccessible dissolved organic matter (DOM), which are necessary nutrients for Hg methylators, and thus, the release of them after re-wetting of peat soils will promote net MeHg production. My third objective was to investigate the effect of soil warming on Hg mobility and MeHg production in northern peatlands. I found that when soil temperature increased by +4.5 °C in the Sphagnum spp. mosses-dominated peatland, concentrations of inorganic Hg and MeHg in soil waters significantly increased. When soil temperature increased by +3.8 °C in the sedge-dominated peatland, inorganic Hg concentrations in soil waters slightly decreased but MeHg concentrations significantly increased. Increases in MeHg concentrations in soil waters were from both MeHg production and releases from peat soils. Overall, my study showed that changes in plant species composition and soil temperature and moisture under climate change will impact Hg input through plant leaves to northern peatlands, Hg mobility, and MeHg production in northern peatlands in the future.
Sun, Ting, "Global Warming Effects on Mercury Cycling in Northern Peatlands" (2021). Electronic Thesis and Dissertation Repository. 8057.