Biogeochemistry of Hg in terrestrial soils A new
Biogeochemistry of Hg in terrestrial soils: A new model Nicole Smith-Downey University of Texas at Austin Jackson School of Geosciences nicole. downey@jsg. utexas. edu Presented by: Elsie Sunderland In collaboration with Team-Hg at Harvard University: Daniel Jacob, Elsie Sunderland, Noelle Selin (now at MIT), Chris Holmes, Elizabeth Sturges Corbitt and funded by:
Driving Questions 1. What controls the lifetime and distribution of Hg in soils? 2. How have anthropogenic emissions of Hg changed soil Hg storage? 3. How sensitive are fluxes from the soil pool to changes in deposition? 4. How sensitive are fluxes from the soil pool to changes in the environment? 5. How do changes in soil storage relate to changes in methylation?
Pools of Hg in Soils • Mineral bound Hg – Derived from rock, parent material rich in Hg yields mineral soils rich in Hg – Subject to re-emission from weathering, volcanoes etc… – v. long timescales – does not interact with the atmosphere on timescale of centuries (except via volcanoes) • Organically bound Hg (O-Hg) – Derived from atmospheric deposition to soil surface or uptake by leaves – Subject to re-emission from combustion, decomposition or photoreduction – This is the pool we are focusing on b/c it interacts with the atmosphere on timescales of days -> seasons -> centuries Andersson 1979
Modeling Approach Hg(II) dry Hg(II) wet Hg 0 Revolitalization Photoreduction Hg 0 Hg Decomposition Fire! OM Hg(II)aq Use Global Terrestrial Mercury Model (GTMM), which is based on the CASA biogeochemical model, to simulate Hg emissions driven by C dynamics, photoreduction and revolitalization Biosphere GTMM Hg 0 dry Atmosphere GEOS-Chem Use GEOS-Chem Hg simulation to estimate deposition
Surface emissions processes • Photoreduction – Parameterized as a function of light intensity based on data from Rolfhus and Fitzgerald (2004) Where fphotored is the monthly fraction of Hg(II) photoreduced • Revolitalization – Because Hg 0 does not build up in the biosphere, we assume any Hg 0 not fixed by leaves is revolitalized at a monthly time step
Soil Carbon Dynamics In CASA CO 2 Based on the CENTURY formulation *tracks pools based on their turnover time rather than their physical location * 13 carbon pools in soil * Q 10 = 1. 5 CO 2 Decomposition Lifetime in soil fast intermediate slow months years decades armored >100 years Increasing Recalcitrance / Carbon: Nitrogen Decomposition is a function of Temperature, Moisture, Litter Quality T response described by: Adapted from Trumbore 1997
CASA Soil Carbon + Hg Dynamics Hg Hg Hg Decomposition Lifetime in soil fast intermediate slow months years decades armored >100 years Increasing Recalcitrance / Carbon: Nitrogen The fraction of Hg lost during decomposition fdecomp controls soil Hg content But…we don’t have measurements of fdecomp so we’ll use observed Hg: C ratios in organic soils to estimate fdecomp
Hg Binding to Soil Organic Material slow armored Case 1 – Hg preferentially binds to younger SOM Case 2 – Hg binds to all pools with equal affinity armored slow From Qian et al. 2002 493 g C/kg soil 1. 9 g reduced S/kg soil Assuming - 1 reduced S: Hg Maximum Limit of 0. 0248 g Hg/g C Deposition + C turnover time control steady state concentration of Hg in soils Assuming Case 2, which appears to be true for aquatic environments (Skyllberg)
Hg fixed by leaves • Field measurements by Rea et al. [2002] show a seasonal increase in leaf Hg concentration at Lake Huron, MI • Used leaf concentration data to tune model Smith-Downey and Jacob (in prep) – JGR Biogeosciences
Deposition from GEOS-Chem
• Compared Hg: C ratio of soils in model to measurements across transect of US (USGS 2005) • Tested a range of models with fdecomp ranging between 0. 01 1. 0 • fdecomp = 0. 16 fit the data best Hg: C ratio of soils Comparison with USGS Soil fdecomp Smith-Downey and Jacob (in prep) – JGR Biogeosciences
Soil Hg Emissions Organic - total Organic -Anthro Mineral Soils Organic - natural Smith-Downey and Jacob (in prep) – JGR Biogeosciences
Soil Hg Storage • Most Hg (90%) is associated with the armored pool • 20% increase in total soil OHg storage for preindustrial -> industrial • 120% increase in respiration emissions of Hg, mostly driven by changes in the fast pool Smith-Downey and Jacob (in prep) JGR Biogeosciences
Anthropogenic Hg is concentrated in most labile C pools Smith-Downey and Jacob (in prep) – JGR Biogeosciences
Summary • Using soil carbon cycling as a framework for Hg cycling in soils is a powerful approach to examine historical and future impact of anthropogenic Hg emissions on soils • We find a 20% increase in Hg associated with organic soils between preindustrial -> industrial and a 120% increase in Hg emissions from respiration • Most labile carbon pools are most affected by anthropogenic Hg • Working on global inventory of fire emissions from model • For more information – contact Nicole at nicole. downey@jsg. utexas. edu
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