1. Biogeochemistry of Hg in terrestrial soils: A new model
Nicole Smith-Downey
University of Texas at Austin
Jackson School of Geosciences
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:

2. Driving Questions
What controls the lifetime and distribution of Hg in soils?
How have anthropogenic emissions of Hg changed soil Hg storage?
How sensitive are fluxes from the soil pool to changes in deposition?
How sensitive are fluxes from the soil pool to changes in the environment?
How do changes in soil storage relate to changes in methylation?

3. Pools of Hg in Soils Mineral bound Hg Organically bound Hg (O-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
This is really to highlight that there is a difference between Hg in rocks – which is relatively inert, and that bound to organic material. Importantantly, all Hg in organic material is derived from the atmosphere. This is the pool we are focusing on -> and I think that this explains some of the difference in magnitude of pool size, because I am only considering Organically bound Hg.
Andersson 1979

4. Modeling Approach Use GEOS-Chem Hg simulation to estimate deposition
dry
Hg(II)
dry
Hg(II) wet
Atmosphere
GEOS-Chem
Hg0
Hg(II)
Hg(II)aq
Hg OM
Photoreduction
Decomposition
Fire!
Revolitalization
Biosphere
GTMM
Here, we use GEOS-Chem to supply deposition to GTMM. GTMM accounts for photoreduction, revolitalization, + decomposition (driven by C dynamics). The fire portion doesn’t exactly work yet because there are serious problems with CASA’s soil burning schemes. All Hg re-emitted to the atmosphere is Hg0, and is fed back into GEOS-Chem. GTMM is a 1x1 degree model with a monthly time step.
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

5. 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 Hg0 does not build up in the biosphere, we assume any Hg0 not fixed by leaves is revolitalized at a monthly time step
Surface emissions processes basically means, mechanisms for Hg to be emitted back to the atmosphere before it has a chance to bind to reduced sulfur groups in O-Hg. This is *roughly* equivalent to Noelle’s prompt recycling…kind of… But is mechanistically driven by sunlight in the case of photoreduction. I just assume that Hg0 doesn’t really accumulate in ecosystems, except via stomatal uptake (in leaves).

6. Soil Carbon Dynamics In CASA
CO2
Based on the CENTURY formulation
*tracks pools based on their turnover
time rather than their physical location
* 13 carbon pools in soil
* Q10 = 1.5
CO2
CO2
CO2
CO2
Decomposition
fast
intermediate
slow
armored
This is just a schematic of how soil C dynamics work in CASA. Basically, organic material enters the system and is decomposed until the most recalcitrant pools are left behind. The decomposition is largely driven by temperature and moisture. I am using a Q10 of 1.5, which if anyone is really a C person, might come up. Also, CASA does not keep track of the physical location (i.e. depth) of soils, rather just pools with different turnover times.
Lifetime in soil months years decades >100 years
Increasing Recalcitrance / Carbon:Nitrogen
Decomposition is a function of Temperature, Moisture, Litter Quality
T response described by:
Adapted from Trumbore 1997

7. CASA Soil Carbon + Hg Dynamics
Decomposition
fast
intermediate
slow
armored
Here there are 2 important things to point out. 1) That unlike C, Hg is added to ALL pools, this explains why later we see Hg concentrated in the most recalcitrant pools. Second is that the fraction of Hg lost during decomposition (f_decomp) will determine the steady state Hg:C ratio. Maybe also mention that using the Hg:C ratio rather than Hg concentration makes for an easier comparison, because you don’t need to know the soil bulk density.
Lifetime in soil months years decades >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

8. 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
Here – this just illustrates why it is important to note that we are assuming Hg binds to all soil pools with equal affinity. If it behaved more like C, where the inputs were to the most labile pools, there would be less Hg overall, and it would accumulate in less recalcitrant pools than the case where Hg binds equally. Also, built into the model is a theoretical limit on Hg storage which is g Hg/ g C. This limit is never reached.
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 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)

9. 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
Hg0 and Hg(II) are taken up by leaves and fixed into leaf tissue. This equation is just the way I parameterize the uptake to match the data collected by Rea et al
Smith-Downey and Jacob (in prep) – JGR Biogeosciences

10. Deposition from GEOS-Chem
Input fields of deposition. The v. low deposition in the arctic means we don’t get any accumulation there….

11. Comparison with USGS Soil
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  1.0
fdecomp = 0.16 fit the data best
Hg:C ratio of soils
I ran the model with a variety of f_decomp values, and found that the best fit is 0.16 – meaning that in each step of decomposition 16% of the Hg bound to soil organic material being decomposed is emitted to the atmosphere. This value gives you the closest Hg:C ratio to that observed in organic soils in the USGS 2005 transect data (only in continental US).
fdecomp
Smith-Downey and Jacob (in prep) – JGR Biogeosciences

12. Soil Hg Emissions Organic - total Organic -Anthro Mineral Soils
Here we have the pre-industrial -> industrial emissions. Secondary soil is everything coming out of Organic soils. Respiration includes both natural and anthropogenic Hg. Notice that the secondary emissions of anthropogenic Hg are ~2/3 the size of primary anthropogenic emissions.
Mineral Soils
Organic - natural
Smith-Downey and Jacob (in prep) – JGR Biogeosciences

13. Soil Hg Storage Most Hg (90%) is associated with the armored pool
20% increase in total soil O-Hg storage for preindustrial -> industrial
120% increase in respiration emissions of Hg, mostly driven by changes in the fast pool
Here you can see that most Hg is stored in the armored pool, and that most of the anthropogenic Hg is in the slow pool (b/c steady state has not yet been reached). There is a 20% increase in total O-Hg storage, but a 120% increase in respiration emissions.
Smith-Downey and Jacob (in prep)
JGR Biogeosciences

14. Anthropogenic Hg is concentrated in most labile C pools
All pools are not impacted equally. The most labile C pools are fractionally more impacted by anthropogenic Hg than the more recalcitrant pools - > implications for methylation and tight coupling between atmospheric deposition and methylation. If fast pools are preferentially methylated, then methylation rates will be tied to changes in fast pools. Although the bulk O-Hg content only goes up 20%, the fast pool increases by 290%, which may explain why there has been such a large increase in methylation despite a small change in bulk Hg.
Smith-Downey and Jacob (in prep) – JGR Biogeosciences

15. 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
Also, maybe mention that this C framework should work for the aquatic environment too. And that someone should do this.