1. Wetland Biogeochemistry
Biology 643: Biogeochemistry
Allison Gill
Nov. 20, 23,

2. Outline Introduction to wetlands II. Wetland C Cycling
Primary Production
Decomposition
Aerobic
Anaerobic
III. Global change in high latitude wetlands
IV: Case Study – SPRUCE Experiment

3. Wetland
Terrestrial system with water at or near the soil surface for all or part of the year
Tropical Peat Bog
Temperate coastal salt marsh
Wetlands are terrestrial systems in which water is at or near the soil surface for all or part of the year
There are lots of different types of wetlands which occur all over the world
Peat forests in the tropics, many of which have been degraded for palm oil plantations, mangrove swamps in subtropical coastal areas, salt marshes in coastal temperate zones, fens and bogs at high latitudes, among many others
All of these are widely different ecosystems, but are united by the transient or continuous characteristic of water saturation, which defines the biogeochemical nature of wetland systems
Water diffuses 10,000 times more slowly in water than air, therefore oxygen is far less abundant than in non-wetland ecosystems
Typically oxygen is consumed faster than it can diffuse through water columns and water-saturated substrates, so systems are typically anoxic and plants and microbes that live there developed to metabolize under those conditions
Subtropical Mangrove Swamp
Boreal Fen

4. Wetland Distribution 5-12 x 10-6 km2 7-15% land area
hard to accurately assess the spatial extent of wetlands
In general, binned into different types based on features of hydrology, vegetation, and soils
but global estimatesa re that they are about 5 to 12 million square kilometers, most are in cold high latitude ecosystems
are a relatively small proportion of the land area, but store the majority of global soil C
are also a major methane source, comprise about 20-30% of methane emissions
5-12 x 10-6 km2
7-15% land area
>50% global soil C
20-33% global CH4 emissions

5. Nature Climate Change Oct 2013
because wetlands play a disproportionate role in the carbon cycle in terms of the size of the belowground pool and the methane flux, are of major interest in to climate change
in particular, a lot of interest to understand the unique biogeochemistry of these systems so we can think about how the giant carbon store might be affected by CC. Will it remain stored as organic material, exported as DOC, or released as gaseous C? Will that gaseous C be CO2 or methane? How fast will it come out?
Nature Climate Change Oct 2013

6. Wetland characteristics are primarily defined by hydrology and water source
Water Enters:
Precipitation
Groundwater
Surface Flow/Run-off
Water Leaves:
Evapotranspiration
Groundwater recharge
Surface outflow
Residence Time is the average amount of time a unit of water remains in the wetland
So lets talk a little bit about the hydrology of wetland systems, as that is a major factor that defines the current state of wetlands and will influence future C storage
Beyond just the presence of water, the source of the water influences the biogeochemical nature of each system
Both the source of the water and the time a particular molecule of water remains in the wetland can vary greatly
Water enters and leaves through a variety of mechanisms

7. Wetland characteristics are primarily defined by hydrology and water source
Brinson et al. 1993 7

8. Wetland characteristics are primarily defined by hydrology and water source
Precip-dominated wetlands
Long residence times
“Closed systems”
Biogeochemically isolated from catchment
Wetlands that get most of the water from precipitation tend to have water that has a very long residence time in the system, thought of as “closed”, are biogeochemically isolated from the catchment. Systems tend to be quite nutrient poor, have slow-growing plant species adapted to oligotrophic conditions
Brinson et al. 1993 8

9. Bog

10. Wetland characteristics are primarily defined by hydrology and water source
Groundwater wetlands
Shorter residence times
Nutrient input from/DOC export to catchment
wetlands with a strong groundwater source tend to have shorter water residence times. The groundwater brings nutrients into the wetland from the surrounding watershed and the wetland tends to export DOC. Because of the nutrient source, these wetlands tend to support different plant communities
Brinson et al. 1993 10

11. Fen

12. Wetland characteristics are primarily defined by hydrology and water source
Surface Flow Wetlands
Shorter residence times
Sediment & nutrient input
Sometimes saline, reduces O2 solubility and species diversity
wetlands that are influenced by surface flow also have short residence times, although it can be variable if it is something like a seasonally-flooded system or a tidal ecosystem
often get sediment and nutrient input from surrounding hydrological system
many tidal systems are saline, tends to reduce O2 solubility and plant species diversity since relatively few plants can deal with fluctuating osmolarity
Brinson et al. 1993 12

13. Tidal Marsh

14. Primary Productivity in Wetlands
Water saturation and anoxia creates unique metabolic environment for present plant and microbial community
Higher plants need oxygen for root growth
Morphological adaptations to access O2
plant growth in wetlands is source of soil C in wetlands
water saturation and anoxia creates unique metabolic environment for present plant and microbial community
higher plants need oxygen for root growth
Some plants have developed physiological adaptations to deal with oxygen limitation – sedges have aerenchymous tissue that allow oxygen to diffuse below the water surface and mangrove species have pneumatophores, or root structures that reach above the surface to access oxygen. In oligotrophic systems, such as bogs, non-vascular Sphagnum spp outcompete vascular plants. Sphagnum do not have true roots and take up water and nutrients though diffusion into hyaline cells, which allows them to proliferate in strongly water-rich and nutrient limited environments. Once present, Sphagnum has extremely recalcitrant litter that decomposes slowly, and also has high hydraulic conductivity, feeding back on water saturation, anoxia, and organic matter accumulation in the wetland.
Sedge aerenchyma
Mangrove pneumatophores
Nonvascular Sphagnum

15. Wetlands and the C Cycle
Photosynthesis and Primary Production
will step through unique aspects of wetland carbon cycling that influence the nature of carbon storage and fluxes
I am going to use this summary figure which does a nice job highlighting all the interacting fluxes
We’ll start with photosynthesis. Plants take up CO2 which they use to generate the complex organic compounds that are a part of plant tissue. O2 is generated as a byproduct

16. Wetlands and the C Cycle
Decomposition impeded in anoxic soils
Many wetlands have accumulated large SOM stocks
Major global carbon store
Plant material still recognizable -> PEAT
Acrotelm: aerobic portion of peat profile above the water table. Aerated with roots, recent plant litter inputs, less decomposed, most microbial activity, higher hydraulic conductivity
Catotelm: portion of the profile below the water surface
Proportion of peat entering the catotelm depends on burial rate and residence time of litter
you guys talked about the carbon cycle already, and are aware that as plants die, the complex organic material is broken down and used as energy by soil microbes
this is fundamentally the case in wetlands, but the processes are somewhat more complex
anoxic wetland conditions impede decomposition, therefore many wetlands have accumulated large SOM stocks, which is the basis of the global carbon store.
When this plant material is still recognizable and accumulates to a certain depth, you have peat
There are two major biogeochemical zones in a peatland that critically affect local decomposition processes
The upper portion of the peat profile, or the area above the water table, is known as the acrotelm. This area has high O2 concentration, is further aerated by plant roots, recievs the most recent plant litter inputs, has peat that is less decomposed, has more microbial activity, and higher hydraulic conductivity
The portion of the peat profile below the water surface is called the catotelm. The proportion of peat entering the catotelm fro m the acrotelm depends on the burial rate and residence time of the litter.
Labile C breaks down in the acroteom and the recalcitrant c is what makes it down to the catotelm, so decomposition is inhibited for two reasons below the water table
Acrotelm
Catotelm

17. SOM Decomposition in Wetlands
Step 1: Depolymerization - Extracellular enzymes cleave large polymers to produce usable monomers that can pass through cell membranes.
Both AEROBIC and ANAEROBIC respiration
Labile:
proteins, carbohydrates, lipids
Complex organic polymers
Recalcitrant: lignin, hemicellulose
Microbially-produced extracellular enzymes
Both aerobic and anaerobic decomposition initiated and constrained by cleavage of oranic monomers from large complex organic polymers by exoenzymes
- large molecules must be broken down extracellularly
Monomers
(can pass thru cell membranes)
Fatty acids, amino acids, monosaccharides

18. SOM Decomposition in Wetlands
Step 1: Depolymerization - Extracellular enzymes cleave large polymers to produce usable monomers that can pass through cell membranes.
Caveat: In ANAEROBIC conditions
Oxidative exoenzymes
degrade phenolics and lignin
Require O2 as an electron acceptor
Inhibited by anoxia
Contributes to accumulation of recalcitrant material at depth
“Enzymatic Latch”
phenol oxidases, laccases, and peroxidases that break down recalcitrant C are inhibited in the absence of oxygen, resulting in a build-up of recalcitrant C and limitation of C substrate when oxygen is not available as an electron acceptor (enzymatic latch, Fenner and Freeman 2006). As labile organic material is quickly metabolized in the aerobic portion of the peat profile above the water table, it is these more recalcitrant fractions that resist initial decomposition and are buried, further amplifying SOM accumulation.

19. SOM Decomposition in Wetlands
Step 1: Depolymerization
Enzymes!

20. SOM Decomposition in Wetlands
Step 2: Aerobic Respiration
“Life is nothing but an electron looking for a place to rest” Albert Szent-Gyorgi (1957)
In the presence of oxygen, the small monomers cleaved by exoenzymes (for simplicity, we will say they are now glucose) are oxidized through aerobic cellular respiration. The carbon monomer goes through glycolysis to form two molecules of pyruvate (glycolysis generates 2ATP), which then enter the citric acid cycle and electron transport chain, generating an additional 36 ATP. The oxidized carbon is respired as CO2, with H2O an additional waste product in the reaction.

21. SOM Decomposition in Wetlands
Aerobic Respiration
O2 is electron acceptor, reduces organic compounds that are oxidized to CO2
Glucose
Pyruvate
CH3COCOO-
Glycolysis
+2 mol ATP
Pyruvate dehydrogenase complex
In the presence of oxygen, the small monomers cleaved by exoenzymes (for simplicity, we will say they are now glucose) are oxidized through aerobic cellular respiration. The carbon monomer goes through glycolysis to form two molecules of pyruvate (glycolysis generates 2ATP), which then enter the citric acid cycle and electron transport chain, generating an additional 36 ATP. The oxidized carbon is respired as CO2, with H2O an additional waste product in the reaction.
Acetyl Co-A
CO2
H2O
Kreb’s Cycle
CO2
+36 mol ATP

22. SOM Decomposition in Wetlands
Step 2: Aerobic Respiration

23. SOM Decomposition in Wetlands
Fermentation
In absence of O2, pyruvate degraded to low molecular weight inorganic compounds. Occurs inside the cell, energy is generated by substrate level phosphorylation
Glucose/Organic monomers
Pyruvate
CH3COCOO-
Glycolysis
+2 mol ATP
Fermentation
First steps same as aerobic respiration, then fermented to organic acids and alcohols. Fermentation occurs iside the cell and energy is generated by substrate level phosphorylation rather than oxidative phosphorylation by electron transport. Rate determining step for further anaerobic decomposition
Extracellular environment
Organic acids, acetate, alcohols

24. SOM Decomposition in Wetlands
Fermentation
Fermentation breaks down pyruvate into other small organic molecules. The fermentation products become substrates for decomp to CO2 by bacteria using alternitive TEAs. Additional respiratory paths have lower energy yields, so support smaller microbial biomass and lower concentrations of exoenzymes

25. Lecture 2

26. SOM Decomposition in Wetlands
Aerobic Respiration
O2 is electron acceptor, reduces organic compounds that are oxidized to CO2
Glucose
Pyruvate
CH3COCOO-
Glycolysis
+2 mol ATP
Pyruvate dehydrogenase complex
In the presence of oxygen, the small monomers cleaved by exoenzymes (for simplicity, we will say they are now glucose) are oxidized through aerobic cellular respiration. The carbon monomer goes through glycolysis to form two molecules of pyruvate (glycolysis generates 2ATP), which then enter the citric acid cycle and electron transport chain, generating an additional 36 ATP. The oxidized carbon is respired as CO2, with H2O an additional waste product in the reaction.
Acetyl Co-A
CO2
H2O
Kreb’s Cycle
CO2
+36 mol ATP
C6H12O6 + 6O2 -> 6CO2 + 6H2O dG= -686 kcal/mol

27. SOM Decomposition in Wetlands
Fermentation
In absence of O2, pyruvate degraded to low molecular weight inorganic compounds. Occurs inside the cell, energy is generated by substrate level phosphorylation
Glucose/Organic monomers
Pyruvate
CH3COCOO-
Glycolysis
+2 mol ATP
Fermentation
First steps same as aerobic respiration, then fermented to organic acids and alcohols. Fermentation occurs iside the cell and energy is generated by substrate level phosphorylation rather than oxidative phosphorylation by electron transport. Rate determining step for further anaerobic decomposition
Extracellular environment
Organic acids, acetate, alcohols

28. SOM Decomposition in Wetlands
ReDox Chemistry
Reduction and Oxidation: the transfer of e- from a donor to an acceptor. One substance is oxidized while another is (in turn) reduced
OIL RIG: Oxidation is Loss, Reduction is Gain
Plant C the most highly reduced C in the biosphere
Standard Reduction Potential (Eo): the tendency of a substance to donate e- and be oxidized or accept e- and be reduced
Greater ΔEo, the greater the free energy yield of the reaction (ΔGo)
ΔGo = -nFΔE
F = Faraday’s constant ( kcal volt-1)
n = number of e-
The products of fermentation are still reduced compounds that can be oxidized by microbes for additonal energy. Anaerobic respiration is fundamentally redox chemistry, and we will do a quick review so that it makes sense
- a reduction-oxidation, or redox, reaction involves the transfer of electrons from a donor to an acceptor. One substance gets oxidized which the other is in turn, reduced
- I was taught to remember that OIL RIG
- plant carbon is the most highly reduced C in the biosphere, so microbes are going to couple it to a series of redox reactions in order to oxidize the C and gain energy
- we think about the standard reduction potential, or the tendency of a substance to donate electrons and be oxidized or accept electrons and be reduced
The greater the reduction potential, the greater the free energy yield of the reaction
Can calculate the free energy yield of the reaction using faradays constant and the dE

29. Copyright © 2013 Elsevier Inc. All rights reserved.
TABLE 7.3 Common Reduction and Oxidation Half Reactions
Copyright © 2013 Elsevier Inc. All rights reserved.

30. Anaerobic Respiration: Metabolic Pathways
SOM Decomposition in Wetlands
Anaerobic Respiration: Metabolic Pathways
Following O2 depletion, fermentation products are oxidized by series of alternate terminal e- acceptors (TEA)
TEA series based on ΔGo of reaction
Use substrate-level phosphorylation rather than e- transport chains to produce ATP
All pathways yield CO2
Dominance in saturated sediments slows decomposition and C losses
Nitrate Reduction/Denitrification
Manganese Reduction (often negligible)
Iron Reduction
Sulfate Reduction
ΔGo

31. SOM Decomposition in Wetlands
Nitrate Reduction
Most energetically favorable anaerobic C mineralization pathway
Performed by facultative anaerobes
Zones of fluctuating ReDox (around water table)
Precipitation dominated wetlands: [NO3-] low, minor pathway
2NO3- + (CH2O) -> 2N2 + CO2 + H2O ΔG° = -649 kcal/mol
Nitrate Reduction/Denitrification
Manganese Reduction (often negligible)
Iron Reduction
Sulfate Reduction
ΔGo

32. SOM Decomposition in Wetlands
Nitrate Reduction
Fermentation breaks down pyruvate into other small organic molecules. The fermentation products become substrates for decomp to CO2 by bacteria using alternitive TEAs. Additional respiratory paths have lower energy yields, so support smaller microbial biomass and lower concentrations of exoenzymes

33. SOM Decomposition in Wetlands
Manganense Reduction
Follows NO3- depletion
Mn4+ scarce in wetlands, minor decomposition pathway
Spatially overlaps with NO3- zone, performed by facultative anaerobes
Nitrate Reduction/Denitrification
Manganese Reduction (often negligible)
Iron Reduction
Sulfate Reduction
ΔGo

34. SOM Decomposition in Wetlands
Iron Reduction
Follows Mn4+ depletion
Below zone of NO3- and Mn4+ depletion
Performed by obligate anaerobes
Fe(OH)3 + 3H+ -> Fe2+ +3H2O ΔG° = -300 kcal/mol
Nitrate Reduction/Denitrification
Manganese Reduction (often negligible)
Iron Reduction
Sulfate Reduction
ΔGo

35. SOM Decomposition in Wetlands
Sulfate Reduction
Follows Iron depletion
Completed by sulfate-reducing bacteria
Produce H2S, although H2S fluxes from wetlands are low as it reacts with iron and is sequestered in sediments
SO (CHO) + 3H+ -> HS- + 2CO2 + 2H2O ΔG° = -190 kcal/mol
Nitrate Reduction/Denitrification
Manganese Reduction (often negligible)
Iron Reduction
Sulfate Reduction
ΔGo

36. SOM Decomposition in Wetlands
Sulfer Reduction
Fermentation breaks down pyruvate into other small organic molecules. The fermentation products become substrates for decomp to CO2 by bacteria using alternitive TEAs. Additional respiratory paths have lower energy yields, so support smaller microbial biomass and lower concentrations of exoenzymes

37. Methanogenesis Follows depletion of all alternative TEAs
Degrade fermentation products and root exudates
Performed by Archea
Obligatate anaerboes
Two methanogenesis pathways:
Acetate splitting
CH3COOH -> CH4 + CO2
ΔG° = -28 kJ/mol
δ13C: -50 to -65 permil
CO2 Reduction
CO2 + 4H2 -> CH4 + H2O
ΔG°= kJ/mol
δ13C: -60 to -100 permil
Assumed acetate, H2, CO2 are three substrates for methanogenesis in freshwater systems. Methylated amines can be substrates in saline systems
Patterns in methane release based on availability of TEAs and substrates, all ultimately plant derived. Therefore methen

38. SOM Decomposition in Wetlands
Methanogenesis
Fermentation breaks down pyruvate into other small organic molecules. The fermentation products become substrates for decomp to CO2 by bacteria using alternitive TEAs. Additional respiratory paths have lower energy yields, so support smaller microbial biomass and lower concentrations of exoenzymes

39. Wetland Methane Flux Whiting and Chanton 1993
Precursors to methanogenesis derived from decomposition of C, acetate from decomposition of labile C such as root exudates. Results in strong association between primary production and methane flux. This figure is across wetlands, not peatlands, so get a broad range in NEP.
High NPP results in acetoclastic methanogenesis and 13C enrichment
Would also expect strong annual cycles in CH4 produciton in these cases
Get more acetoclastic at surface and more H2/CO2 at depth, corresponds with more recalcitrant C and further distance from root exudates
Whiting and Chanton 1993

40. CH4 + 2O2 -> CO2 + H2O ΔG° = -193.5 kcal/mol
Methane Oxidation
Up to 50% of methane produced below WT is oxidized by methanotrophic bacteria in aerobic zone
CH4 + 2O2 -> CO2 + H2O ΔG° = kcal/mol

41. SOM Decomposition in Wetlands
Methanotrophy
Fermentation breaks down pyruvate into other small organic molecules. The fermentation products become substrates for decomp to CO2 by bacteria using alternitive TEAs. Additional respiratory paths have lower energy yields, so support smaller microbial biomass and lower concentrations of exoenzymes

42. Methane Release to Atmosphere
Relationship between NEP and methane flux also due to facilitated gas exchange with wetland plants, aerenchymous tissue are the hollow stems that bring O2 below surface, also transport methane up
Ebullition, diffusion, plant transport thru aerenchyma
In bogs, methanotrophy limited by low soil bulk density, CH4 ebullates before it has a chance to get oxidized

43. Steady state methane flux (left) and methane ebullition (right)
Demonstrates ebullative flux

44. Peatlands and Climate Change
Changes in state factors: [CO2] -> temperature -> water availability induces physical and biogeochemical changes in the system
Plant community dynamics, production, and ecosystem nutrient availability
SOM decomposition and nature of greenhouse gas emission

45. Atmospheric [CO2]
Less consistent fertilization effect compared to upland systems
Explored in small scale field, mesocosm, lab studies
Limited responses due to N limitation
(Berendse et al. 2001, Hutchin et al 1995, Hoosbeek et al. 2001, Kang et al. 2001, Saarnio et al. 1998, 2000)
Exacerbated N limitation increases root production/exudation
Increases dominance of acetoclastic methanogenesis
Increased t will increase
Increased root production/exudation, increase in acetoclastic methanogenesis

46. Temperature Increases
Permafrost thaw, opens barriers to drainage
T results in N mineralization, mediates N limitation
SOM decomposition and GG production:
Recalcitrant C has higher activation energy, therefore higher temp sensitivity
Recalcitrant C turnover more affected by T than labile C
Methanogenesis more T sensitive than aerobic respiration

47. Water table reduction WT reduction increases vascular plant abundance
Loss of non-vascular plants -> less recalcitrant SOM -> slower peat accumulation
Boreal forest expansion into peatland has net warming effect on radiation balance (Bonan 2008)
Peatland expansion onto tundra -> CO2 sink
Drier peat-> initial permafrost expansion
Sphagnum Lawn
~0-5cm
High Water Table
~15-25cm
Low Water Table
~40-50cm

48. Atmospheric Deposition
Well studied in European peatlands
Increases TEA abundance, decreases methane production (Erikkson and Nilsson 2009)
N deposition increases vascular plant abundance relative to Sphagnum
Increases decomposition rate due to more recalcitrant litter, ultimately reduces peat accumulation rate

49. Spruce and Peatland Responses Under Climactic and Environmental Change