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Biogeochemistry of C

NREM 665

CO2

diffusion

CH4CO2

CO

& ebullition

2

DOC CH4methanogenesis

Organic-C accretion

C Biogeochemistry

I.Forms of C in WTLs & Coastal Ecosystems

Molecule Oxidation State Form/PhaseMolecule Oxidation State Form/PhaseCarbon dioxide, CO2Methane, CH4Carbonate, CO3

2-, 3Bicarbonate,HCO3

-

Carbonic Acid, H2CO3

Operationally-defined forms of C:

Dissolved org. C (DOC) <0.7 µm P ti l t C (POC) >0 7Particulate org. C (POC) >0.7 µm

Plant biomass, microbial biomass, litter, soil C

2

Carbonate Buffer System

CO2 + H2O H2CO3 H+ + HCO3- 2H+ + CO3

2-

3

II. Aerobic conditions: PSN & Resp. dominant rxns

A 6CO 12H O li ht C H O 6O 6H OA. 6CO2 + 12H2O + light → C6H12O6 + 6O2 + 6H2O

B. C6H12O6 + 6O2 → 6CO2 + 6H2O + energy

C. C storage = Production– Decomposition

4

Ratio of aboveground to belowground biomassbelowground biomass can range from 0.2 to 3.9 (Gopal & Massing 1990)

1. Decomp inhibited in flooded soils → high C storage in WTLs, SGBs

2. NPP, Decomp, C storage vary across WTL types

a due to climate hydrology veg type soil type nutrienta. due to climate, hydrology, veg type, soil type, nutrientavailability, microbial community/activity, etc.

b. lability of FW marsh litter TSM littery

3. Litter/detritus processing in WTLs/SGBs is complex: involvesmicrobes, shredders, deposit feeders, filter feeders, release of DOC & POC

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Decomposition of Freshwater Marsh Vegetation

7(Mitsch & Gosselink 1993)

Sagittaria latifolia(arrowhead(arrowhead, duck potato)

Pontederia cordata(pickerelweed)

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Nuphar advena (yellow pond lilly)http://plants.ifas.ufl.edu/nulupic.html

9http://aquat1.ifas.ufl.edu/zizaqu1m.jpg

Decomposition of Salt Marsh Vegetation

10(McKee & Seneca 1982)

Spartina alterniflora(saltmarsh cordgrass)

Juncus roemerianus(black needlerush)(black needlerush)

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Plant N & P

CO2CH4

N & P

4

Bioavailable Soil organic

matterN & P

matter accumulation Decomposition

Hydrology Electron acceptors

Nutrients

RainfallEvapotranspiration External loading

acceptors

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Evapotranspiration External loading

Regulators of Decomposition (Reddy & DeLaune 2008)

Detrital plant biomass

D t it Decomposition}ble

Detritus Decomposition}}W

ater

tab

PeatBurial}W

Compaction} Compaction}13Decomposition & burial of OM (Reddy & DeLaune 2008)

III. Anaerobic conditions: Fermentation (Ferm) & Methanogenesis (MTG) dominantMethanogenesis (MTG) dominant

A. Ferm occurs when organic matter is TEA in anaerobic respirationrespiration

1. C6H12O6 → 2CH3CH2OCOOH (Ferm) ∆Go = 239 kJ mol-1

2. C6H12O6 → 2CH3CH2OH + 2CO2 (Ferm) ∆Go = 122 kJ mol-1

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B. MTG occurs when methanogens use CO2 (or other simple C compounds) as TEA in respiration & produce CH4

1. CO2 + 8e- + 8H+ ⇒ CH4 + 2H2O

2 CH COO + 4H O → 2CH + 2H O2. CH3COO + 4H2O → 2CH4 + 2H2O

3. MTG rates:

a. summer >> winter in temperate zones, less seasonal var. in tropicsp

b. ƒ(flood freq. & duration, pres/abs of veg)

c. permanently flooded >> intermittentl flooded

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>> intermittently flooded

4. MTG in FW WTLs >> MTG in tidal WTLs?

a.

b. C flux in GA salt marsh vs WI lake sed (M&G 2000)

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IV. CH4 oxidation: Methanotrophs convert CH4 to methanol formaldehyde & COmethanol, formaldehyde, & CO2

CH4 → CH3OH → HCHO- → HCOOH → CO2

1. CH4 produced @ depth oxidized by methanotrophs @ surface (↓CH4 flux)

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2. Net CH4 emission = ƒ(methanogenesis – methane oxidation)

a. rates of CH4 emission for different wetland types

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http://www.ace.mmu.ac.uk/eae/Global_Warming/Older/GWPs.html

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Long-term Accumulation of Organic Matter in Selected Wetlands

Wetland LocationC Accumulation

(g m-2 yr-1) Reference

Everglades Florida Reddy et al 1993EvergladesTypha sp.Cladium sp.

Florida163-38786-140

Reddy et al. 1993

S lt M h L i i 200 300 H tt t l 1993Salt Marsh Louisiana 200-300 Hatton et al. 1993

Mangrove Mexico 100 Twilley 1992

Cypress Swamp Georgia 45 Cohen 1974

Peatland Alaska 22 122 Billings 1987Peatland Alaska 22-122 Billings 1987

Bog Wisconsin 34-75 Kratz & Dewitt 1986

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Sh t t i b h b l t & l l d ti

3. C sinks in WTLs

a. Short-term: immob, herb. plant & algal production

b. Long-term: woody production, C seq. in soil/sediment

V C Cyling in Tidal WTLs vs FW WTLsV. C Cyling in Tidal WTLs vs FW WTLs

A. Tidal WTLs have high prod & frequent tidal exchange → export higher amounts of C than FW WTLs

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VI. Case Study: Are Phragmites-dominated wetlands a net source or net sink for greenhouse gases?

23(Brix et al. 2001)

CO2C cycling in wetlands

photosynthesis

2

is

diffusion & ebullition

CO2 CH4

CO2 metha

diffusion & ebullition

thane-oxidationre

spira

tion

DOC CH4methanogenesis

r

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Organic-C accretion

CO2 & CH4 dynamics in a Phragmites-2 4 y gwetland

Net CO2-assimilation

CH4-emission

CO2-emission

25(Brix et al. 2001)

CO2

Phragmites wetland:Units: mol m-2 year-1

photosynthesis 45 4s

diffusion & ebullition

CO2 CH498

CO2 methanen

ane-oxidationre

spira

tion

14DOC CH4methanogenesis

O i C ti 1849

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Organic-C accretion 18(Brix et al. 2001)

Phragmites C budget:g g

N t CO A i il ti 98 l 2 1Net CO2 Assimilation: 98 mol m-2 yr-1

CO2 Emission: 45 mol m-2 yr-1

CH Emission: 4 mol m-2 yr-1CH4 Emission: 4 mol m 2 yr 1

Net C Fixation: 98 - 45 - 4 = 49 mol m-2 yr-1Net C Fixation: 98 45 4 49 mol m yr

Ratio CH4 Emission:Net C Fixation: 0.08

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Th ti f CH l d tThe ratio of CH4 released to annual net C fixed variesannual net C fixed varies

generally from 0.05 to 0.13(Whi i & Ch 199 )(Whiting & Chanton 1997)

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Question:

Are wetlands a net source or a net sink for greenhouse gases?sink for greenhouse gases?

Answer:

Depends on the ratio of CH4emitted to annual net C fixed & timeemitted to annual net C fixed & time

scale evaluated.

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Greenhouse effect as a function of time scale

E

0.20 Ratio between CH4emitted & net C fixed

SOU

RC

E

0 10

INK

0.10

0.05

SI

•Wetlands may be sources of GHGs on short time scales (<10 yr) but ( )

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sinks over longer time scales (>100 yrs). Lower the ratio, sooner a site becomes a sink (Brix et al. 2001).

VII. C Biogeochemistry ConclusionsA M jA. Major processes

Production: Fixation of C from atmosphere via PSN

Decomposition: Conversion of complex molecules into simpler molecules via activities of aerobes & anaerobes

Fermentation: Use of organic matter as TEA in anaerobic respiration

Methanogenesis: Bacteria use CO2 as TEA in anaerobic respirationg 2 p

CH4 Oxidation: Methanotrophs convert CH4 to methanol, formaldehyde, & CO2 in aerobic surface soil zones

B. Although WTLs cover ~ 7% of Earth’s land area, they have a disproportionate effect on global C cycle

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