Biogeochemical processes of methane

1
Biogeochemical processes of methane emission and
uptake
Edward Hornibrook Bristol Biogeochemistry
Research Centre Department of Earth
Sciences University of Bristol
2
Outline
1. Methanogenesis methanotrophy
2. Anaerobic C mineralisation in wetlands -
uncertainties?
3. Stable isotopes methane
4. Current BBRC research
3
Alessandro Volta (1776) "Combustible Air"
Wolfe (1993)
4
Universal Phylogenetic Tree of Life (16S 18S
RNA)
Madigan et al (2003)
5
C6H12O6 6 O2 ? 6 CO2 6 H2O DG0 -2870 kJ/mol
C6H12O6? 3 CO2 3 CH4 DG0 -418 kJ/mol
6
Methanogenic Substrates
II. Methyl substrates Methanol, CH3OH
Methylamine, CH3NH3 Dimethylamine,
(CH3)2NH2 Trimethylamine, (CH3)3NH
Methylmercaptan, CH3SH Dimethylsulphide,
(CH3)2S
7
Diversity of methanogenic Archaea
Methanobacteriales 5 Genera 25 species
Substrates mainly H2 CO2, formate
Methanosphaera methanol, Methanothermus
reduction of S0
Methanococcales 5 Genera 9 species Substrates
mainly H2 CO2, formate Methanococcus
pyruvate
Methanomicrobiales 8 Genera 22 species
Substrates mainly H2 CO2, formate
Methanocorpusculum, Methanoculleus
Methanolacinia alcohols
Methanopyrales 1 Genera 1 species
Methanopyrus hyperthermophile (110C)
Substrates H2 CO2
8
Anaerobic Chain of Decay
complex organics (cellulose, hemicellulose)
homoacetogenic bacteria
9
The importance of syntrophy
4 H2 2 HCO3- H ? CH3COO- 4 H2O
-105
DG ? typical in situ abundance of reactants
products VFAs 1 mM HCO3- 5 mM
glucose 10 mM CH4 0.6 atm H2 10-4 atm
Madigan et al (2003)
10
Methanotrophic Bacteria

• Aerobic methane oxidation (Proteobacteria)
• Low affinity methanotrophs (culturable)
• High affinity methanotrophs (no isolates to
  date)

2. Anaerobic methane oxidation Marine
environments Methanogen/ sulphate-reducer
consortia
11
Substrates used by methylotrophs methanotrophs
Methane, CH4 Methanol, CH3OH Methylamine,
CH3NH3 Dimethylamine, (CH3)2NH2
Trimethylamine, (CH3)3NH Tetramethylammonium,
(CH3)4N Trimethylamine N-oxide, (CH3)3NO
Trimethylsulphonium, (CH3)3S
Formate, HCOO- Formamide, HCONH2 Carbon
monoxide, CO Dimethyl ether, (CH3)2O Dimethyl
ether, (CH3)2O Dimethyl carbonate, CH3OCOOCH3
Dimethyl sulphoxide, (CH3)2SO Dimethylsulphide,
(CH3)2S
12
Methanotrophic Bacteria
Type I (Ribulose monophosphate C-assimilation
pathway) Methylomonas, Methylomicrobium,
Methylobacter, Methylococcus Type II (Serine
C-assimilation pathway) Methylosinus,
Methylocystis, Methylocella, Methylocapsa acid
ophiles isolated from peat bogs (Dedysh et al.
2000 2002)
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Anaerobic C Mineralisation in Wetlands
Tenet 1 Methanogenesis is the terminal step in
anaerobic decay of organic matter in freshwater
wetlands.
Tenet 2 In most freshwater systems, 2/3 of
methanogenesis occurs via acetate fermentation
and 1/3 by CO2 reduction (H2).
Vile et al. (2003). Global Biogeochem. Cycles
17(2), 1058. anaerobic C mineralisation in
freshwater wetlands along a natural sulphate
gradient 36 to 27 SO42- reduction vs. ltlt1
methanogenesis ? fermentation of organic acids
? CO2
Wieder Lang (1988). Biogeochemistry 5, 221-242.
anaerobic C mineralisation in West Virginian
Sphagnum bog 38 to 64 SO42- reduction vs. 2.8
to 11.7 methanogenesis
Bridgham et al. (1998). Ecology 79, 1545-1561.
anaerobic C mineralization via methanogenesis
0.5 in bogs and lt2 in fens
14
Decoupling of Terminal Carbon Mineralisation
Pathway
Lansdown et al. (1992). Geochim. Cosmochim. Acta
56(9), 3493-3503. Kings Lake Bog, Washington
State (ombrotrophic peatland) CH4 derived
mainly from CO2/H2 confirmed with 14C tracer
experiments
Hines et al. (2001). Geophys. Res. Lett. 28(22),
4251-4254. northern wetlands CH4 derived
mainly from CO2/H2 Acetate accumulation to high
levels ultimately degraded aerobically to CO2
?contribution to high levels of DOC/ organic
acids in ombrotrophic bogs
15
Buck Hollow Bog (Michigan, USA)
-45
-50
d13C-CH4 ()
-55
-60
CR
CR
AF
-65
Jun
Jul
Apr
Nov
Jan
Feb
20
15
soil (peat) temperature (C)
10
5
0
Jul
Jun
Apr
Nov
Jan
Feb
Avery et al. (1999)
16
Turnagain Bog (ombrotrophic peatland, Anchorage
Alaska pH 4.6 to 5.1)
Duddleston et al. (2002). Geophys. Res. Lett.
28(22), 4251-4254.
17
'Underachieving' northern wetlands?
O2
CO2
SO42-
H2S
Possible causes? (i) temperature (ii)
pH (iii) vegetation (iv) trophic level
acetate ? CH4
What is the mechanism of acetate production?
(i) heterotrophic or (ii) autotrophic
VFAs
CH4 flux VFAs? (Christensen et al. 2003)
H2/CO2 ? CH4
18
d-values
-

0
?D, ?13C, ?15N, ?18O, ?34S ()
19
Stable Carbon Isotopes
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
10
?13C ()
after Hoefs (1997)
20
?13C of CH4 Sources
?13Cwt. avg. -54.4 ?13Catmosphere -47.3
Biomass Burning
-243
Coal Mining
Natural Gas
-502
Landfills
Ruminants
-605
Termites
Rice Paddies
-635
Oceans
Gas Hydrates
-40 to -86
Freshwater
-705
-605
Natural Wetlands
0
5
10
15
20
25
Methane Flux ( of total)
Tyler et al. (1988), Wahlen (1994), Quay et al.
(1991, 1999), Breas et al (2002)
21
20
10
0
-10
?13C-?CO2 ()
-20
-30
-40
-50
-120
-110
-100
-90
-80
-70
-60
-50
-40
-30
?13C-CH4 ()
Whiticar M. J., Faber E., and Schoell M. (1986)
Biogenic methane formation in marine and
freshwater environments CO2 reduction vs.
acetate fermentation - Isotope evidence.
Geochimica et Cosmochimica Acta 50,
693-709.
22
d13C of CH4 with pathway confirmed with 14C
tracers
acetate d13C-CH4
CO2-reduction d13C-CH4
Study
Environment
coastal marine
-62
Alperin et al. (1992)
-39 to -37
peatland
Lansdown et al. (1992)
-73 4
n/a
rice paddy
Sugimoto Wada (1993)
-43 to -30
-77 to -60
-62 to -58
-43 10
coastal marine
Blair et al. (1993)
freshwater estuary
Avery (1996)
-72 2.2
n/a
peatland (May)
Avery et al. (1999)
-43.8 12
-72 1.3
peatland (June)
Avery et al. (1999)
-71 1.3
-44.5 5.4
23
(r2 0.64 n 55)
Sifton Bog
20
DC 54
DC 86
10
DC 40
0
?13C-?CO2 ()
-10
-20
CR
AF
-30
-90
-30
-40
-50
-60
-70
-80
?13C-CH4 ()
Hornibrook et al. (2000)
24
C3 compost (soybean meal rice straw) ?13C
-26.5
dried rice plants ?13C (CH3COOH) -32.1
kudzu (fresh green leaves) ?13C (CH3COOH)
-32.9
?13C (CH3-)
?????13C (COOH)
dried rice plants -39.7
-24.4
kudzu -42.9
-22.9
intersection -42.3 (CH4)
-21.3 (?CO2)
Sugimoto Wada (1993)
25
Other Wetlands
20
CR
AF
10
0
?13C-?CO2 ()
-10
-20
-30
-40
-90
-30
-40
-50
-60
-70
-80
?13C-CH4 ()
Hornibrook et al. (2000)
26
Other Wetlands
20
CR
AF
10
0
?13C-?CO2 ()
-10
-20
-30
-40
-90
-30
-40
-50
-60
-70
-80
?13C-CH4 ()
Aravena et al. (1993), Lansdown et al. (1992),
Waldron et al. (1999)
27
20
CO2 reduction
10
acetate fermentation
deep
?13C-?CO2 ()
0
-10
shallow
shallow
-20
-90
-30
-40
-50
-60
-70
-80
?13C-CH4 ()
Hornibrook et al. (2000)
28
UK Sites
determine the prevalence of these d13C
distributions in different classes of natural
wetlands (SW England Wales)
determine CH4 pathway predominance using 14C
tracers
determine relationship between pore water
distribution and d13C signature of CH4 emissions
Ms. Helen Bowes (NERC Ph.D. student)
29
Field sites
1.Cors Caron 2.Tor Royal, Dartmoor 3.Llyn
Mire 4.Blanket bog, Elan Valley 5.Gors Lywd, Elan
Valley 6.Crymlyn Bog 7.Wicken Fen
7
5
4
1
3
6
2
30
Summary
The relative proportions of anaerobic processes
in freshwater wetlands needs to be better
characterised.
How wide spread is decoupling of terminal
stages of anaerobic C mineralisation in northern
wetlands?
What controls decoupling? Can systems switch
TCM processes?
Can stable isotope signatures of CH4 be used as
an accurate proxy for biogeochemical and physical
processes?
Models
Better understanding of anaerobic C flow needed
to represent microbial activity accurately in
process-based models
Integrated models of gas abundance/ emission
accurate simulation of stable isotope signatures.