Stable Isotope Ecology Day 2, 18.1.2021: Carbon - Prof. Nina Buchmann, Institute of Agricultural Sciences
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Stable Isotope Ecology
Day 2, 18.1.2021: Carbon
Prof. Nina Buchmann, Institute of Agricultural Sciences
nbuchmann@ethz.ch
Carbon isotopes
Fate of stable carbon isotopes from the atmosphere through
the plant into the soil and back into the atmosphere
• Atmosphere: Source air for photosynthesis
- Intra-canopy profiles, spatio-temporal variability
• Plants
- Leaf discrimination; Link to ecophysiology (C3)
- Leaf discrimination (C4)
- Leaf compounds; allocation
(- Leaf respiration, post-photosynthesis Day 3)
• Soils: Bulk soil signatures; Decomposition of litter
• Biospheric feedback to the atmosphere
Outline
Isotopomers and abundances
< 15 cm
(Dawson et al. 2002)
Repetition: Isotopic jargon
depleted enriched
("light") ("heavy")
Standard
„C4 plants are
_____ enriched
than air.“
= δ13Csoil is
_____ positive
than δ13CCH4.
C dynamics of terrestrial ecosystems
(after Trumbore 2006)
Introduction ◦ Concept ◦ Study Sites ◦ Methods ◦ Time Plan ◦ Open Questions
Source air for photosynthesis: [CO2]
For
• 37 m,
• 14 m,
• 1 m,
• 0.02 m
(Buchmann et al. 1997)
Atmosphere
Source air for photosynthesis: δ13C
For
• 16 m,
• 3 m,
• 0.3 m,
• 0.02 m
Both, [CO2] and δ13C vary in
time and space. These
variations strongly depend
on canopy structure,
vegetation activity and land
use history.
(Buchmann et al. 1997)
Atmosphere
δ13C and [CO2] into the troposphere
Height [m]
Hainich NP, August 2002
1500
1000
CBL
500
50
40
30 Canopy
20
10
Soil
0
-20 -10 -9 -8 -7 360 380 5000 15000
δ13C [‰] CO2 conc. [ppm] (unpubl. dataset)
Atmosphere
Intra-canopy variations of 13C signatures
Paracou, French Guiana
Dry season 1994
40 Troposphere -7.7 • δ13Cleaf integrates over
daytime variations in δ13Cair.
leaves (21.9) • Vertical variations of δ13Cleaf
30 ∆
can range a couple of ‰.
Canopy height [m]
20 • The impact of δ13Cair is
(26.8)
much smaller (about 30%)
∆
than that of ecophysiology.
10
(26.4)
∆
0
-36
soil -26
-31 -21 -16 -11 -6
(Buchmann et al. 1997;
δ13C [‰] Buchmann et al. 2002)
Atmosphere
Discrimination
In ecophysiology, we often use the term discrimination ∆ to
talk about and calculate fractionation (in the promil notation)
In a first approximation: ∆ [‰] ≈ δSubstrate - δProduct
δ13Cleaf ≈ -28 ‰ (C3)
δ13Cair = -8.3 ‰ -14 ‰ (C4)
Atmospheric CO2 Assimilation
∆leaf δ13Cleaf
PlantsCarbon isotope discrimination during PS
∆ = (α – 1) x 1000
Since α = RSource/RProduct ∆ = (RS/RP –1) x 1000
In the case of photosynthesis (PS), the source is CO2 in air
and the product is plant dry matter.
∆ = (Rair/Rplant –1) x 1000
Since δair = Rair/Rstd – 1 and δplant = Rplant/Rstd – 1, then
∆ = (δair - δplant) / (1 + δplant)
PlantsCarbon isotope discrimination during PS
∆ = (δair - δplant) / (1 + δplant)
Assume: δair = -8.3 ‰ and δplant = -27.4 ‰
Calculate: ∆ in ‰
What is the value for ∆?
PlantsCarbon discrimination in C3 leaves
(Wingate et al. 2007)
PlantsFractionation factors for C3 plants
ab 2.9‰ diffusion of CO2 through the boundary layer to stomata
a 4.4‰ diffusion of CO2 in air through stomatal pore
am (=al + es) mesophyll CO2 transport and transfer
al 0.7‰ diffusion of dissolved CO2 through water
es 1.1‰ dissolution of CO2 into water (at 25°C)
b ~27‰ net C3 fixation by Rubisco/PEPc reactions
f ~10‰ photorespiration
e -6 to 6‰ dark respiration (= daytime mitochondrial respiration)
∆leaf
(Farquhar et al. 1989, Tscherkez 2006, Wingate et al. 2007)
PlantsFull equation of ∆leaf for C3 plants
(based on Farquhar et al. 1989)
PlantsSimplified equation of ∆leaf for C3 plants
(Farquhar et al. 1989)
PlantsSimplified equation of ∆leaf for C3 plants
(Farquhar et al. 1989, Wingate et al. 2007)
Plants∆leaf = f(ci/ca) Measured on-line
ci/ca = (Farquhar et al. 1989)
PlantsChanges in ∆leaf
Chloroplast Chloroplast Chloroplast
Cytoplasm Cytoplasm Cytoplasm
Intercellular Intercellular Intercellular
Space Space Space
ci ci ci
Stomata Stomata Stomata
ca ca ca
Stomata close
gs, ci
ci /ca
∆leaf = a + (b - a) * ci/ca
∆
δ13C
PlantsControls on ci/ca and thus ∆
precipitation
input
soil water
Everything that controls
transpiration
A and gs influences ∆: leaf water potential
- Air humidity
light
- Soil moisture stomatal conductance
- Precipitation Air humidity
- Radiation
∆ ci/ca CO2
- Photosynthetic capacity
- ….
Assimilation
δ13C of
fixed carbon Amax
PlantsSoil moisture
C3 plants
discriminate
less when
exposed to
water stress
drought
(Condon et al. 1992)
PlantsPrecipitation
Increase in C3
discrimination
along precip
gradients
(Stewart et al. 1995)
PlantsIncoming radiation
C3 carbon isotope
discrimination
decreases with
increased sunlight
(PPFD)
(Ehleringer et al. 1986)
Plantsδ13Cleaf in vegetation stands = f(ht)
Abies amabilis What is the reason for
Young-high these patterns?
15
Beech forest
1-yr-old
Height (m)
10
Height [m]
current
5
0
-32 -30 -28 -26
δ13Cleaf (‰)
(Buchmann et al., unpubl) (Knohl et al. 2005)
Plantsδ13Cleaf in a canopy: Confounding factors
Abies amabilis
-26 (a) (b) (c) -26
current
δ13Cleaf (‰)
-28 -28
-30 1-yr-old -30
-32 -32
0 1 2 3 4 0.5 1.0 100 200
N concentration (g N m-2) Needle thickness (mm) Branch length (cm)
Many factors affect ci/ca along a height gradient:
• Assimilation, light and [N]: ci ↓, ∆ ↓, δ13C ↑
• rH: ci↑, ∆ ↑, δ13C ↓
Even if ∆ is known, need to measure auxiliary variables
(Buchmann et al., unpubl)
Plantsδ13Cleaf, ∆ and the link to WUE
WUE = A/g
(Farquhar et al. 1989)
Plants∆ and WUE
Demand
• assume simple model
A = g l ⋅ (ca − ci )
Supply
• and rearrange
ci A A
= 1− = intrinsic water use efficiency
ca g l ⋅ ca gl
PlantsLinking ∆leaf to water use efficiency
(Ehleringer et al. 1993)
PlantsC4 photosynthesis
Xylem Phloem
Bundle sheath cell
PEP Carboxylation Decarboxylation
in mesophyll cell: in bundle sheath
ATP + NADPH cell, Rubisco
C4 sugar Carboxylation:
CO2 from photo- CO2 Glucose,
respiration refixed Photorespiration
PlantsC4 vs. C3 photosynthesis = f(PAR, CO2)
(Larcher 1994, Ehleringer et al. 1991)
Plants∆leaf for C4 plants
(Brugnoli and
Farquhar 2000)
= leakiness
(Farquhar 1983)
Plants∆leaf for C4 plants: Leackiness/leackage Φ
Leakiness ↑,
thus ∆ increases
(Ehleringer, unpubl.)
Plants∆leaf in C4 plants: Effects of stress
• Leakiness ↑ with
increasing light or
water stress
• Leakiness differs
for different C4
subtypes
(Buchmann et al. 1996)
PlantsDifferent aspects of ∆leaf of bulk foliage
δ13Cleaf = δ13Cair - (a + (b - a) * ci/ca) What we measure
with IRMS
What we learn about
∆leaf = (δair - δplant) / (1 + δplant)
the impacts of the
environment on leaves
What we learn about
∆leaf = (a + (b - a) * ci/ca)
Ecophysiology
Many applications
PlantsCarbon allocation: Use of stable isotopes
E.g., Pulse-labelling
with 13CO2
Insights into allocation aspects
• Fate of recently assimilated carbon
• Time-lags of allocation
• Environmental impacts on allocation
• Underlying mechanisms
(Brüggemann et al. 2011)
PlantsTechniques to trace carbon
• In the field
13CO pulse labelling
2
under controlled
conditions
Girdling
Large-scale 13CO2 pulse labeling
Small-scale 13CO2 pulse labelling
• Under controlled conditionsEffect of drought on coupling: Beech
control
drought
Under controlled conditions
in the greenhouse
(Rühr et al. 2009)
PlantsEffect of drought on coupling: Beech
control
drought
Drought slows down allocation throughout the tree system
(Rühr et al. 2009)
PlantsEffect of drought on coupling: Beech
control
drought
… and even affects soil CO2 fluxes (Rühr et al. 2009)
PlantsEffect of drought on coupling: Grassland
Drought reduces
incorporation into
shoots, but
increases relative
allocation to roots
…
(Burri et al. 2014)
PlantsEffect of drought on coupling: Grassland
… but less newly fixed C was lost via root respiration.
(Burri et al. 2014)
PlantsDecomposition of plant materials: δ13C
δ13Cleaf ≈ -28‰ (C3)
δ13Cair = -8.3‰ -14‰ (C4)
Atmospheric CO2 Assimilation
δ13Cleaf
Senescence
δ13Clitter
Decomposition,
SOM formation
δ13CSOC ≈ -22‰ δ13CSOC
Numbers given are an approximation, in reality they vary….
SoilRelationship between δ13Cfoliage, litter, SOC
Steady enrichment of about 2-3‰ in δ13C from foliage to soil due to
multiple reasons, not completely understood. (Buchmann et al. 1997)
SoilTypical soil δ13CSOC and %C profiles
LI = litter
Enrichment: Suess effect plus soil/ microbial effects
(Balesdent et al. 1993)
SoilGlobal patterns of soil δ13CSOC vs. %C
(Bird and Pousai 1997)
SoilWhere does the enrichment come?
from?
• Decrease of δ13C in
atmospheric CO2,
Suess effect
(Friedli et al. 1986)
SoilChanges in atmospheric [CO2] & δ13C
• Profound changes in
atm. composition
• Anthropogenic origin
• Large biotic effects
• [CO2] ↑ from 280 ppm
to now-a-days 400 ppm
• δ13C ↓ to < -8.3‰
MLO = Mauna Loa
SPO = South Pole
(IPCC, WG1, 2013)
SoilChanges in atmospheric [CO2] & δ13C
Jungfraujoch, January 2009 to July 2017
(Steinbacher et al., unpubl.)
SoilEffect of δ13Cair/plant on δ13Csoil organic carbon
FACE = Free Air Carbon dioxide
Enrichment
(Søe and Buchmann 2004)
SoilOrganic δ13C in soils is related to inputs
n = 1000 C4 plants
80
Absolute frequencies
C3 plants
60
40
20
CO2
0
-35 -30 -25 -20 -15 -10 -5
δ13Cleaf [‰]
Distance [m]
(Schwartz et al. 1996)
SoilOrganic δ13C in soils is related to inputs
n = 1000 C4 plants
80
Absolute frequencies
C3 plants
60
40
20
CO2
0
-35 -30 -25 -20 -15 -10 -5
δ13Cleaf [‰]
Relevance of land
use history!!
(Ehleringer et al. 2000)
SoilCarbon turnover is size dependent
(Mariotti and Balesdent 1996)
SoilWhere does new carbon go?
In general:
• Up to 2-3‰ difference in
δ13C among different
fractions
• Highest percentage if new
carbon sources in coarse/
Fig 1 bales 96 large size fractions.
• Old carbon sources in fine
fractions.
(Balesdent 1996)
SoilDecomposition of plant materials
CO2 released
respiration
residue C input decomposition
(e.g., leaf or root litter) decomposer biomass C
immobilization
SOM formation
physically and/or
chemically protected
organic matter C
SoilSoil CO2, respired CO2 and δ13C
fractionation with
diffusion of CO2 out
of the soil: 4.4 ‰.
Thus, soil CO2 is
4.4 ‰ more
enriched than soil
respired CO2.
(Cerling et al. 1991)
SoilChanges in δ13C during decomposition?
• Still an important hot topic
• Many parallel processes (see complications), plus
• Memory effect: δ13C of atmospheric CO2 from
decades and centuries ago (Süess effect)
Introduction ◦ Concept ◦ Study Sites ◦ Methods ◦ Time Plan ◦ Open Questions SoilChanges in δ13C during decomposition?
• Still an important hot topic
• Many parallel processes, plus
• Memory effect: δ13C of atmospheric CO2 from
decades and centuries ago (Suess effect)
• „apparent“ fractionation: substrate preference of
microbes
• Internal recycling/mixing of carbon compounds (soil –
microbes)
• Fast and slow respiration processes, respiring
different pools with different turnover times
• …..
Introduction ◦ Concept ◦ Study Sites ◦ Methods ◦ Time Plan ◦ Open Questions SoilCarbonate and its isotopic signature
• Carbonate precipitates in
equilibrium with CO2 from the
decomposition of organic
matter
• Offset is sum of equilibrium
effects during CO2 solution
(dependent on temperature)
and kinetic effects during
diffusion
Carbonates are well
preserved and can be used
for C3/C4 reconstructions
(Quade and Cerling 1995)
SoilC dynamics of terrestrial ecosystems
(after Trumbore 2006)
Introduction ◦ Concept ◦ Study Sites ◦ Methods ◦ Time Plan ◦ Open Questions FeedbackIntegrating the canopy? "Keeling plot"
[CO2] d13C Two source mixing model of
CO2 molecules from
Atmosphere
Ecosystem respiration
[CO2]canopy = [CO2]atm + [CO2]R
[CO2]canopy δ13Ccanopy = [CO2]atm δ13Catm + [CO2]R δ13CR
FeedbackMass balance equation
δ13Ccanopy = δ13CR + (δ13Catm - δ13CR) * [CO2]atm * 1/[CO2]canopy
-10
st
1 source
δ13C [‰]
-15
atmosphere
-20
-25 y=b+m x
nd
2 source: 0.000 0.001 0.002 0.003
respiration -1
1/[CO2] [ppm ]
Linear geometric regression:
with b = δ13C of ecosystem respiration δ13CR
FeedbackShort-term variations in δ13CNEE: diurnal
Hainich National Park
23.05.01 24.07.01
-22
a b
0
-24
[µmol m-2 s-1]
δ13CE [‰]
1.9‰
NEE
-26 -10
3.8‰
-28
δ13CE δ13C E -20
NEE NEE
30
Air temperature c Air temperature d
QP QP
25
1500
Air temperature [°C]
QP [µmol m-2 s-1]
20
1000
15
500
10
0
5
00:00 06:00 12:00 18:00 00:00 00:00 06:00 12:00 18:00 00:00
(Knohl et al. 2005)
FeedbackEnvironmental effects on δ13CR
Hainich National Park 24.07. - 16.08.2002
-25
a • Timelag between δ13CR and
-26
climatic conditions/canopy
δ13CR [‰]
-27
stress
-28
• „ecosystem memory“
-29
b
20 • For comparison:
timelag for conifers: 5 - 10
D [hPa]
10 days (Bowling et al. 2002),
timelag for soil respiration:
2000 3 – 4 days (Ekblad and
c
Högberg 2001)
QP [µmol m-2 s-1]
1500
1000
500
0
26.7. 31.7. 6.8. 11.8. 16.8. (Knohl et al. 2005)
Feedbackδ13CR is linked to humidity, 5 to 10 days prior
-22
-24
δ13C of respired CO2 (‰)
-26 more closed
dR (‰)
-28
stomata
P. menziesii - C
P. menziesii - D
-30 P. ponderosa - E
J. occidentalis - F more open
-32
0 1 2 3 4
Time-lagged vapor pressure deficit (kPa)
humid dry
(Bowling et al. 2002)
FeedbackEcosystem discrimination
∆ecosystem = (δ13Ctrop - δ13CR) / (1 + δ13CR)
(Kaplan et al. 2002)
FeedbackEcosystem discrimination
Biospheric feedback to the atmosphere (Kaplan et al. 2002)
FeedbackOcean and land C uptake
Stable isotope measure-
ments of CO2 beginning
in the early 1990s allows
partitioning of land and
ocean sinks.
• Land photosynthesis D is approx. 18 ‰ (dep on C3:C4
distributions), but variable over time
• Ocean uptake involves approx. 1-2 ‰ equilibrium
fractionation
• Note: C4 plant fractionation is similar to that of oceans!
(Tans and White 1998)
FeedbackSink strength depends on assumptions
Land Dashed lines denote
constant terrestrial D
Ocean Solid lines are variable D
ocean
Land GPP
0.2‰ change in ∆ can
affect estimate of
terrestrial sink by 25 %!
(Randerson et al. 2002)
Knowing processes controlling ∆ is important at global scale
FeedbackCarbon isotopes
Fate of stable carbon isotopes from the atmosphere through
the plant into the soil and back into the atmosphere
• Atmosphere: Source air for photosynthesis
- Intra-canopy profiles, spatio-temporal variability
• Plants
- Leaf discrimination; Link to ecophysiology (C3)
- Leaf discrimination (C4)
- Leaf compounds; allocation
(- Leaf respiration Day 3)
• Soils: Bulk soil signatures; Decomposition of litter
• Biospheric feedback to the atmosphere
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