Stable Isotope Ecology

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Stable Isotope Ecology

• Lecture 2
• Carbon Isotope Ratios

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Carbon Cycle
Each of the above pools and fluxes have their own
respective isotopic signature and fractionation.
The largest pool of carbon is in the ocean.
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Range of d13C Values in the Biosphere
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Lets Suppose that atmospheric CO2 is at
equilibrium with the ocean carbon pool.
Bulk of carbon is in the HCO3- form with a d13C
value 0.0. The isotopic fractionation factor
between HCO3- and CO2 at 25 oC as per the
fractionation table is aHCO3-CO21.0079. What is
the expected d13C of atmospheric CO2? What is
the observed d13C of atmospheric CO2?
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What about Photosynthesis and Respiration?
Sucrose
Water
Vapor O2
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• Large variation in fractionation caused by
  photosynthetic modes
• Smaller variations caused by plasticity of
  photosynthetic systems within a photosynthetic
  type.

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C4
C3
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aCO2-CH2O1.029
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C4
aHCO3-mal1.002
Bundle Sheath
aCO2-CH2O1.029
Leakiness
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HORSE SENSE!
Wang et al. 1994
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Crassulacean Acid Metabolism
Titratable Acidity CO2 uptake
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Winter Holtum 2005.
Mesembryanthemum cristallinum
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Variation within C3 plants.
aCa-Cia
aCi-CH2Ob
Fin
Fbchm
CO2(Ca)
(Ci)
(CH2O)n
Fout
Two Steady State Equations to Derive the Plant
d13C equation.
Ci
Fbchm Fin -Fout
Fbchm(dCi-b) Fin(dCa-a) Fout(dCi-a)
CO2 (Ca)
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C3 Plant d13C Equation
d13Cplantd13Catm a (b-a)Ci/Ca
Farquhar et al. 1982
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d13C of plant is related to instantaneous water
use efficiency.
(Ca-Ci)gs/1.6
(Ca-Ci)
W.U.E. Assimilation/Transpiration

(ei-ea)gs
(ei-ea)1.6
Ca(1 Ci/Ca)

(ei-ea)1.6
d13Cplantd13Catm a (b-a)Ci/Ca
So that the greater Ci/Ca (low d13C value) the
lower the W.U.E. Assuming the vapor pressure
gradient remains the same.
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Exercise

• Given that the d13C value of atmospheric CO2 is
  -7.8 what is the Ci of two species of plants
  growing next to each other with d13C value of -25
  and -28 for species A and B respectively?
• How many times greater is the W.U.E. of one over
  the other?

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Using carbon isotope ratios for selection of
water use efficient wheat varieties.
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Carbon isotope ratios of C3 plants are not always
related to water relations. Any factor which can
decrease Ci relative to Ca will increase plant
d13C values. Examples High light levels
increase internal assimilation of CO2 driving Ci
down (high d13C values). Nitrogen increase
enzymatic activity driving Ci down (high d13C
values). Internal damage to photosynthetic
apparatus drives Ci up (low d13C values).
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Internal Processes Respiration and Lipid
Biosynthesis.
Cellulose
d13Clipidsltltltd13Ccellulose
Pataki 2005
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Utilization of Lipid Respiration During the
Heating of Philodendron selloum Spikes.
Walker et al. 1983.
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Source or Physiological Effect?
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The pendulum swings to interpreting all
differences in d13C values of C3 plants being
caused by physiological effects i.e. source
effects are minimal even in tropical forests. i.e
depleted value d13C at the ground flora are
solely due to physiological effects (low light
levels).
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Medina et al. 1986
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The Keeling Plot
The Problem In order to know the contribution
of respired CO2 to the forest ambient air we need
to know the isotopic composition of the respired
CO2. There are several other cases where it is
important to know the isotopic composition of a
CO2 source without actually measuring its
isotopic composition, but just the contaminated
air. We need to take into consideration how the
contamination affects the CO2 concentration in
ambient air.
CO2 (ambient) CO2(atmospheric) CO2(source)
d13C (ambient)CO2 (ambient) d13C(atmospheric)CO2(
atmospheric) d13C(source)CO2(source)
A linear relationship between d13C (ambient) and
1/CO2(ambient) with an intercept equal to
d13C(source)
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Greaver et al. 2005
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Sternberg et al. 1989
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Exercise
  You have just caught your first hummingbird and
you want to know the carbon isotope ratios of
its diet without plucking a feather out. So you
put the bird in a box and monitor the CO2
concentration and the d13C value of the CO2.
  Here are your results
CO2 d13C val 380 -8.4 450 -11.4 500 -1
3.1 600 -15.1 650 -16.5
What is the d13C value of the CO2 from the birds
metabolism? If the concentration of atmospheric
CO2 is 370ppm what is the d13C value of
atmospheric CO2?
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C3 plants have a physiology such that
environmental factors can affect their carbon
isotope ratios. Therefore, they are a poor proxy
for changes in the d13C values of atmospheric
CO2.
What about C4 plants?
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Equation for d13C values of C4 plants.
d13Cplant d13Catm - a - (cb?-a)pi/pa
a and b are as before for the C3 equation c is
the combined effect of carbonic anihydrase and
Pep carboxylase ? is a leakage factor.
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