principles and environmental applications of stable isotopes
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Principles and Environmental Principles and Environmental Applications of Stable IsotopesApplications of Stable Isotopes
The whirlwind tour
Elizabeth SulzmanOregon State University,
Dept. Crop & Soil Sci
Part 1: the basics
The difference among isotopesThe difference among isotopes• Isotopes are atoms of the same element
with different numbers of neutrons
• Same chemistry, but different physics e.g., Heavy molecules form stronger bonds,
diffuse more slowly, evaporate last . . .
What makes a stable isotope useful for environmental studies?
Low Atomic Mass (measurable separation)
Large mass difference: 100%, 8.3%, & 12.5% for D/H, 13C/12C, 18O/16O, respectively
Large diff. in natural abundance
• Preparation system • Inlet system• Collector system
Delta notation and why standards are used
Absolute abundances are VERY low !!
Rstandard :• 2H:1H = 0.00015576• 13C:12C = 0.0112372• 15N:14N = 0.0036765• 18O:16O VSMOW= 0.0020052
the value for all standards is 0 !
1000*)1R
R(‰)(
standard
sample −=
Typical range of 13C values
So if atmospheric CO2 is the base of the food chain, why is this pool so much less variable than, and
sometimes different from, the other pools?
What is fractionation? When do you observe it?
Separation of isotopes in the environment
Observe when reactions do not go to completion (open system) or with precise techniques (closed system)
Mazor 1991
Slope: f(RH, T)
Temperature Dependence of Fractionation Factors
Part 2: APPLICATIONS of Isotopes in Ecology & Environmental Sciences
Isotopes record biological responses to Earth’s changing environmental condition
Isotopes trace the origin and movement of key elements and substances
Isotopes indicate the presence and magnitude of key processes
Isotopes integrate ecological processes in space and time
Some examples
Paleoclimate reconstruction (e.g., Ice enriched in 18O= warm and wet)
Food web studies (What do the wolves eat?; What did paleo-humans eat?)
Food purity (Does the beer have corn in it? Is the orange juice from concentrate?)
Environmental quality / human health: What is the source of nitrate in our ground water?
Examples in biogeochemistry
Plant C
Soil C
Ecosystem respiration (terrestrial contribution to the global C budget)
History of plant isotope studies Nier and Gulbransen (1939) discovered plant
samples exhibit lower 13C/12C than background air
Extensive “surveys” of plant material through 1940s and 1950s (e.g., Craig 1953)
First model postulating leaf fractionation must occur (Park and Epstein 1960)
All this inquiry carried out by geochemists and geologists – ecologists/plant physiologists didn’t pick this back up until the 1980s! (O’Leary, Vogel, Farquhar)
More recent history
Farquhar (1982) showed that the C isotope ratio of an individual plant was correlated with its intercellular [CO2] it was concluded that this could be used in
selective breeding for a high C acquisition efficiency and low water use (i.e., WUE)
Basis for 13C variations in plants
irreversible steps in the metabolic process, where not all of the substrate is consumed
metabolic branch points
opportunities where diffusion is a fundamental step in the process
secondary fractionation events associated with common pools
There are …There are …
C3 photosynthetic pathway
O’Leary 1988
transportdissolution
fixation
C4 photosynthetic pathway
O’Leary 1988
fixation
transport transport
2º fixation doesn’t fractionate
Cerling et al (1997)
C3 and C4 plants differ in their carbon isotope ratios
C3/C4 distribution a link to past climate, important for models of C sinks
C4 evolved under low CO2, is more moisture conservative
Ehleringer et al. (1997)
Vegetation shifts as a “natural” tracer experiment
Balesdent and Mariotti, 1996
Change in SOM over time: conversion to C4
Balesdent et al. 1987
Calculation of turnover times from “natural” tracer experiments
One of many formulations:
f = (1-X)i + Xn
where i is initial soil, f is final soil, n is new vegetation, and X is the proportion of C coming from the new vegetation
Wedin et al. 1995
Isotopic data suggest soils are not homogeneous
Townsend et al. 1995
Soil and leaf contribution to the atmospheric 13C
Ehleringer et al. 2000
As scale of observation increases, system becomes more heterogeneous and increasingly difficult to characterize isotopically
The terrestrial end-member is not a single pool!
Isotopic disequilibrium
1. Plant SOM: 13C of SOM at any point in time does not
necessarily match 13C of current biomass, especially with land use/land cover changes
2. Now 10-100 years ago:
As SOM decomposes, it releases some CO2 that was fixed at a time when atmospheric isotopic composition was different (heavier) than it is today
More complications
Differences as great as 10‰ in 13C of tissues near forest floor and those at the top of a forest canopy
The proportion of C3 and C4 vegetation can change seasonally in some places We know these are diff w. respect to 13C; Gillon and
Yakir (2001) showed discrimination against 18O also radically different for C3 vs. C4
CO2
“The Solution” A Keeling plotKeeling (1958)
-30
-25
-20
-15
-10
-5
0 0.0005 0.001 0.0015 0.002 0.0025 0.003
13C
(‰
)
1/[CO2] (mol mol-1)
13CR
carbon isotope ratio ofrespired CO2
background forest air
Cmeas = Cbackground + Crespired
measCmeas = backgroundCbackground + respiredCrespired
meas = (slope)*(1/Cmeas) + respired
13CR or R
Keeling-derived 13CR values reflect real processes
Rochette and Flanagan 1997
Pataki et al. (2003)
13C of ecosystem respiration responds to drought across biomes
Effect of vapor pressure deficit on the 13C of ecosystem respiration
Bowling et al. 2002
more closed
more open
sto
mat
a
humid dry
Cautions…
Keeling plots often require extrapolation of the intercept far from the actual measurements
Small errors in measurement of either isotopic composition or concentration can yield large errors
Hard to account for potential CO2 recycling (tho modified equations exists – and they don’t agree!)
What if you are wrong?? A difference of 3‰ in the calculated e
leads to a 20% overestimate of the terrestrial sink strength!! (Buchmann and Kaplan 2001)
Ciais et al. 2000
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