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Principles and Environmental Applications of Stable Isotopes

Principles and Environmental Applications of Stable Isotopes. The whirlwind tour Elizabeth Sulzman Oregon State University, Dept. Crop & Soil Sci. Part 1: the basics. The difference among isotopes. Isotopes are atoms of the same element with different numbers of neutrons

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Principles and Environmental Applications of Stable Isotopes

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  1. Principles and Environmental Applications of Stable Isotopes The whirlwind tour Elizabeth Sulzman Oregon State University, Dept. Crop & Soil Sci

  2. Part 1: the basics

  3. The 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 . . .

  4. 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

  5. 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 d value for all standards is 0 !

  6. Typical range of d13C 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?

  7. Slope: f(RH, T) Mazor 1991 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)

  8. Temperature Dependence of Fractionation Factors

  9. 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

  10. 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?

  11. Examples in biogeochemistry • Plant C • Soil C • Ecosystem respiration (terrestrial contribution to the global C budget)

  12. 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)

  13. 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)

  14. Basis for 13C variations in plants There are … • 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

  15. C3 photosynthetic pathway fixation dissolution transport O’Leary 1988

  16. 2º fixation doesn’t fractionate C4 photosynthetic pathway fixation transport transport O’Leary 1988

  17. C3 and C4 plants differ in their carbon isotope ratios Cerling et al (1997)

  18. 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)

  19. Vegetation shifts as a “natural” tracer experiment Balesdent and Mariotti, 1996

  20. Change in dSOM over time: conversion to C4 Balesdent et al. 1987

  21. One of many formulations: df = (1-X)di + Xdn 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 Calculation of turnover times from “natural” tracer experiments Wedin et al. 1995

  22. Isotopic data suggest soils are not homogeneous Townsend et al. 1995

  23. Soil and leaf contribution to the atmospheric d13C As scale of observation increases, system becomes more heterogeneous and increasingly difficult to characterize isotopically The terrestrial end-member is not a single pool! Ehleringer et al. 2000

  24. Isotopic disequilibrium • Plant  SOM: • d13C of SOM at any point in time does not necessarily match d13C of current biomass, especially with land use/land cover changes • 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

  25. More complications • Differences as great as 10‰ in d13C 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

  26. background forest air d13C (‰) d13CR carbon isotope ratio of respired CO2 1/[CO2] (mol mmol-1) CO2 d13CR or dR “The Solution” A Keeling plot Keeling (1958) Cmeas = Cbackground + Crespired dmeasCmeas = dbackgroundCbackground + drespiredCrespired dmeas = (slope)*(1/Cmeas) + drespired

  27. Keeling-derived d13CR values reflect real processes Rochette and Flanagan 1997

  28. 13C of ecosystem respiration responds to drought across biomes Pataki et al. (2003)

  29. more closed stomata more open Effect of vapor pressure deficit on the d13C of ecosystem respiration humid dry Bowling et al. 2002

  30. 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!)

  31. Ciais et al. 2000 What if you are wrong?? • A difference of 3‰ in the calculated De leads to a 20% overestimate of the terrestrial sink strength!! (Buchmann and Kaplan 2001)

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