1. 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

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, 13 C/ 12 C, 18 O/ 16 O, 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 !! R standard : 2 H: 1 H = 0.00015576 13 C: 12 C = 0.0112372 15 N: 14 N = 0.0036765 18 O: 16 O VSMOW= 0.0020052 the  value for all standards is 0 !

6. Typical range of  13 C values So if atmospheric CO 2 is the base of the food chain, why is this pool so much less variable than, and sometimes different from, the other pools?

7. 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)

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 18 O= 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 13 C/ 12 C 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 [CO 2 ]  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 13 C 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 …

15. C 3 photosynthetic pathway O’Leary 1988 transport dissolution fixation

16. C 4 photosynthetic pathway O’Leary 1988 fixation transport 2º fixation doesn’t fractionate

17. Cerling et al (1997) C 3 and C 4 plants differ in their carbon isotope ratios

18. C 3 /C 4 distribution a link to past climate, important for models of C sinks  C 4 evolved under low CO 2, is more moisture conservative Ehleringer et al. (1997)

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

20. Change in  SOM over time: conversion to C 4 Balesdent et al. 1987

21. Calculation of turnover times from “natural” tracer experiments One of many formulations:  f = (1-X)  i + X  n 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

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

23. Soil and leaf contribution to the atmospheric  13 C 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!

24. Isotopic disequilibrium 1.Plant  SOM:   13 C of SOM at any point in time does not necessarily match  13 C of current biomass, especially with land use/land cover changes 2.Now  10-100 years ago:  As SOM decomposes, it releases some CO 2 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  13 C of tissues near forest floor and those at the top of a forest canopy  The proportion of C 3 and C 4 vegetation can change seasonally in some places  We know these are diff w. respect to 13 C; Gillon and Yakir (2001) showed discrimination against 18 O also radically different for C 3 vs. C 4

26. CO 2 “The Solution” A Keeling plot Keeling (1958)  13 C (‰) 1/[CO 2 ] (mol  mol -1 )  13 C R carbon isotope ratio of respired CO 2 background forest air C meas = C background + C respired  meas C meas =  background C background +  respired C respired  meas = (slope)*(1/C meas ) +  respired  13 C R or  R

27. Keeling-derived  13 C R values reflect real processes Rochette and Flanagan 1997

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

29. Effect of vapor pressure deficit on the  13 C of ecosystem respiration Bowling et al. 2002 more closed more open stomata humid dry

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 CO 2 recycling (tho modified equations exists – and they don’t agree!)

31. 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