1. What we learned about isotopes Elizabeth Sulzman

2. The ground water in your area exceeds the EPA standard of 10 ppm of nitrate. What nitrate sources are responsible? Problem 1: Sources of nitrate in ground water δ 15 N (‰)δ 18 O (‰) ground water14 (sd=1)3 (sd=1) fertilizer1 (sd=1)25 (sd=1) soil organic N3 (sd=1)1 (sd=1) livestock manure 16 (sd=1)1 (sd=1) 3 possible sources 2 isotopic signatures measured in sources and groundwater

3. Problem 1: Sources of nitrate in ground water

6. Problem 2 What is the isotopic composition of soil respiration from a 40-yr-old stand dominated by Douglas-fir in the Cascade mountains of central Oregon.

7. Problem 2 What is the isotopic composition of soil respiration from a 40-yr-old stand dominated by Douglas-fir in the Cascade mountains of central Oregon. Differences north / south slopes? Spatial aggregation to watershed level Small scale Keeling plots Soils & Foliage North & South facing slopes At different elevations Watershed level Keeling plot

8. Keeling plots Keeling (1958) C mix = C bg + C source  mix C mix =  bg C bg +  source C source  mix C mix =  bg C bg +  source (C mix – C bg )  mix C mix = (  bg -  source ) C bg +  source C mix  mix = (  bg -  source ) C bg / C mix +  source  mix = (slope)*(1/C mix ) +  source

9.  Keeling plot:  mix = (slope)*(1/C mix ) +  source Hypothesis: North facing slope has a more negative (lighter) isotopic signature due to: 1.Lower water stress 2.Lower temperature

10. Isotopic signature soil respiration

11. Difference in isotopic signatures between north and south facing slope not large enough ?

12. Problem 3: Contributions of C3 and C4 to ecosystem respiration Miranda et al. 1997

13. Problem 3: Contributions of C3 and C4 to ecosystem respiration Miranda et al. 1997

14. Problem 3: Contributions of C3 and C4 to ecosystem respiration Miranda et al. 1997  13 C veg sd C3 plants-28.36672.084067 C4 plants-13.150.353553 Ecosystem respiration -21.41070.977209 Two source mixing model: 54 +/- 7 % of ecosystem respiration stems from C3 sources.

15. On to radioactive isotopes …