what we learned about isotopes elizabeth sulzman

What we learned about isotopes

Elizabeth Sulzman

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

δ15N (‰) δ18O (‰)

ground water 14 (sd=1) 3 (sd=1)

fertilizer 1 (sd=1) 25 (sd=1)

soil organic N 3 (sd=1) 1 (sd=1)

livestock manure

16 (sd=1) 1 (sd=1)

3 possible sources

2 isotopic signatures measured in sources and groundwater

Problem 1: Sources of nitrate in ground water

Problem 1: Sources of nitrate in ground water

Problem 1: Sources of nitrate in ground water

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.

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

Keeling plotsKeeling (1958)

Cmix = Cbg + Csource

mixCmix = bgCbg + sourceCsource

mixCmix = bgCbg + source (Cmix – Cbg)

mixCmix = (bg - source)Cbg + source Cmix

mix = (bg - source)Cbg / Cmix+ source

mix = (slope)*(1/Cmix) + source

Keeling plot: mix = (slope)*(1/Cmix) + source

Hypothesis:North facing slope has a more negative (lighter) isotopic signature due to:1.Lower water stress2.Lower temperature

Isotopic signature soil respiration

Isotopic signature soil respiration

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

Problem 3: Contributions of C3 and C4 to ecosystem respiration

Miranda et al. 1997

Sampling height (m)

13C of canopy

air

canopy air [CO2]

Species 13C veg

15-7.9 351

Andropogon spp.

-12.9

13.5-8.5 370

Syagrus comosa

-28.8

9

-9.6 410

Annona crassiflora

-30.2

5

-10.3 430

Melinis minutiflora

-13.4

1-10.9 450

Palicourea rigida

-26.1

Problem 3: Contributions of C3 and C4 to ecosystem respiration

Miranda et al. 1997

Sampling height (m)

13C of canopy

air

canopy air [CO2]

Species 13C veg

15-7.9 351

Andropogon spp.

-12.9

13.5-8.5 370

Syagrus comosa

-28.8

9

-9.6 410

Annona crassiflora

-30.2

5

-10.3 430

Melinis minutiflora

-13.4

1-10.9 450

Palicourea rigida

-26.1

Problem 3: Contributions of C3 and C4 to ecosystem respiration

Miranda et al. 1997

Sampling height (m)

13C of canopy

air

canopy air [CO2]

Species 13C veg

15-7.9 351

Andropogon spp.

-12.9

13.5-8.5 370

Syagrus comosa

-28.8

9

-9.6 410

Annona crassiflora

-30.2

5

-10.3 430

Melinis minutiflora

-13.4

1-10.9 450

Palicourea rigida

-26.1

13C veg sd

C3 plants -28.3667 2.084067

C4 plants -13.15 0.353553

Ecosystem respiration

-21.4107 0.977209

Two source mixing model:54 +/- 7 % of ecosystem respiration stems from C3 sources.

On to radioactive isotopes …