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 …