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What we learned about isotopes
Elizabeth Sulzman
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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
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Problem 1: Sources of nitrate in ground water
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Problem 1: Sources of nitrate in ground water
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Problem 1: Sources of nitrate in ground water
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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.
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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
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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
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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
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Isotopic signature soil respiration
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Isotopic signature soil respiration
Difference in isotopic signatures between north and south facing slope not large enough ?
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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
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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
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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.
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On to radioactive isotopes …