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Understanding Isotope Signatures in Groundwater Nitrate and Soil Respiration Patterns

This study delves into the sources of elevated nitrate levels in groundwater, identifying three key sources and their isotopic signatures. It explores the isotopic composition of soil respiration in a 40-year-old Douglas-fir stand in Central Oregon's Cascade Mountains, comparing north and south slopes and assessing variations at different elevations. Additionally, it addresses the contributions of C3 and C4 plants to ecosystem respiration, utilizing a two-source mixing model to quantify their respective roles. The insights gained enhance our understanding of nutrient cycling and environmental impacts.

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Understanding Isotope Signatures in Groundwater Nitrate and Soil Respiration Patterns

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Presentation Transcript

  1. What we learned about isotopes Elizabeth Sulzman

  2. Problem 1: Sources of nitrate in ground water The ground water in your area exceeds the EPA standard of 10 ppm of nitrate. What nitrate sources are responsible? 3 possible sources 2 isotopic signatures measured in sources and groundwater

  3. Problem 1: Sources of nitrate in ground water

  4. Problem 1: Sources of nitrate in ground water

  5. 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) Cmix= Cbg+ Csource dmixCmix= dbgCbg+ dsourceCsource dmixCmix = dbgCbg + dsource (Cmix – Cbg) dmixCmix = (dbg- dsource)Cbg + dsourceCmix dmix = (dbg- dsource)Cbg / Cmix+ dsource dmix= (slope)*(1/Cmix)+ dsource

  9. Keeling plot: dmix = (slope)*(1/Cmix) + dsource Hypothesis: North facing slope has a more negative (lighter) isotopic signature due to: Lower water stress Lower temperature

  10. Isotopic signature soil respiration

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

  15. On to radioactive isotopes …

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