1. Tracing Atmospheric Nitrate Deposition in a Complex Semiarid Ecosystem Using  17 O by Michalski, G., Meixner, T., Fenn, M., Hernandez, L., Sirulnik, A., Allen, E., and Thiemens, M.

2. Ranges of  15 N and  18 O values of different sources of NO 3 (from Kendall et al., 1998)

3. Background-mass dependent vs. mass independent fractionation Mass dependent fractionation that can alter  18 O values will not affect the value of  17 O. Mass dependent fractionation that can alter  18 O values will not affect the value of  17 O. Photochemical systems do not always follow the mass-dependent relationship for d17O Photochemical systems do not always follow the mass-dependent relationship for d17O  17 O signal is generated during the formation of ozone.  17 O signal is generated during the formation of ozone.  17 O is NOT affected by terrestrial fractionating processes (ie. nitrate transformation)  17 O is NOT affected by terrestrial fractionating processes (ie. nitrate transformation)  17 O more sensitive tracer for atmospheric nitrate than  18 O measurements!!!  17 O more sensitive tracer for atmospheric nitrate than  18 O measurements!!!

4. Relationship between  18 O and  17 O values. Modified from Michalski et al., 2002.

5. Mass Independent Fractionation   17 O =  17 O -0.52 *  18 O Nitrate with  17 O > 0 o / oo is atmospheric  17 O up to 31 permil for aerosols and rainwater (Michalski and Thiemens, 2000)  17 O = 0 for fertilizer and bacterial nitrate

6. Objectives of the study by Michalski and others (2004) Use  17 O to quantify atmospheric nitrate in soils and streams compared with  18 O techniques. Use results for process level understanding of fate and transport of nitrogen in the soil and catchment.

7. Study Area: Southern California N Dep = 35-45 kg/ha/yr N N Dep = 5 kg/ha/yr N

8. Oxygen Isotopic Composition of Stream and Soil NO 3. (From Michalski et al., 2004)

9. Oxygen three isotope plot of nitrate (from Michalski et al., 2004)

10. NO 3 from stream (o) and soil ( ) samples and  18 O and mass balance mixing lines between atmospheric NO 3 and nitrification NO 3 (from Michalski et al., 2004).

11. Mass balance estimates of the % of atmospheric NO 3 in terrestrial NO 3 (from Michalski et al., 2004) (from Michalski et al., 2004)

12. Streamflow hydrograph during Nov and Mar in Devil Canyon Watershed. (Michalski et al., 2004)

13. Conclusions  17 O can be used as a conserved tracer  17 O can be used as a conserved tracer 4-40% unassimilated atmospheric nitrate in runoff (all sites) and 20-40% (Devils Canyon) 4-40% unassimilated atmospheric nitrate in runoff (all sites) and 20-40% (Devils Canyon) More robust indicator than  18 O More robust indicator than  18 O Previous work may be underestimating atmospheric nitrate Previous work may be underestimating atmospheric nitrate Powerful new technique to augment the dual isotope technique Powerful new technique to augment the dual isotope technique

14. Future Work with  17 O Kendall et al., in progress Kendall et al., in progress Analyzing  17 O,  18 O and  15 N of nitrate from NADP for source identification. Analyzing  17 O,  18 O and  15 N of nitrate from NADP for source identification. Nanus et al., in progress Nanus et al., in progress Analyzing  17 O,  18 O and  15 N of nitrate Analyzing  17 O,  18 O and  15 N of nitrate in alpine lakes in 5 Rocky Mountain National Parks. Compare to Kendall et al.