Tracing Atmospheric Nitrate
Deposition in a Complex Semiarid
Ecosystem Using D17O
by Michalski, G., Meixner, T., Fenn, M.,
Hernandez, L., Sirulnik, A.,
Allen, E., and Thiemens, M.
Ranges of d15N and d18O values of different sources of
NO3 (from Kendall et al., 1998)
Background-mass dependent vs.
mass independent fractionation
• Mass dependent fractionation that can alter d18O
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values will not affect the value of D17O.
Photochemical systems do not always follow the
mass-dependent relationship for d17O
D17O signal is generated during the formation of
ozone.
D17O is NOT affected by terrestrial fractionating
processes (ie. nitrate transformation)
D17O more sensitive tracer for atmospheric
nitrate than d18O measurements!!!
Relationship between d18O and d17O values.
Modified from Michalski et al., 2002.
Mass Independent Fractionation
D17O = d17O -0.52 * d18O
Nitrate with D17O > 0 o/oo is atmospheric
D17O up to 31 permil for aerosols and
rainwater (Michalski and Thiemens, 2000)
D17O = 0 for fertilizer and bacterial nitrate
Objectives of the study by Michalski
and others (2004)
Use D17O to quantify atmospheric
nitrate in soils and streams compared
with d18O techniques.
Use results for process level
understanding of fate and transport of
nitrogen in the soil and catchment.
N Dep = 35-45 kg/ha/yr N
N Dep = 5 kg/ha/yr N
Study Area: Southern California
Oxygen Isotopic Composition of Stream and Soil
NO3. (From Michalski et al., 2004)
Oxygen three isotope plot of nitrate (from
Michalski et al., 2004)
NO3 from stream (o) and soil ( ) samples and d18O
and mass balance mixing lines between atmospheric
NO3 and nitrification NO3 (from Michalski et al., 2004).
Mass balance estimates of the % of
atmospheric NO3 in terrestrial NO3
(from Michalski et al., 2004)
Streamflow hydrograph during Nov and Mar in
Devil Canyon Watershed. (Michalski et al., 2004)
Conclusions
• D17O can be used as a conserved tracer
• 4-40% unassimilated atmospheric nitrate in
runoff (all sites) and 20-40% (Devils Canyon)
• More robust indicator than d18O
• Previous work may be underestimating
atmospheric nitrate
• Powerful new technique to augment the dual
isotope technique
Future Work with D17O
• Kendall et al., in progress
Analyzing D17O, d18O and d15N of nitrate
from NADP for source identification.
• Nanus et al., in progress
Analyzing D17O, d18O and d15N of nitrate
in alpine lakes in 5 Rocky Mountain
National Parks. Compare to Kendall et al.