Tracer Approaches

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Tracer Approaches

• Natural Abundance
• Integrative over large time and space scales
• Requires much inference creative minds
• Tracer-
• Direct approach
• Demonstrate process on short time scale
• Perturbation of system potential artifact
• e.g. bottle effects, enzyme kinetics

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N Tracers

• Radioisotopes
• 13N - positron decay, high specific activity
• 10 min half-life
• Proton bombardment of H20 cyclotron
• Limited use
• Stable isotopes
• 15N available in enriched (and depleted)
  forms
• Emission or mass spectrometry
• Used widely in ecology, oceanography

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Tracer Terminology

• Atom Percent Excess APE, AE
• Measured ratio natural abundance

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Two Fundamental Approaches
Direct Tracer uptake of NH4
Isotope Dilution (NH4 regeneration)
Spike
15NH4 14NH4
PO15N
PON (0.366 atm )
Enriched (5-25)
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Calculations direct uptake

• v (t-1) (atomexcessproduct)tn
• 
  atomexsubstrate tn
• fractional uptake/ unit time
• ? (umol N(L h)-1 v PN (umol NL-1)
• volume-normalized rate of mass uptake

N uptake (umol/t) AEproduct x
PNsample(umol) AEsubstrate
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N Regeneration

• Ammonification DON/PON -gt NH4
• hydrolytic deamination
• Amino Acid oxidation aa -gt NH4
• recent, cell surface enzymes
• Protein, peptide hydrolysis -gt aas
• Redfield Model
• Stoichiometry of regeneration
• CN demand vs. CN of regeneration availability
• Bacterial/ Algal CN demand lt 101

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Isotope Dilution Model

• 1. Measure change in concentration
• dNH4 NH4t NH40 P- I
  Production - Incorp
• dt t (Net flux)
• or
• NH4t NH40 (P-I)t
  Slope (P-I)

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2 Measure change in isotopic ratio of
pool Where R 15N ratio _at_ t0 ( 0.1), 15n
0.00366 d15NH4 P(15n) IR dt d14NH4
P(1-15n) I(1-R) dt dR (15n-R)P dt
NH40 ln (Rt 15n) ln (Ro 15n)
P/(P-I) ln NH4_at_t Slope
P/(P-I) NH4_at_0
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Nitrification
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Methods
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Spike
Direct Tracer uptake of NO3
Isotope Dilution (Nitrification NH4 ox)
15NO3 14NO3
PO15N
NH4 (0.366 atm )
Enriched (5-25)
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Nitrification Rates
Oceanic nitrification should roughly balance new
production based on NO3- Range 500 (Eppley)-
2500 (Falkowski) Tg
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Nitrification (cont)
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Denitrification, Nitrate Reduction (dissimilatory)
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Denitrification (cont)
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Methods

• Direct N2 gas flux - (difficult,N2 contamination)
• C2H2 blockage method
• C2H2
• NO3 -gt NO2 -gt NO -gt N2O --gt N2
• Direct tracer 15NO3 or 15NO2 --gt 15N2
• Natural abundance ?15N of residual NO3 increased
• N2/ Ar ratios
• Immunofluorescence
• Gene probes

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Rates of Denitrification
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Nitrification-Denitrification Coupling

• Sediment denitrification- two sources
• NO3 diffusion in
• In situ nitrification (i.e. NO3 production)
• NH4 ---gt NO3 -gt -gt N2
• Isotope Pairing (Nielsen et al.,
• Spike with 15NO3, measure 28N, 29N, 30N

Exogenous
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Calculations

• D15 (14N15N) 2(15N15N)
• D14 (14N15N) D15
• 2(15N15N)
• Dt D14 D15
• Dw D15/ e where e 15N labelled fraction
  of effluent NO3
• Dn (D14 D15) - Dw

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MIMS- N2/Ar ratios

• Changes in N2 relative to Ar
• Traditionally w/ GC
• High precision with mass spec
• Quadrupole or isotope ratio mass spec