Nitrogen Cycle
• Most of Nitrogen is in
the atmosphere.
• 14N = 99.6% 15N = 0.4%
• Air is standard for d15N
• Range is –20 to +20 ‰
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Assimilation and Dissimilation Nitrogen
Cycling Pathways
Assimilation
NO3 and NH4 uptake (NO3 or
NH4  org N)
N2 Fixation (N2  org N)
Dissimilation
-Mineralization (org N  NH4)
-Nitrification (NH4  NO3)
-Denitrification (NO3  N2)
-Anammox (NO2 + NH4  N2)
(in marine systems)
2
d15N Range in Terrestrial Plants
Non N-fixers = -6 to +6 ‰ N-Fixers = 0±3 ‰
3
KIE during Nitrate Assimilation by
Terrestrial Plants
• Large range of KIE values measured in culture
experiments
• Range: - 3 to –24 ‰ (Mariotti, 1980)
-3 to –7 ‰ (Kohl and Shearer, 1980)
-0.3±0.6 ‰ (38 species) (Mariotti, 1982)
• Measured KIE depends significantly on culture
conditions (growth rate, NO3 concentration)
• Typically, KIE increased with increased nitrate
concentration.
4
N2 Fixation in Plants
• Plants have d15N = 0±3 ‰ similar to d15N of N2 in air.
-molecular diffusion of N2 is air (soil pockets) yields KIE =
-9 ‰. N2 diffusion in water has KIE = -1.3 ‰
-also you would expect a significant KIE during the
breaking of a N-N bond to convert to NH4
-this seems to imply that aqueous N2 diffusion limits N2
fixation. What other explanation?
• Haber Process (N2 + 3H2  2NH3) yields commercial
fertilizers with d15N of ~ 0±3 ‰
(high temp (>300ºC) and complete conversionlittle KIE)
5
KIE during NO3
and NH4 Uptake
in Plankton
Cultures
KIE ranged from 0 to
–15 ‰ with a mean
of ~ –7 ‰
6
Effect of growth rate of KIE in cultures
NO3 Substrate
NH4 Substrate
+
A
A
As growth rate increased, KIE decreased. High concentrations
(mmolar) of NO3 and NH4.
7
KIE during
N2, NO3 and
NH4 uptake
(same species
Anabaena)
KIEs
N2 = -2 ‰
NO3 = -11 to –13 ‰
NH4 = -14 ‰
8
KIE during NO3 assimilation under closer
to ambient concentrations (uM vs mM)
How useful are culture derived KIEs?
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d15N variations in marine plankton
Three Ocean Regimes
NO3 replete (I)
NO3 limited (II)
N2 fixation (III)
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d15N of Plankton (PON) in Mixed Layer
depends on NO3 Concentration
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Effect of KIE on d15N of NO3
Southern Ocean
Sta Papa
KIE during nitrate assimilation enriches d15N of remaining NO3 pool
Both regions yields a KIE = -9.1 ‰. What is advantage of this approach?
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d15N of NH4 in
Delaware
Estuary
KIE for NH4
uptake = -20±? ‰
(Rayleigh)
-NH4 uptake
-nitrification
-sewage NH4 input
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Summary of
KIE during N
Assimilation
How useful are
culture KIE
determinations?
14
Steps in nitrate assimilation
~15 ‰ offset between internal and external NO3 pools
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d15N based N Budget at ALOHA and BATS
ALOHA near Hawaii
BATS near Bermuda
At ALOHA, N2 fixation contributes 50% of new nitrogen.
At BATS, N2 fixation is a negligible N source.
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Assimilation and Dissimilation Nitrogen
Cycling Pathways
Assimilation
NO3 and NH4 uptake (NO3 or
NH4  org N)
N2 Fixation (N2  org N)
Dissimilation
-Mineralization (org N  NH4)
-Nitrification (NH4  NO3)
-Denitrification (NO3  N2)
-Anammox (NO2 + NH4  N2)
(in marine systems)
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d15N of OM
trends in soils
Are these trends a
result of KIE
during
remineralization?
d15N vs %N
d15N vs C/N
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d15N of NO3 and NH4 released during a Soil
Incubation Experiment
The NO3 produced during
OM degradation is 15N
depleted, whereas the NH4
is enriched (initially)
relative to OM.
NO3
NH4
Complicated interpretation
because there is OM
mineralization, nitrification
d15N
and denitrification
occurring simultaneously,
all of which likely have
different KIEs.
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d15N of Organic N in Sediment Traps
N. Atlantic Bloom Conditions
Equatorial Pacific
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Denitrification
• Converts nitrate (NO3)  Nitrite (NO2) Nitrous
Oxide (N2O)  Nitrogen gas (N2)
• Major NO3 loss pathway in the ocean.
• Anammox is another pathway to convert combined N
to Nitrogen gas (NH4 + NO2  N2 + 2H2O)
• Denitrification occurs under low Oxygen
concentrations (e.g., sediments, soils, certain ocean
regions (OMZs), etc.)
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Denitrification
• If none of the NO2 or N2O produced via
denitrification is removed, diluted with other
sources or affected by reactions other than
denitrification (e.g., anammox), then the 15N/14N
of the N2 produced during denitrification would
equal the 15N/14N of nitrate times the KIE for NO3
reduction to NO2.
(15N/14N )NO3 *NO3-NO2 = (15N/14N )NO2 *NO2-N2O
(15N/14N )NO2 *NO2-N2O = (15N/14N )N2O *N2O-N2
(15N/14N )N2O *N2O-N2 = (15N/14N )N2
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Culture Measurements of KIE for Denitrification
Typically, the KIE for denitrification is measured by
the measuring the 15N/14N trend of nitrate pool
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Summary of KIE for Denitrification in
Cultures, Soils and Ocean
(Granger, Limnol&Oceanogr, 2008)
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Depth Profiles of
NO3 deficit, d15N of
NO3 and N2 in
ETNP and Arabian
Sea
Similar depth for:
- Nitrate deficit max.
- d15N NO3 max.
- d15N N2 min.
25
Estimate of KIE for Denitrification
in ETNP and AS: field data
• = ETNP
•
= AS
Closed System: KIE
= -25 ‰ (ETNP) and
–22 ‰ (AS)
Open System: KIE = -30±3 ‰
(ETNP) and –25±5 to –28±4 ‰ (AS)
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KIE during Nitrification in Cultures
Previous lab cultures = -18 to –40 ‰ (Mariotti 1981)
Field Study (Chesapeake Bay) = -13 to –16 ‰ (Horrigan et al., 1990)
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Nitrogen Cycling in Sediments
Ocean Water
Sediments
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Ocean’s Combined Nitrogen Budget
Large uncertainties in the rates of N2 fixation and denitrification.29
Combined N and 15N Budgets
ΔN/Δt = River + Atm + Fix – Denitr – Burial
Δ15N/Δt = River*(15N/14N)riv + Atm*(15N/14N)atm +
Fix*(15N/14N)fix – Denitr(15N/14N)dentr –
Burial*(15N/14N)orgN
• Estimate three of the rates: river, atmos and burial
• Estimate the 15N/14N of all the terms
• Solve for N2 fixation and denitrification rates.
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KIE for denitrification
One significant complication
is that the KIE assigned for
denitrification depends
significantly on the proportion
of denitrification that occurs in
the water column (KIE~ -20
‰) versus in the sediments
(KIE ~ -1 ‰).
Water
column
sediments
Thus the solution to the N and
15N budget depends on the
proportion of sedimentary vs
water column denitrification.
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Combined N and 15N Budgets
Results: The only way to get an isotopic balance is to have very high
rates of denitrification and N2 fixation.
Sedimentary Denitrification Rate = 200 - 280 Tg/yr
N2 Fixation Rate = 100 - 330 Tg/yr (direct meas. << 100Tg/yr)
32
-substantial uncertainty in budget approach.
d15N of Organic N in Ocean Sediments
Correlation between
changes in d15N of
organic N in sediments
in the Arabian Sea
with ice core d18O over
last few glacial cycles.
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Correlation between
marine d15N and
atmospheric pCO2
changes
Is there a link between
denitrification
controlling NO3
concentrations in the
ocean and biological
drawdown of
atmospheric CO2?
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