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Nitrogen Cycle. Most of Nitrogen is in the atmosphere. 14 N = 99.6% 15 N = 0.4% Air is standard for d 15 N Range is –20 to +20 ‰ . Assimilation and Dissimilation Nitrogen Cycling Pathways. Assimilation NO 3 and NH 4 uptake (NO 3 or NH 4  org N) N 2 Fixation (N 2  org N)

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nitrogen cycle
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 ‰
assimilation and dissimilation nitrogen cycling pathways
Assimilation and Dissimilation Nitrogen Cycling Pathways


NO3 and NH4 uptake (NO3 or NH4 org N)

N2 Fixation (N2 org N)


-Mineralization (org N  NH4)

-Nitrification (NH4 NO3)

-Denitrification (NO3 N2)

-Anammox (NO2 + NH4 N2)

(in marine systems)

d 15 n range in terrestrial plants
d15N Range in Terrestrial Plants

Non N-fixers = -6 to +6 ‰ N-Fixers = 0±3 ‰

kie during nitrate assimilation by terrestrial plants
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.
n 2 fixation in plants
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)

kie during no 3 and nh 4 uptake in plankton cultures
KIE during NO3 and NH4 Uptake in Plankton Cultures

KIE ranged from 0 to –15 ‰ with a mean of ~ –7 ‰

effect of growth rate of kie in cultures
Effect of growth rate of KIE in cultures

NO3 Substrate

NO3 Substrate

NH4 Substrate




As growth rate increased, KIE decreased. High concentrations (mmolar) of NO3 and NH4.

kie during n 2 no3 and nh4 uptake same species anabaena
KIE during N2, NO3 and NH4 uptake (same species Anabaena)


N2 = -2 ‰

NO3 = -11 to –13 ‰

NH4 = -14 ‰

kie during no 3 assimilation under closer to ambient concentrations um vs mm
KIE during NO3 assimilation under closer to ambient concentrations (uM vs mM)

How useful are culture derived KIEs?

d 15 n variations in marine plankton
d15N variations in marine plankton

Three Ocean Regimes

NO3 replete (I)

NO3 limited (II)

N2 fixation (III)

effect of kie on d 15 n of no 3
Effect of KIE on d15N of NO3

Sta Papa

Southern Ocean

KIE during nitrate assimilation enriches d15N of remaining NO3 pool

Both regions yields a KIE = -9.1 ‰. What is advantage of this approach?

d 15 n of nh 4 in delaware estuary
d15N of NH4 in Delaware Estuary

KIE for NH4 uptake = -20±? ‰ (Rayleigh)

-NH4 uptake


-sewage NH4 input

summary of kie during n assimilation
Summary of KIE during N Assimilation

How useful are culture KIE determinations?

steps in nitrate assimilation
Steps in nitrate assimilation

~15 ‰ offset between internal and external NO3 pools

d 15 n based n budget at aloha and bats
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.

assimilation and dissimilation nitrogen cycling pathways17
Assimilation and Dissimilation Nitrogen Cycling Pathways


NO3 and NH4 uptake (NO3 or NH4 org N)

N2 Fixation (N2 org N)


-Mineralization (org N  NH4)

-Nitrification (NH4 NO3)

-Denitrification (NO3 N2)

-Anammox (NO2 + NH4 N2)

(in marine systems)

d 15 n of om trends in soils
d15N of OM trends in soils

d15N vs %N

Are these trends a result of KIE during remineralization?

d15N vs C/N

d 15 n of no 3 and nh 4 released during a soil incubation experiment
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.

Complicated interpretation because there is OM mineralization, nitrification and denitrification occurring simultaneously, all of which likely have different KIEs.




d 15 n of organic n in sediment traps
d15N of Organic N in Sediment Traps

N. Atlantic Bloom Conditions

Equatorial Pacific


• 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.)


• 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

culture measurements of kie for denitrification
Culture Measurements of KIE for Denitrification

Typically, the KIE for denitrification is measured by the measuring the 15N/14N trend of nitrate pool

summary of kie for denitrification in cultures soils and ocean
Summary of KIE for Denitrificationin Cultures, Soils and Ocean

(Granger, Limnol&Oceanogr, 2008)

depth profiles of no 3 deficit d 15 n of no 3 and n 2 in etnp and arabian sea
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.

estimate of kie for denitrification in etnp and as field data
Estimate of KIE for Denitrification in ETNP and AS: field data


 = AS

Open System: KIE = -30±3 ‰ (ETNP) and –25±5 to –28±4 ‰ (AS)

Closed System: KIE = -25 ‰ (ETNP) and –22 ‰ (AS)

kie during nitrification in cultures
KIE during Nitrification in Cultures

Previous lab cultures = -18 to –40 ‰ (Mariotti 1981)

Field Study (Chesapeake Bay) = -13 to –16 ‰ (Horrigan et al., 1990)

nitrogen cycling in sediments
Nitrogen Cycling in Sediments

Ocean Water


ocean s combined nitrogen budget
Ocean’s Combined Nitrogen Budget

Large uncertainties in the rates of N2 fixation and denitrification.

combined n and 15 n budgets
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.

kie for denitrification
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 ‰).

Thus the solution to the N and 15N budget depends on the proportion of sedimentary vs water column denitrification.

Water column


combined n and 15 n budgets32
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)

-substantial uncertainty in budget approach.

d 15 n of organic n in ocean sediments
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.

correlation between marine d 15 n and atmospheric pco 2 changes
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?