1

Nitrogen Cycle

• Most of Nitrogen is in the atmosphere.

• 14N = 99.6% 15N = 0.4%• Air is standard for 15N • Range is –20 to +20 ‰

2

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)

3

15N Range in Terrestrial Plants

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

4

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.

5

N2 Fixation in Plants

• Plants have 15N = 0±3 ‰ similar to 15N 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 15N of ~ 0±3 ‰

(high temp (>300ºC) and complete conversionlittle KIE)

6

KIE during NO3 and NH4 Uptake

in Plankton Cultures

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

7

Effect of growth rate of KIE in cultures

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

NO3 SubstrateNO3 Substrate

+

NH4 Substrate

A

A

8

KIE during N2, NO3 and NH4 uptake

(same species Anabaena)

KIEs

N2 = -2 ‰

NO3 = -11 to –13 ‰

NH4 = -14 ‰

9

KIE during NO3 assimilation under closer to ambient concentrations (uM vs mM)

How useful are culture derived KIEs?

10

15N variations in marine plankton

Three Ocean Regimes

NO3 replete (I)

NO3 limited (II)

N2 fixation (III)

11

15N of Plankton (PON) in Mixed Layer depends on NO3 Concentration

12

Effect of KIE on 15N of NO3

KIE during nitrate assimilation enriches 15N of remaining NO3 pool

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

Southern Ocean Sta Papa

13

15N of NH4 in Delaware Estuary

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

-NH4 uptake

-nitrification

-sewage NH4 input

14

Summary of KIE during N Assimilation

How useful are culture KIE determinations?

15

Steps in nitrate assimilation

~15 ‰ offset between internal and external NO3 pools

16

15N based N Budget at ALOHA and BATS

At ALOHA, N2 fixation contributes 50% of new nitrogen.

At BATS, N2 fixation is a negligible N source.

ALOHA near Hawaii BATS near Bermuda

17

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)

18

15N of OM trends in soils

15N vs %N

15N vs C/N

Are these trends a result of KIE during remineralization?

19

15N of NO3 and NH4 released during a Soil Incubation Experiment

NO3

NH4

15N

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.

20

15N of Organic N in Sediment Traps

N. Atlantic Bloom Conditions Equatorial Pacific

21

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.)

22

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

23

Culture Measurements of KIE for Denitrification

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

24

Summary of KIE for Denitrification in Cultures, Soils and Ocean

(Granger, Limnol&Oceanogr, 2008)

25

Depth Profiles of NO3 deficit, 15N of

NO3 and N2 in ETNP and Arabian

Sea

Similar depth for:

- Nitrate deficit max.

- 15N NO3 max.

- 15N N2 min.

26

Estimate of KIE for Denitrification in ETNP and AS: field data

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

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

• = ETNP

= AS�

27

KIE during Nitrification in Cultures

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

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

28

Nitrogen Cycling in Sediments

Ocean Water

Sediments

29

Ocean’s Combined Nitrogen Budget

Large uncertainties in the rates of N2 fixation and denitrification.

30

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.

31

KIE for denitrificationOne 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.

sediments

Water column

32

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.

33

15N of Organic N in Ocean Sediments

Correlation between changes in 15N of organic N in sediments in the Arabian Sea with ice core 18O over last few glacial cycles.

34

Correlation between marine 15N and

atmospheric pCO2 changes

Is there a link between denitrification controlling NO3 concentrations in the ocean and biological drawdown of atmospheric CO2?