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)

-Annamox (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)

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)

14

Summary of KIE during N Assimilation

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