Lecture 18 The Ocean Nitrogen Cycle

Denitrification Reactions Distributions

Nitrogen Fixation Reactions Distributions

Why is this important for chemical oceanography?What controls ocean C, N, P?g ≈ 1.0

Mass Balance for whole ocean:C/ t = VRCR – f B

CS = 0; CD = CD

VU = VD = VMIX

Negative Feedback Control:if VMIX ↑VUCD ↑B ↑f B ↑ (assumes f will be constant!)assume VRCR then CD ↓ (because total ocean balanceVUCD ↓ has changed; sink > source)B ↓

CS

CD

if VMIX = m y-1 and C = mol m-3

flux = mol m-2 y-1

The nutrient concentration of the deep ocean will adjust so thatthe fraction of B preserved in thesediments equals river input!

Nitrogen species:NO3

- ; NO2- ; N2O; N2 ; NH4

+

(V) (III) (I) (0) (-III)

Nitrogen Isotopes:14N 99.634%15N 0.366%

Isotopic Composition:15 15

14 14 tan15 315

14 tan

( ) ( )10

( )[ ]sample s dard

s dard

N NN NN

NN

‰

The standard is atmospheric N2

Fractionation factors , where is the isotopic enrichment factor

1000 1

FractionationHeavier stable isotope forms stronger bond.

Microbial Enzymes break light isotope bonds more easily.

Reactants become heavier (enriched) (e.g. NO3- → N2)

Products become lighter (depleted)

Partial versus total reaction (products have same values as reactants)

The Global Nitrogen Budget-one example(Brandes et al, 2002)

Main Ocean Source of N

Nitrogen FixationEnzyme catalyzed reduction of N2

N2 + 8H+ + 8e- + 16 ATP → 2NH3 + H2 + 16 ADP + 16Pi

Mediated by a two protein (Fe and Fe-Mo) complex called nitrogenase

Inactivated when exposed to O2

Main Ocean Sink of N

Fixed Nitrogen (NO3-, NO2

-, NH4+) is

converted to N2 in low oxygen zones of the ocean

Two Pathways

Denitrification ( <2 to 10 M O2):

2 NO3- + organic matter → N2

Anammox (<2 M O2) NH4

+ + NO2- → N2 + H2O

Schematic of Ocean Nitrogen Cycle

Gruber (2005) Nature 436, 786

% of Export Production (as N) at HOT derived from N2 Fixation

(N-P mass balance model of Karl et al (1997) Nature 388, p. 533)

Coupling of N sources and sinks (Deutsch et al, 2007, Nature, 445, 163)

Capone and Knapp (2007) Nature, 445, 159

N* is defined asN* = [NO3] – 16 x [PO4] +2.9

Global distribution of O2 at the depth of the oxygen minimumGruber and Sarmiento, 1997

N* at 200m in the Pacific (Gruber and Sarmiento, 1997)

Kuypers (2003) Nature 422: 608-611.

Nitrogen Cycle w/ anammox

PO4 versus Nitrate (GEOSECS data)Insert shows the effect of nitrification, photosynthesis, N2 fixationand denitrification.

The solid line shows thelinear equationP = 1/16 N + 0.345(equivalent to N* = 0)

Values to the right havepositive N*, to the left have negative N*

What is N*?

N* is defined asN* = [NO3] – 16 x [PO4] +2.9

The Global Nitrogen Budget-one example(Brandes et al, 2002)

Spatial coupling of N2 fixation and denitrification (Deutsch et al, 2007)

Deutsch et al, 2004)

Downcore records of 15N-orgN from several sitesHigh values of 15N-OrgN suggest more extensive denitrification

Deutsch et al, 2004