The Ocean Nitrogen Cycle

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Lecture 18 The Ocean Nitrogen Cycle
Denitrification Reactions
Distributions Nitrogen Fixation Reactions
Distributions
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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 balance VUCD ? has
changed sink gt source) B ?
The nutrient concentration of the deep ocean
will adjust so that the fraction of B preserved
in the sediments equals river input!
CS
CD
if VMIX m y-1 and C mol m-3 flux mol m-2 y-1
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Nitrogen species
NO3- NO2- N2O N2 NH4 (V) (III)
(I) (0) (-III)
Nitrogen Isotopes
14N 99.634 15N 0.366
Isotopic Composition

The standard is atmospheric N2
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Fractionation factors
, where e is the isotopic enrichment factor
Fractionation Heavier 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)
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Schematic of Ocean Nitrogen Cycle
Gruber (2005) Nature 436, 786
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Main Ocean Source of N Nitrogen Fixation Enzyme
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
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Main Ocean Sink of N Fixed Nitrogen (NO3-, NO2-,
NH4) is converted to N2 in low oxygen zones of
the ocean Two Pathways Denitrification ( lt2 to
10 mM O2) 2 NO3- organic matter ?
N2 Anammox (lt2 mM O2) NH4 NO2- ?
N2 H2O
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What is N?
The solid line shows the linear equation P 1/16
N 0.345 (equivalent to N 0) Values to the
right have positive N, to the left have
negative N
N is defined as N NO3 16 x PO4 2.9
PO4 versus Nitrate (GEOSECS data) Insert shows
the effect of nitrification, photosynthesis, N2
fixation and denitrification.
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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)
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Nitrogen Cycle w/ anammox
Kuypers (2003) Nature 422 608-611.
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N is defined as N NO3 16 x PO4 2.9
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Global distribution of O2 at the depth of the
oxygen minimum Gruber and Sarmiento, 1997
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N at 200m in the Pacific (Gruber and Sarmiento,
1997)
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Spatial coupling of N2 fixation and
denitrification (Deutsch et al, 2007)
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Coupling of N sources and sinks (Deutsch et al,
2007, Nature, 445, 163)
Capone and Knapp (2007) Nature, 445, 159
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The Global Nitrogen Budget-one example (Brandes
et al, 2002)
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Downcore records of 15N-orgN from several
sites High values of 15N-OrgN suggest more
extensive denitrification
Deutsch et al, 2004)
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Deutsch et al, 2004