Lecture 18 The Ocean Nitrogen Cycle Denitrification Reactions Distributions Nitrogen Fixation...
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Transcript of Lecture 18 The Ocean Nitrogen Cycle Denitrification Reactions Distributions Nitrogen Fixation...
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