Download 18-Nitrogen

N cycling in the world’s
oceans
Nitrogen

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N is an essential nutrient for all living
organisms (nucleic acids and amino
acids)
N has many oxidation states, which
makes speciation and redox chemistry
very interesting
NH4+ is preferred N nutrient
Marine N
Libes, 1992
Bioavailable/Fixed (oxidation state)
NO3- 5.7*105 Tg N (+5)
NO2- 500 Tg N (+3)
NH4+ 7.0*103 Tg N (-3)
Organic N 5.3*105 Tg N (-3)
Non-bioavailable
N2O 200 Tg N (+1)
N2 2.2*107 Tg N (0)
Marine Fixed N Budget
Hypothetical Fixed N Evolution
Codispoti et al. (2001)
Fixed N (Tg)
8.E+05
Marine Reservoir: 6.3*105 Tg N
6.E+05
Sources: 287 Tg N/yr
4.E+05
2.E+05
Sinks: 482 Tg N/yr
0.E+00
-2.E+05
0
500
1000
1500
2000
2500
3000
3500
Time (years)
Atmospheric
deposition: 86
Tg N/yr
N2O
loss: 6
Tg N/yr
N2 fixation:
125 Tg N/yr Water Column
denitrification:
150 Tg N/yr
River
Input: 76
Tg N/yr
Organic
N export:
1 Tg N/yr
Sedimentation:
25 Tg N/yr
Benthic
denitrification:
300 Tg N/yr
Fixation
N2
Nitrification
NH4
NO3
Uptake
Phytoplankton
Grazing
Chlorophyll
Zooplankton
Mortality
Water column
Mineralization
Susp.
particles
Large
detritus
Nitrification
N2
NH4
NO3
Denitrification
Sediment
Organic matter
Aerobic mineralization
Nitrogen Cycle
http://www.petsforum.com/personal/trevor-jones/nitrogencycle.gif
Organic Matter Oxidation
Sequence
Morel & Herring, 1993
Respiration
1
4
CH 2O  O2  CO2  H 2O
1
4
1
4
1
4
Denitrification


1
1
1
1
1
1
4 CH 2O  5 NO3  5 H  4 CO2  10 N 2  2 H 2O
MnO2 reduction

2
1
1
1
1
CH
O

MnO

H

CO

Mn
 34 H 2O
2
2
2
4
8
4
2
Fe oxide reduction

2
1
CH
O

Fe
(
OH
)

2
H

Fe
 14 CO2  114 H 2O
2
3
4
Sulfate reduction
2


1
1
1
1
1
1
4 CH 2O  8 SO4  8 H  8 HS  4 CO2  4 H 2 O
Methanogenesis
1
4
CH 2O  18 CH 4  18 CO2
ΔG° (kJ/mol)
-119
-113
-96.9
-46.7
-20.5
-17.7
Alternative pathways to N2
Microbially mediated
Nitrification
Anammox
NH 4  2O 2  NO3  H 2 O  2H 
Heterotrophic Denitrification
5CH 2 O  4NO 3  4H   2N 2  5CO 2  7H 2 O
NH 4  NO 2  N 2  2H 2 O
OLAND
NH 4  34 O 2  12 N 2  32 H 2 O  H 
Nitrogen Fixation
N 2  5H 2 O  2NH 4  2OH   32 O 2
Chemical Reactions
MnO2 Reduction
3MnO 2  2NH 4  4H   3Mn2  N 2  6H 2 O
4MnO 2  NH 4  6H   4Mn 2  NO3  5H 2 O
Mn2+ Oxidation
5Mn2  2NO3  4H 2 O  5MnO 2(solid)  N 2  8H 
Marine Fixed N Budget Unbalanced
WHY??????????????????????
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N Fixation may have been underestimated
Limited data on Trichodesmium and other N fixers;
variability in abundances and fixation rates of
organisms
Recent estimates of N fixation rates have increased
(Gruber and Sarmiento, 1997; Karl et al., 1997)
Denitrification may have been overestimated
Stoichiometric and model-based estimates used;
limited data on direct denitrification measurements
My research

Denitrification describes the removal of fixed
N, mostly NO3-, resulting in the formation of
non-biologically available N, primarily N2 gas

Continental shelf sediments are responsible
for up to 67% of marine denitrification
estimates
Sandy sediments comprise 70% of continental
shelves; global estimates of denitrification are
mostly based on muddy sediments
Sands contain less organic matter and
nutrients, and high oxygen concentrations in
overlying water
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Benthic primary production
(BPP)
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Sandy sediments have low organic matter
content, substrate for heterotrophic
denitrification
BPP supplies reactive organic matter through
remineralization
Organisms compete with microbes for
nutrients such as NH4+
Organisms also produce oxygen during
photosynthesis
Role of BPP remains unclear
Isotope tracer experiments
15
15
A. Experiment 1
14N14N
POM
1E
1A
15NH +,
4
14NO 3
ation
NO 3  14,15 NO 3 denitrific

 29,30 N 2
ion
ation
NH 4 nitrificat
 
15 NO 3  14,15 NO 3 denitrific

 29,30 N 2
15NO 3
1B
14N15N,
15N15N
1C
1F
1D
15N15N
14N15N
B. Experiment 2
15N15N
2E
15NO -,
3
14NH +
4
2A
14NO 3
POM
2D
14N14N
14N15N
14N15N,
14N14N
2C
2F
2B
Possible outcomes of amendment
experiments. 1A = Aerobic nitrification
of 15NH4+; 1B = Heterotrophic
denitrification with 14NO3- and/or
15NO3-; 1C = OLAND with 15NH4+ or
partial nitrate reduction to nitrite
followed by anammox with 15NH4+;
1D = Same as 1C except with standard
nitrate; 1E = Heterotrophic
denitrification with standard nitrate; 1F
= Assimilation. 2A = Aerobic
nitrification of standard ammonium; 2B
= Heterotrophic denitrification with
14NO3- and/or 15NO3-; 2C = OLAND
with standard ammonium or partial
nitrate reduction to nitrite followed by
anammox with standard ammonium;
2D = Same as 2C except with 15NO3-;
2E = Heterotrophic denitrification of
15NO3-; 1F = Assimilation
Sampling
Sampling
Membrane Inlet Mass Spec.
(MIMS)
Results
xp
er
im
R
en
4t1
Ex
-C
pe
or
ri m
e1
R
en
4Ex
t1
-C
pe
or
ri m
e2
R
en
4Ex
t2
-C
pe
or
ri m
e3
en
t2
R
-C
4or
C
e4
on
tro
l-C
R
4or
C
e5
on
tro
W
27
l-C
-A
or
m
e6
en
de
d
C
or
W
e
27
-C
on
tro
l
4E
R

W27 and Experiment 2
results suggest the
presence of
denitrification
Experiment 1 results
suggest that within the
48-hr timescale of the
experiment, no
alternative pathway to
N2 exists in these
sediments
mM N2 Produced

3
2.5
2
1.5
1
0.5
0
29N2
30N2
Denitrification Rates
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W27 Experiment provided a rate of 21.6 µmole
N m-2 d-1
R4-Experiment 2 provided rates of 22.8 & 23.2
µmole N m-2 d-1
Rates obtained from other continental shelf
studies of denitrification yielded 700-3200
µmole N m-2 d-1
Other continental shelf sites studied contain
higher organic matter content than Georgia
sediments
Georgia continental shelf sediments are oxic to
at least 1-cm depth, thus inhibiting higher rates
of denitrification
Discussion of results
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Sandy, continental shelf sediments are
potentially important sites of denitrification that
may have been overlooked
These environments may have similar rates to
current study site and if so, similar techniques
can be used to measure such low rates of
denitrification
Denitrification was not completely inhibited by
low organic matter content or benthic primary
production
BPP varies seasonally and spatially, yet
denitrification rates were very close between
two different stations during different seasons
Future work
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Impact of BPP can be explored further by
monitoring nutrient and dissolved O2
concentrations and benthic primary
production rates (monitored by SABSOON)
Compare rates to Gulf of Mexico shelf
denitrification rates (Nov. – Dec. 2004)
Further explore the presence of alternative
pathways in salt marsh sediments by using
isotope tracers, 15N isotopic analyses, and
HgCl2 (Oct. – Nov. 2004)
Future work (cont’d)
NO3 + Hg Samples
400
400
350
350
300
250
NO3
200
NO2
150
NH4
100
Concentration (uM)
Concentration (uM)
NO3 Samples
300
250
NO3
200
NO2
150
NH4
100
50
50
0
0
0
10
20
30
40
0
50
10
20
40
50
Time (Days)
Time (Days)
NO3 Samples
NO3 + Hg Samples
7
7
6
6
5
N29
4
N30
3
2
1
Concentration (uM-N)
Concentration (uM-N)
30
5
N29
4
N30
3
2
1
0
0
0
10
20
30
Time (Days)
40
50
0
10
20
30
Time (Days)
40
50