N cycling in the world’s oceans
Nitrogen
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
Non-bioavailableN2O 200 Tg N (+1)
N2 2.2*107 Tg N (0)
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)
Libes, 1992
Marine Reservoir: 6.3*105 Tg N
Sources: 287 Tg N/yr
Sinks: 482 Tg N/yr
Water Column denitrification:
150 Tg N/yr
Atmospheric deposition: 86 Tg N/yr
N2O loss: 6 Tg N/yr
N2 fixation: 125 Tg N/yr
Sedimentation: 25 Tg N/yr
Benthic denitrification: 300 Tg N/yr
Organic N export: 1 Tg N/yr
River Input: 76 Tg N/yr
Codispoti et al. (2001)Hypothetical Fixed N Evolution
-2.E+05
0.E+00
2.E+05
4.E+05
6.E+05
8.E+05
0 500 1000 1500 2000 2500 3000 3500
Time (years)
Fix
ed N
(T
g)
Marine Fixed N Budget
NO3
Chlorophyll
Largedetritus
Organic matter
N2 NH4 NO3
Water column
SedimentSediment
Phytoplankton
NH4
Mineralization
Uptake
Nitrification
Nitrification
Grazing
Mortality
Zooplankton
Susp.particles
Aerobic mineralizationAerobic mineralizationDenitrificationDenitrification
N2
Fixation
Nitrogen Cycle
http://www.petsforum.com/personal/trevor-jones/nitrogencycle.gif
Respiration ΔG° (kJ/mol)
-119
Denitrification-113
MnO2 reduction-96.9
Fe oxide reduction-46.7
Sulfate reduction-20.5
Methanogenesis-17.7
281
481
241 COCHOCH
OHCOOOCH 241
241
241
241
OHNCOHNOOCH 221
2101
241
51
351
241
OHCOFeHOHFeOCH 2411
2412
3241 2)(
OHCOHSHSOOCH 241
241
81
812
481
241
OHMnCOHMnOOCH 2432
21
241
281
241
Organic Matter Oxidation Sequence Morel & Herring,
1993
Alternative pathways to N2
OHCONHNOOCH 22232 752445
HOHNOONH 22 2324 OHNNONH 2224 2
HOHNONH 223
221
243
4
OHNMnHNHMnO 222
42 63423
Nitrification Anammox
OLAND
MnO2 Reduction
Microbially mediated
Chemical Reactions
Heterotrophic Denitrification
223
422 225 OOHNHOHN Nitrogen Fixation
OHNOMnHNHMnO 232
42 5464
HNMnOOHNOMn )solid( 85425 22232
Mn2+ Oxidation
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
Marine Fixed N Budget UnbalancedWHY??????????????????????
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
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
Benthic primary production (BPP)
Isotope tracer experiments
23029
31514
315 NNONO ,ationdenitrific,
23029
31514
315
415 NNONONH ,ationdenitrific,ionnitrificat
15NH4+,
14NO3-
1A 15NO3-
14N15N, 15N15N
1B
15N15N
1C
14N15N
1D
14N14N 1E
POM1F
A. Experiment 1
15NO3-,
14NH4+
14NO3-
2A14N15N, 14N14N
2B
14N14N
2C
14N15N
2D
15N15N2E
POM
B. Experiment 2
2F
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
SamplingSampling
Membrane Inlet Mass Spec. (MIMS)
00.5
11.5
22.5
3
R4-Exp
erim
ent 1
-Core
1
R4-Exp
erim
ent1
-Cor
e2
R4-Exp
erim
ent2
-Cor
e3
R4-Exp
erim
ent2
-Cor
e4
R4-Con
trol-C
ore5
R4-Con
trol-C
ore6
W27-
Amend
ed Cor
e
W27-
Contro
l
m M N
2 P
rod
uce
d
29N2 30N2
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
Results
Denitrification Rates
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
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
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)
NO3 Samples
0
50
100
150
200
250
300
350
400
0 10 20 30 40 50
Time (Days)
Co
nce
ntr
atio
n (
uM
)
NO3
NO2
NH4
NO3 + Hg Samples
0
50
100
150
200
250
300
350
400
0 10 20 30 40 50
Time (Days)
Co
nce
ntr
atio
n (
uM
)
NO3
NO2
NH4
NO3 Samples
0
1
2
3
4
5
6
7
0 10 20 30 40 50
Time (Days)
Co
nce
ntr
atio
n (
uM
-N)
N29
N30
NO3 + Hg Samples
0
1
2
3
4
5
6
7
0 10 20 30 40 50
Time (Days)
Co
nce
ntr
atio
n (
uM
-N)
N29
N30
Future work (cont’d)