Marine Nitrogen Cycle

1
Marine Nitrogen Cycle

• Matthieu Heine

2
Marine Nitrogen Cycle

• Global patterns of marine nitrogen fixation and
  denitrification. Nicolas Gruber and Jorge L.
  Sarmiento (1997)
• The oceanic fixed nitrogen and nitrous oxide
  budgets Moving targets as we enter the
  anthropocene? L.A. Codispoti, Jay A. Brandes,
  J.P. Christensen, A.H. Devol, S.W.A. Naqvi, Hans
  W. Paerls and T. Yoshinari (2001)

3
Schedule

• General overview of the marine nitrogen cycle.
• Review Global patterns of marine nitrogen etc.
• Review The oceanic fixed nitrogen etc.
• Overall conclusion.

4
Overview N-cycle

• Nitrification
• Step 1 Ammonia oxidation
• NH3 1½ O2 ? NO2- H H2O
• Step 2 Nitrite oxidation
• NO2- 1½ O2 ? NO3-
• Denitrification
• NO3- 6H 5e- ? ½N2 3H2O
• Nitrogen fixation
• N2 8H 8e- 16ATP --gt 2NH3
• H2 16ADP 16Pi (biological)
• Assimilation
• NO3- preferred
• Production/reduction of N20
• Depends on circumstances (O2, NO2, etc)

5
Global patterns of marine nitrogen fixation and
denitrification.

• Introduction
• The concept of N
• Data considerations
• Global observations
• Global marine N-budget

6
Introduction

• Uncertainties estimating marine N-budget
• Ocean system Steady State/Dynamic?
• Differences in residence times
• Data of small spatial and temporal scale
• New method in town!
• N N rnp P constant

7
Concept of N

• Effects on biogeochemical cycles
• Nitrification Jnitr(N)
• Denitrification J denitr(N)
• N-fixation of N-rich O.M. by diazotrophic
  organisms J N-rich nitr(N)
• Formulas
• G(N) Jnitr(N) J denitr(N) J N-rich
  nitr(N) (1)
• G(P) Jnitr(P) J denitr(P) J N-rich
  nitr(P) (2)
• - G(Tracer) dT/dt u dT d (D-dT) (3)

8
Concept of N

• Substitution of (1), (2) in (3)
• Values of Stoichiometric ratios
• rNP nitr. 16 (Redfield et al.)
• rNP denitr. -108,8 (Takahashi et al.)
• rNP n-rich nitr. 125 (Karl et al.)

9
Concept of N

• Final definition N
• N (N-16P 2,90 µmol kg-1) 0,87 (4)
• G(N) J denitr(N) 0,76 J N-rich nitr(N) (5)

10
Concept of N

• Absolute value of N is arbitrary!
• Only deviation is from the solid line is
  important Redfield ratio (16) ? deviation is 0.
• Left-hand side net effect of
• N2-fixation (excess P).
• Right-hand side net effect of denitrification.

11
Data Consideration

• Sampling in Atlantic, Pacific and Indian Ocean.
• Data used from the GEOSECS, TTO NAS, TTO TAS,
  SAVE, Atlantis 109 and AJAX-cruises.
• Data has been consistent over a longer period of
  time.

12
Global observations
13
Global marine N-budget
14
Global marine N-budget

• Conclusions
• N-budget from previous literature needs to be
  adjusted ? more N-fixation than expected
  (especially in the Atlantic)
• Not in compliance with other studies that the
  ocean as a whole is losing fixed nitrogen.

15
Marine Nitrogen Cycle

• Global patterns of marine nitrogen fixation and
  denitrification. Nicolas Gruber and Jorge L.
  Sarmiento (1997)
• The oceanic fixed nitrogen and nitrous oxide
  budgets Moving targets as we enter the
  anthropocene? L.A. Codispoti, Jay A. Brandes,
  J.P. Christensen, A.H. Devol, S.W.A. Naqvi, Hans
  W. Paerls and T. Yoshinari (2001)

16
The oceanic fixed nitrogen and nitrous oxide
budgetsMoving targets as we enter the
anthropocene?

• Excluded reactions N-cycle
• Excluded interactions N-cycle
• Dynamics
• Conclusion

17
Excluded reactions N-cycle

• Labile components
• NO
• Hydroxylamine (NH2OH).
• Bacteria
• Transport NO3- to sulphide-rich sediments
• Denitrification linked to oxidation by HS-
• Excess N2 production.

18
Excluded interactions N-cycle

• Dependence on O2 concentration
• High O2 ? Low N-fixation due to polluted
  enzyme system
• High O2 ? Low denitrification
• Low O2 ? High N2O production with
  nitrification.
• Interactions with Iron
• Fe/NO3- ratio with N-fixation is 601.
• Oxygen Minimum Zone (OMZ) ? High denitrification
  ? high N-fixation ? High Fex requirements ? Ocean
  becomes Iron-depleted
• Iron limitation constrains N-fixation thus
  primary production
• Increase in CO2 in the atmosphere.

19
Excluded interactions N-cycle

• Different C/N ratios
• - Less carbon necessary for denitrification than
  for N-fixation.

20
Dynamics

• Ocean fixed nitrogen sink
• Heavily dominated by denitrification.
• Ocean fixed nitrogen source
• - N-fixation is largest biological source
• Large effect input from rivers.

21
Dynamics
22
Conclusion

• Difference in fixed nitrogen budgets last long
  enough to have an significant impact on the CO2
  concentrations in the atmosphere.
• New calculations 15-40 more N2 produced than
  nitrate.
• Still excess in N2.
• Possible sources of excess N2
• Sedimentary denitrification
• Bacteria oxidise HS- with NO3- producing NH3.
  Which under suboxic conditions use Mn to reduce
  to N2.

23
Overall conclusion

• Balance in marine N-budget
• Existing estimates of water column
  denitrification are too low
• Underestimated sources
• Anthropogenic sources and
• Input through groundwater.