Marine Nitrogen Cycle

Matthieu Heine

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)

Schedule

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

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 --> 2NH3

+ H2 + 16ADP + 16Pi (biological)

• Assimilation;NO3

- preferred

• Production/reduction of N20

Depends on circumstances ([O2], [NO2], etc)

“Global patterns of marine nitrogen fixation and denitrification.”

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

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 – rn:p P + constant

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- Γ(N) = Jnitr(N) + J denitr(N) + J N-rich nitr(N) (1)

- Γ(P) = Jnitr(P) + J denitr(P) + J N-rich nitr(P) (2)

- Γ(Tracer) = dT/dt + u + dT – d * (D-dT) (3)

Concept of N*

• Substitution of (1), (2) in (3)

• Values of Stoichiometric ratiosrN:P nitr. = 16 (Redfield et al.)

rN:P denitr. = -108,8 (Takahashi et al.)

rN:P n-rich nitr. = 125 (Karl et al.)

Concept of N*

• Final definition N*;

N* = (N-16P + 2,90 μmol kg-1) * 0,87 (4)

Γ(N*) = J denitr(N) + 0,76 J N-rich nitr(N) (5)

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.

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.

Global observations

Global marine N-budget

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.

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)

“The oceanic fixed nitrogen and nitrous oxide budgets:Moving targets as we enter the anthropocene?”

• Excluded reactions N-cycle

• Excluded interactions N-cycle

• Dynamics

• Conclusion

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.

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 ± 60:1.

- 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.

Excluded interactions N-cycle

• Different C/N ratios;

- Less carbon necessary for denitrification than for N-fixation.

Dynamics

• Ocean fixed nitrogen sink: Heavily dominated by denitrification.

• Ocean fixed nitrogen source:- N-fixation is largest biological source;

- Large effect input from rivers.

Dynamics

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.

Overall conclusion

• Balance in marine N-budget;• Existing estimates of water column denitrification

are too low;• Underestimated sources:- Anthropogenic sources; and- Input through groundwater.