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: NH 3 + 1½ O 2 → NO H + + H 2 O Step 2 Nitrite oxidation: NO ½ O 2 → NO 3 - Denitrification; NO H + + 5e - → ½N 2 + 3H 2 O Nitrogen fixation; N 2 + 8H + + 8e ATP --> 2NH 3 + H ADP + 16Pi (biological) Assimilation; NO 3 - preferred Production/reduction of N 2 0 Depends on circumstances ([O 2 ], [NO 2 ], 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 – r n:p P + constant

Concept of N* Effects on biogeochemical cycles ; -Nitrification J nitr (N) -Denitrification J denitr (N) -N-fixation of N-rich O.M. by diazotrophic organisms J N-rich nitr (N) Formulas -Γ(N) = J nitr (N) + J denitr (N) + J N-rich nitr (N)(1) -Γ(P) = J nitr (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 ratios r N:P nitr. = 16 ( Redfield et al.) r N:P denitr. = -108,8 ( Takahashi et al.) r N: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 N 2 -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

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 (NH 2 OH). Bacteria: -Transport NO 3 - to sulphide-rich sediments; -Denitrification linked to oxidation by HS - ; -Excess N 2 production.

Excluded interactions N-cycle Dependence on O 2 concentration: -High [O 2 ] → Low N-fixation due to polluted enzyme system; -High [O 2 ] → Low denitrification; -Low [O 2 ] → High N 2 O production with nitrification. Interactions with Iron: -Fe/NO 3 - ratio with N-fixation is ± 60:1. -Oxygen Minimum Zone (OMZ) → High denitrification → high N- fixation → High Fe x requirements → Ocean becomes Iron-depleted; -Iron limitation constrains N-fixation thus primary production; -Increase in CO 2 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 CO 2 concentrations in the atmosphere. New calculations; 15-40% more N 2 produced than nitrate. Still excess in N 2. Possible sources of excess N 2 : -Sedimentary denitrification; -Bacteria oxidise HS - with NO 3 - producing NH 3 +. Which under suboxic conditions use Mn to reduce to N 2.

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