1. Lecture 18 The Ocean Nitrogen Cycle Denitrification Reactions Distributions Nitrogen Fixation Reactions Distributions

2. Why is this important for chemical oceanography? What controls ocean C, N, P? g ≈ 1.0 Mass Balance for whole ocean:  C/  t = V R C R – f B C S = 0; C D = C D V U = V D = V MIX Negative Feedback Control: if V MIX ↑ V U C D ↑ B ↑ f B ↑ (assumes f will be constant!) assume V R C R  then C D ↓ (because total ocean balance V U C D ↓ has changed; sink > source) B ↓ CSCS CDCD if V MIX = m y -1 and C = mol m -3 flux = mol m -2 y -1 The nutrient concentration of the deep ocean will adjust so that the fraction of B preserved in the sediments equals river input!

3. Nitrogen species: NO 3 - ; NO 2 - ; N 2 O; N 2 ; NH 4 + (V) (III) (I) (0) (-III) Nitrogen Isotopes: 14 N 99.634% 15 N 0.366% Isotopic Composition: ‰ The standard is atmospheric N 2

4. Fractionation factors, where  is the isotopic enrichment factor Fractionation Heavier stable isotope forms stronger bond. Microbial Enzymes break light isotope bonds more easily. Reactants become heavier (enriched) (e.g. NO 3 - → N 2 ) Products become lighter (depleted) Partial versus total reaction (products have same values as reactants)

5. The Global Nitrogen Budget-one example (Brandes et al, 2002)

6. Main Ocean Source of N Nitrogen Fixation Enzyme catalyzed reduction of N 2 N 2 + 8H + + 8e - + 16 ATP → 2NH 3 + H 2 + 16 ADP + 16P i Mediated by a two protein (Fe and Fe-Mo) complex called nitrogenase Inactivated when exposed to O 2

7. Main Ocean Sink of N Fixed Nitrogen (NO 3 -, NO 2 -, NH 4 + ) is converted to N 2 in low oxygen zones of the ocean Two Pathways Denitrification ( <2 to 10 m M O 2 ): 2 NO 3 - + organic matter → N 2 Anammox (<2 m M O 2 ) NH 4 + + NO 2 - → N 2 + H 2 O

8. Schematic of Ocean Nitrogen Cycle Gruber (2005) Nature 436, 786

9. % of Export Production (as N) at HOT derived from N 2 Fixation (N-P mass balance model of Karl et al (1997) Nature 388, p. 533)

10. Coupling of N sources and sinks (Deutsch et al, 2007, Nature, 445, 163) Capone and Knapp (2007) Nature, 445, 159

11. N* is defined as N* = [NO 3 ] – 16 x [PO 4 ] +2.9

12. Global distribution of O 2 at the depth of the oxygen minimum Gruber and Sarmiento, 1997

13. N* at 200m in the Pacific (Gruber and Sarmiento, 1997)

14. Kuypers (2003) Nature 422: 608-611. Nitrogen Cycle w/ anammox

15. PO4 versus Nitrate (GEOSECS data) Insert shows the effect of nitrification, photosynthesis, N2 fixation and denitrification. The solid line shows the linear equation P = 1/16 N + 0.345 (equivalent to N* = 0) Values to the right have positive N*, to the left have negative N* What is N*? N* is defined as N* = [NO 3 ] – 16 x [PO 4 ] +2.9

17. The Global Nitrogen Budget-one example (Brandes et al, 2002)

18. Spatial coupling of N2 fixation and denitrification (Deutsch et al, 2007)

19. Deutsch et al, 2004) Downcore records of 15N-orgN from several sites High values of 15N-OrgN suggest more extensive denitrification

20. Deutsch et al, 2004