1. Sedimentary denitrification
The marine fixed N budget & the role of sediments
Sedimentary organic matter oxidation by NO3-
Anaerobic NH4+ oxidation

2. Nitrogen cycling: reactions

3. Denitrification in the water column: restricted to low-O2 areas

4. But low-O2 areas are not geographically restricted in sediments…
O2 penetration depths in
Sediments:
N. Atlantic Ocean (high
bw O2)
NE Pacific (low bwO2)

5. Global fixed N budgets: 1. Gruber and Sarmiento, 1997

6. Other estimates of sedimentary denitrification rates Middelburg et al

7. And an alternate marine fixed N budget

8. Nitrification - Denitrification
Organic matter decomposition in sediments NO3-: the second electron acceptor
Measure: NO3-, N2
s.w.i.
O O2
o.m.--> NH > NO3-
o.m.
o.m. ---> NH4+
FeO2, SO42- N2
Nitrification - Denitrification

9. Pore water NO3 profiles
Goloway and Bender 1982
L&O 27,

10. “Group I”

11. “Group II”

12. “Group III”

14. NO3 as an electron acceptor for Corg ox. Model results
Middelburg et al., 1996
G.B.C. 10,

15. Estimating sedimentary denitrification rates 1
Estimating sedimentary denitrification rates 1. Direct measurement of N2 flux Devol, 1991 Nature 349,
Data from Washington continental margin
High Corg ox. Rate ! Low BW O2 !

16. Estimating sedimentary denitrification rates 2
Estimating sedimentary denitrification rates 2. Pore water NO3- Profiles “High N, Low O2” bottom water Reimers et al., 1992 GBC 6,

17. Summary, Northeast Pacific “High N, Low O2” bw

18. Calculation of sedimentary denitrification rates 2
Calculation of sedimentary denitrification rates 2. Pore water NO3- profiles “High O2, Low N” bottom water
NO3- from oxic degradation --
Some lost to bottom water !
NO3- for denitrification

19. Summary: denitrification in sediments From Martin and Sayles, 2004 DSR I 51, 457-489
NW Atlantic
MAB
depocenter
NE Pac
Calif margin
NE Pac
Wash margin
NW Atlantic
70°W

20. The anaerobic oxidation of NH4+ to N2
Two mechanisms:
NO3- diffuses down from oxic layer
NH4+ diffuses up from Fe(III) and SO42-
reduction zones
anammox: NO2- a product of organic
matter oxidation by NO3-
Demonstrated using sediment incubations
Thamdrup et al Appl. Env. Microbiol. 68,
Evidence in steady-state systems?

21. Sediment incubations Procedure: -- section anoxic zone of sediments
-- add (1) 15N-ammonium + unlabeled nitrate (filled symbols)
(2) 15N-ammonium only (open symbols)
-- measure NO3- (A: circles), NO2- (A: triangles), 15N-NH4 (B: circles),
15N-N2 (B: triangles)
O2 pen max [NO3]
(µM)

22. Evidence for anaerobic NH4
Oxidation in untreated
Samples?
e.g., NW Atlantic slope / rise
… it’s difficult to tell, but
maybe…

23. An interpretation of
the curvature of the
profiles.