Denitrification and the sedimentary N cycle 1.The marine fixed N budget 2.Reactions and cartoons 3.“classic” denitrification 4.Anaerobic NH 4 + oxidation 5.Fluxes and pore water profiles of nitrate 15 N

Gruber and Sarmiento 1997, and Codispoti and Christensen , sinks 2X sources; 1997, balanced budget with larger fluxes Why do we care about the benthic N cycle? It is the largest sink for fixed Nitrogen

Brandes and Devol 2002 A more recent estimate: Fixed nitrogen sinks exceed sources by up to 200 Tg N / yr (more than the entire 1985 budget!) Sed denitrification 3X water column rate

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

But low-O 2 areas are not geographically restricted in sediments… O 2 penetration depths in Sediments: N. Atlantic Ocean (high bw O 2 ) NE Pacific (low bwO 2 )

Codispoti & Christensen 1985 Denitrification: NO 3 -  NO 2 -  NO  N 2 O  N 2

Goloway and Bender 1982 oxygen respiration (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 138O 2 => 106HCO NO HPO H H 2 O nitrate reduction (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) NO 3 - => 13.6CO HCO N 2 + HPO H 2 O Oxidation state of N?

Devol, 1991 Benthic flux chambers on WA margin: N 2 efflux, NO 3 - uptake

oxygen respiration (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 138O 2 => 106HCO NO HPO H H 2 O nitrate reduction (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) NO 3 - => 13.6CO HCO N 2 + HPO H 2 O MnO 2 reduction (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 236MnO H + => 236Mn HCO N 2 + HPO H 2 O Fe 2 O 3 reduction (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 212Fe 2 O H + => 424 Fe HCO NH HPO H 2 O sulfate reduction (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) + 53SO 4 -2 => 106HCO NH HPO HS H + fermentation (CH 2 O) 106 (NH 3 ) 16 (H 3 PO 4 ) => 53CO CH NH 3 + H 3 PO 4 What happens to the ammonium?

Burdige, 2006 * * *

1. hydrolysis 2. DON uptake 3. ammonification 4. nitrification (aerobic) 5. denitrification 6. dissimilatory reduction of nitrate to ammonium 7. nitrogen fixation 8. anammox (anaerobic ammonium oxidation by nitrite) 9., 10. anaerobic ammonium oxidation by MnOx

Burdige, 2006

The anaerobic oxidation of NH 4 + to N 2 Two mechanisms: NO 3 - diffuses down from oxic layer NH 4 + diffuses up from Fe(III) and SO 4 2- reduction zones anammox: NO 2 - a product of organic matter oxidation by NO 3 - Demonstrated using sediment incubations Thamdrup et al Appl. Env. Microbiol. 68, Evidence in steady-state systems?

2002 filled: 15 N-NH 4 + NO 3 Open 15 N-NH 4 only circles: nitrate (NO 3 -) Triangles: nitrite (NO 2 -) spike w labeled NH 4, w and w/o NO 3 Nitrate-dependent ammonium oxidation (MnOx alone are not enough to oxidize NH 4, at these sites): Without nitrate (open), label doesn’t move from NH 4 into N 2. With nitrate + labeled NH 4 label moves into N 2. circles: d 15 N of NH 4 triangles: d 15 N of N 2

Degree of labeling reflects mechanism: 15N-NH4 and unlabeled NO3 (top), produces only 29N-N2 (suggests 1 NH4 N + 1 NO3 N) 15N-NO3 (middle) (with unlabeled NH4 from ammonification) produces both 29N and 30N N2 (“classic” denitrification alone of 15N-NO3 would yield only 30N-N2) 15N-NH4 + 15N-NO3 (bottom) increases yield of 30N-N2 (still some 29N-N2 formed, using NH4 from ammonification) open circles: d 29 N N 2 fillled circles: d 30 N N 2

Thamdrup and Dalsgaard, 2002 Anaerobic ammonium oxidation is significant at both Skagerrak sites; it is the most important sink for nitrate at one of the site. And note that anammox rates are highest at the shallower two sites, even though the fraction is lower.

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

Brandes and Devol, 1997; 2002 What can we learn from natural abundances of 15N in nitrate? Pore water data (circles) suggest a much smaller fractionation for sedimentary denitrification than for water column denitrification (triangles).

Sigman & Casciotti, 2001  15 N Shifts in the balance between water column and sedimentary denitrification can change the  15 N of the oceans

Benthic flux chambers in Santa Monica Basin to estimate the d15N signature of benthic denitrification.

No change in d15N, consistent with very little net N isotopic fractionation (small  (epsilon)) during benthic denitrification..Does this reflect different intrinsic isotope effects, or different “expression”?

Lehmann et al. submitted Pore water data from Bering Sea multicores; oxygen penetration 1 to several cm; nitrate profiles indicate denitrification (Goloway “type II”, “type III” (?), irrigation(?)) (NH4 fluxes; some pore water nitrite)

Pore water d15N-NO3 does increase as NO3 decreases – a strong intrinsic isotope effect for sediment denitrification. But, d15N gradients are very small just below the sediment-water interface; The denitrification-driven fractionation is not expressed in the flux across the swi.

Lehmann et al. submitted The fractionation is not expressed (in large part) because the NO3 molecules are on a one- way ride – they’re diffusing down, and wind up being ~100% reduced, so no net fractionation. However, what about other N-cycle processes, and their isotopic signals? A role for anammox? For aerobic nitrification just below the SWI? For NH4 that escapes oxidation and diffuses into bottom water…