Denitrification sedimentary

In coastal sediments where organic carbon concentrations are high, the redox boundary is at or near the sediment-water interfece. Under these conditions, denitrification acts as a sink for nitrate. In some settings, the rate of sedimentary denitrification is fast enough to drive a diffusive flux of nitrate from the bottom waters into the sediments. Remineralization of organic matter imder suboxic and anoxic conditions releases... 

It is obvious that sedimentary denitrification and water-column nitrification seem to be the major N2O formation processes. However, the yield of N2O from both processes strongly depends on the local O2 concentrations, thus dissolved O2 is the key factor regulating N2O production (and its subsequent emissions to the atmosphere). Additionally, N2O distributions in estuaries show a pronounced seasonal variability. 

Sedimentary denitrification rates have been estimated from measured pore-water solute profiles using diagenetic models, determined direcdy via sediment incubation both on deck and in situ, and determined from N-incubation techniques. Sedimentary diagenetic process can be thought of as a simple reaction—diffusion-transport system (Berner, 1980 Boudreau, 1997). In a simple fine-grained sediment system, transport is via molecular diffusion and the diagenetic equation describing this system can be expressed as ... 

The small isotopic effect expressed during sedimentary denitrification could be due to diffusion limitation as proposed by Brandes and Devol (1997), but it could also result from a balance between the isotopic effects of denitrification being offset by an input of low N03 from sedimentary nitrification. However in an... 

The amount of sedimentary denitrification occurring in the ocean today is one of the most poorly quantified terms in the marine combined nitrogen budget. Most modem measurements of denitrification rate have utilized pore-water N03 profiles (estimated from diffusion calculations—reaction models) whole-sediment incubations, either on deck or in situ, or by the isotope paring technique. It appears that shelf and upper slope sediments are quantitatively the most important sites of sedimentary denitrification (Christensen, et al., 1996 Christensen et al., 1987 Devol, 1991 Devol and Christensen, 1993 Gruber and Sarmiento, 1997 Kristensen, et al., 1999 Middelburg et al., 1996). Typical denitrification rates in these areas... 

The continental shelves receive N from the open ocean (820 x 10 molyear ), from estuaries (250 x 10 mol year ), from major rivers (350 x 10 mol year ) and from atmospheric deposition (130 x 10 mol year ). Some is lost to the sediments (120 x 10 mol year ) and fish catch (32 x 10 mol year ), but the majority is removed from the system via sedimentary denitrification (1400 x 10 mol year ). Nitrogen introduced to the shelves from the open ocean appears to contribute the most to shelf denitrification (Seitzinger and Gibhn, 1996). 

Potential processes that may be called upon to explain the large discrepancy between excess N2 and NOs deficits within the SNM are (1) sedimentary denitrification, (2) anammox, (3) metal (Fe, Mn)-catalyzed denitrification, and (4) non-Redfieldian organic matter mineralization (Codispoti et al., 2001 Devol et al., 2006a,b). The first of these possibilities requires a decoupling between nitrogen and phosphorus cycles (e.g. possible greater burial of phosphorus in continental margin sediments). The less likely additional requirement is that the maximal inputs from the sediments should occur at the same depth as the water column peak of N02. In other words, the exact coincidence of the extrema in NOs , N02, ... 

The major modes of nitrogen losses from the Bay of Bengal are sedimentary denitrification and burial. Note that the estimate for sedimentary denitrification (3 Tg N year ) is based on only one measurement at the outer shelf off Chennai (Naik, 2003), while the burial in sediments was computed from the carbon burial using a C N value of 10. 

Tanaka, T., Guo, L, Deal, C., Tanaka, N., and Whitledge, T. (2004). N deficiency in a weU-oxygenated cold bottom water over the Bering Sea shelf influence of sedimentary denitrification. Cont. Shelf Res. 24, 1271-1283. 

The tr ectories for denitrification, nitrate uptake, and sedimentary denitrification were each calculated using a closed-system Rayleigh model with an isotope discrimination factor ( ) in the middle of the range given in Rg. 29.2. The two nitrification tr ectories shown were calculated with a mass balance model for remineralization and nitrification of organic matter produced from average deep water N03, or diazotrophic organic matter with a of —2%o (Montoya etal,2m2). 

Denitrification is the largest loss term for oceanic nitrogen and plays a key role in regulating the avadabifity of combined nitrogen in the ocean on long time scales (Codispoti et al., 2001 Codispoti, 2007) (see Devol, Chapter 6, this volume). While sedimentary denitrification is unhkely to alter the of oceanic NOs" (Fig. 29.3), pelagic denitrification plays a central role in the isotopic balance of the ocean since it does not typically go to completion and therefore produces residual with an... 

Sedimentary denitrification Sea level (shelf area) (+) Organic matter rain (+) Oxygen supply (-)... 

Sedimentary denitrification, in contrast, shows very little expression of the enzymatic isotope effect. Benthic flux experiments have shown that, even with high rates of sedimentary denitrification, minimal increase of <5 Nni ate is observed (Brandes and Devol, 1997, 2002 Lehmann et al., 2004). The common explanation is that nitrate is nearly completely consumed at the locus of sedimentary denitrification. Because no nitrate escapes the denitrification zones, there is no expression of the isotope effect. This view is supported by the observation of extreme enrichment... 

It seems safe to assume that N2 fixation, water column denitrification and sedimentary denitrification have been the dominant input/output terms of the marine N budget throughout, at least, the past 2 Myr. With this assumption in place, one can begin to consider how potential changes would have altered the of marine nitrate, both within a given region and at the scale of the whole ocean. 

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