Sedimentary denitrification rates

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 ... 

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... 

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... 

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... 

Sediment deposition on the seafloor traps interstitial water. After deposition, complex reactions take place in the sediment, most of them fueled by the decay of organic matter, such as sulfate reduction, denitrification,... Because of fast diffusion rates of most cations in seawater, the presence of interstitial water makes exchange between overlying sedimentary layers a much easier process than if sediment deposition was dry. The book by Berner (1980) is entirely dedicated to these processes and only a short example is given here. 

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