Denitrification sediments

Estuary export Atmosphere N fixation wastewater sources inputs column Denitrification Sedimentation landings Other losses Net... 

Christensen J. P., Murray, J. W., Devol, A. H. and Codispoti, L. A. (1987). Denitrification in continental shelf sediments has major impact on oceanic nitrogen cycle. Glob. Biogeochem. Cycles 1,97-116. 

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. 

Bruchert V, Knoblauch C, Jorgensen BB (2001) Controls on stable sulfur isotope fractionation during bacterial sulfate reduction in Arctic sediments. Geochim Cosmochim Acta 65 763-776 Bryan BA, Shearer G, Skeeters JL, Kohl DH (1983) Variable expression of the nitrogen isotope effect associated with denitrification of nitrate. J Biol Chem 258 8613-8617 Canfield DE (2001) Biogeochemistry of sulfur isotopes. Rev Mineral Geochem 43 607-636 Chau YK, Riley JP (1965) The determination of selenium in sea water, silicates, and marine organisms. Anal Chim Acta 33 36-49... 

In sediments that lie in coastal waters, organic carbon levels are high enough to support denitrification, iron respiration, sulfate reduction and methanogenesis. As shown in the idealized profile presented in Figure 12.3b, the depth of O2 penetration in organic-rich sediments is typically so shallow as to make the zones of aerobic respiration. 

Compared to denitrification rates, burial in the sediments is a minor loss of nitrogen from the oceans. Current estimates suggest that shelf sediments are the major site of denitrification (14TgNy ) with a minor contribution from the sediments in the open ocean (0.8TgN ). 

The biogeochemical cycling of nitrogen is very much controlled by redox reactions. This perspective is presented in Figure 24.3 for the redox reactions that take place in the water column and sediments. The major pathways of reduction are nitrogen fixation, assimilatory nitrogen reduction, and denitrification. The major oxidation processes are nitrification and anaerobic ammonium oxidation (anammox). Each of these is described next in further detail. 

During denitrification, nitrate is reduced to N2(g) by heterotrophic bacteria. The stoichiometry for this process was presented in Eq. 8.11 and Table 12.1 for the water column and the sediments, respectively. Denitrification is thought to be performed by fecultative aerobes that switch to nitrate respiration when O2 concentrations are less than 5 p-M (0.1 ml/L). 

Before the recent discovery of anammox, most of the loss of fixed nitrogen from the ocean was assumed to be occurring via denitrification. We now know that some, if not most, of this loss is truly due to anammox. Anammox occurs in coastal sediments, anoxic marginal seas, and in OMZs. The bacteria responsible for anammox are similar to the ones found in wastewater bioreactors (Planctomycetales). 

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... 

Environments N flux across sediment-water interface (mgNm d Denitrification N requirements of primary production ... 

Reddy KR, Patrick WH, Jr., Lindau CW. 1989. Nitrification-denitrification at the plant root-sediment interface in wetlands. Limnology and Oceanography 34 1004-1013. 

Estuaries have been shown to be active sites of N2O production. DIN loading to sediments was positively correlated with N2O fluxes, indicating that eutrophication may be linked with increases in global emissions of N2O. In fact, a model of DIN transport by world rivers and estuaries indicates that 90% of the denitrification in rivers and estuaries occurs in the northern hemisphere. 

Figure 10.6 Global budget of N in the coastal zone (fluxes are in 1012 g N yr 1). = More recent estimates of denitrification in estuaries and continental shelf sediments range from 200 to 300 Tg N y-1 (Seitzinger and Giblin, 1996 Codispoti et al., 2001). (From Wollast, 1993, with permission.)...

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