Denitrification in marine sediments: A model study

Abstract

The rate and factors controlling denitrification in marine sediments have been investigated using a prognostic diagenetic model. The model is forced with observed carbon fluxes, bioturbation and sedimentation rates, and bottom water conditions. It can reproduce rates of aerobic mineralization, denitrification, and fluxes of oxygen, nitrate, and ammonium. The globally integrated rate of denitrification is estimated by this model to be about 230–285 Tg N yr−1, with about 100 Tg N yr−1 occurring in shelf sediments. This estimate is significantly higher than literature estimates (12–89 Tg N yr−1), mainly because of a proposed upward revision of denitrification rates in slope and deep‐sea sediments. Higher sedimentary denitrification estimates require a revision of the marine nitrogen budget and lowering of the oceanic residence time of nitrogen down to about 2×103 years and are consistent with reported low N/P remineralization ratios between 1000 and 3000 m. Rates of benthic denitrification are most sensitive to the flux of labile organic carbon arriving at the sediment‐water interface and bottom water concentrations of nitrate and oxygen. Denitrification always increases when bottom water nitrate increases but may increase or decrease if oxygen in the bottom water increases. Nitrification is by far the most important source of nitrate for denitrification, except for organic‐rich sediments underlying oxygen‐poor and nitrate‐rich water.

Origin of the deep Bering Sea nitrate deficit: Constraints from the nitrogen and oxygen isotopic composition of water column nitrate and benthic nitrate fluxes

“…The decrease in area-normalized denitrification rate with depth in the lower 2000 m is qualitatively consistent with the expected downward decrease in organic carbon rain rate and with the downward increase in bottom water [O 2 ] [Middelburg et al, 1996]. However, the model simplifies the real situation as it ignores vertical mixing and advection.…”

“…Nitrate flux-based estimates of denitrification tend to underestimate the actual denitrification rate because they do not account for the coupling of nitrification and denitrification [Christensen et al, 1987]. Given that, in most environments, nitrificationdenitrification accounts for the major portion of total denitrification [e.g., Christensen et al, 1987;Middelburg et al, 1996;Seitzinger, 1988], we can expect the total denitrification rates in the BS to be much higher. We used the O 2 consumption rates calculated from electrode O 2 profiles to obtain estimates of the potential rate of nitrate production due to aerobic mineralization and nitrification [Froelich et al, 1979] (Table 2).…”

“…There has been a drastic upward revision in estimates of the rate of denitrification in marine sediments, such that this is now the largest single flux in the N budget [Brandes and Devol, 2002;Codispoti et al, 2001;Middelburg et al, 1996]. However, the new estimates for sedimentary denitrification are highly uncertain [Deutsch et al, 2004].…”

Strong and Dynamic Benthic-Pelagic Coupling and Feedbacks in a Coastal Upwelling System (Peruvian Shelf)

“…Although, sediments underlying oxygen deficient waters account for less than 1% of the total seafloor, globally they account for 10% of benthic N loss . On average, 1 mole of fixed N is transformed to N 2 for every 4 moles of POC oxidized at the time-series station (Table 3), compared to a ratio of 1:10 in oxic shelf settings (Middelburg et al, 1996;Bohlen et al, 2012). As explained above, benthic denitrification must modulate the biogeochemistry of the water column on the Peruvian shelf.…”

“…These allow sediment fluxes to be parameterized using zerodimensional time-dependent approaches (i.e., a single box) at the lower ocean boundary, or by the implementation of transfer functions to predict the immediate response of sediments to changes occurring at the sediment-seawater boundary. These models can be derived from statistical analysis of verticallyresolved benthic models or be purely empirical in nature (Middelburg et al, 1996;Soetaert et al, 2000;Capet et al, 2016).…”