Simple transfer functions for calculating benthic fixed nitrogen losses and C:N:P regeneration ratios in global biogeochemical models

Empirical transfer functions are derived for predicting the total benthic nitrate loss (L-NO3) and the net loss of dissolved inorganic nitrogen (L-DIN) in marine sediments, equivalent to sedimentary denitrification. The functions are dynamic vertically integrated sediment models which require the rain rate of particulate organic carbon to the seafloor (RRPOC) and a proposed new variable (O-2-NO3)(bw) (bottom water O-2 concentration minus NO3- concentration) as the only input parameters. Applied globally to maps of RRPOC and (O-2-NO3)(bw) on a 1 degrees x 1 degrees spatial resolution, the models predict a NO3- drawdown of 196 Tg yr(-1) (L-NO3) of which 153 - 155 Tg yr(-1) is denitrified to N-2 (L-DIN). This is in good agreement with previous estimates using very different methods. Our approach implicitly accounts for fixed N loss via anammox, such that our findings do not support the idea that the relatively recent discovery of anammox in marine sediments might require current estimates of the global benthic marine N budget to be revised. The continental shelf (0 - 200 m) accounts for >50% of global L-NO3 and L-DIN, with slope (200 - 2000 m) and deep-sea (>2000 m) sediments contributing ca. 30% and 20%, respectively. Denitrification in high-nitrate/low-oxygen regions such as oxygen minimum zones is significant (ca. 15 Tg N yr(-1); 10% of global) despite covering only similar to 1% of the seafloor. The data are used to estimate the net fluxes of nitrate (18 Tg N yr(-1)) and phosphate (27 Tg P yr(-1)) across the sediment-water interface. The benthic fluxes strongly deviate from Redfield composition, with globally averaged N:P, N:C and C:P values of 8.3, 0.067 and 122, respectively, indicating world-wide fixed N losses (by denitrification) relative to C and P. The transfer functions are designed to be coupled dynamically to general circulation models to better predict the feedback of sediments on pelagic nutrient cycling and dissolved O-2 distributions.