Coupled nitrification-denitrification in sediment of the eastern Bering Sea shelf leads to 15N enrichment of fixed N in shelf waters

We studied the nitrogen biogeochemistry of the ice-covered eastern Bering Sea shelf using the isotope ratios (N-15/N-14 and O-18/O-16) of NO3- and other N species. The N-15/N-14 of late winter NO3- on the shelf decreases inshore and is inversely correlated with bottom water [NH4+], consistent with an input of low-N-15/N-14 NO3- from partial nitrification of NH4+ remineralized from the sediments. An inshore N-15/N-14 increase in total dissolved N (TDN) suggests that (1) the sediment-derived NH4+ is elevated in N-15 due to the same partial nitrification that yields the low-N-15/N-14 NO3-, and (2) N-15-deplete NO3- from partial nitrification within the sediments is denitrified to N-2. The proportion of newly nitrified NO3- on the shelf, evidenced by an inshore decrease in NO3- O-18/O-16, is correlated with the N deficit, further implicating nitrification coupled to denitrification; however, a simple N isotope budget indicates a comparable rate of denitrification supported by diffusion of NO3- into the sediments. The isotopic impact of benthic N loss is further demonstrated by a correlation between the N-15/N-14 of shelf surface sediment and the N deficit of the overlying water column, both of which increase inshore and northward, as well as by Arctic NO3- isotope data indicating that the fixed N transported through Bering Strait has a N-15/N-14 higher than is found in the open Bering Sea. The significant net isotope effect of benthic N loss on the Bering shelf, 6-8 parts per thousand, is at odds with previous assumptions regarding the global ocean's N isotope budget.