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 (¹⁵N/¹⁴N and ¹⁸O/¹⁶O) of NO₃^- and other N species. The ¹⁵N/¹⁴N of late winter NO₃ ^- on the shelf decreases inshore and is inversely correlated with bottom water [NH₄⁺], consistent with an input of low- ¹⁵N/¹⁴N NO₃^- from partial nitrification of NH₄⁺ remineralized from the sediments. An inshore ¹⁵N/¹⁴N increase in total dissolved N (TDN) suggests that (1) the sediment-derived NH₄⁺ is elevated in ¹⁵N due to the same partial nitrification that yields the low- ¹⁵N/¹⁴N NO₃^-, and (2) ¹⁵N-deplete NO₃^- from partial nitrification within the sediments is denitrified to N₂. The proportion of newly nitrified NO₃^- on the shelf, evidenced by an inshore decrease in NO₃^-18O/¹⁶O, 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 ¹⁵N/¹⁴N 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 ¹⁵N/¹⁴N higher than is found in the open Bering Sea. The significant net isotope effect of benthic N loss on the Bering shelf, 6-8 ‰, is at odds with previous assumptions regarding the global ocean's N isotope budget.