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

Abstract

We studied the nitrogen biogeochemistry of the ice‐covered eastern Bering Sea shelf using the isotope ratios ( 15 N/ 14 N and 18 O/ 16 O) of NO 3 − and other N species. The 15 N/ 14 N of late winter NO 3 − on the shelf decreases inshore and is inversely correlated with bottom water [NH 4 + ], consistent with an input of low‐ 15 N/ 14 N NO 3 − from partial nitrification of NH 4 + remineralized from the sediments. An inshore 15 N/ 14 N increase in total dissolved N (TDN) suggests that (1) the sediment‐derived NH 4 + is elevated in 15 N due to the same partial nitrification that yields the low‐ 15 N/ 14 N NO 3 − , and (2) 15 N‐deplete NO 3 − from partial nitrification within the sediments is denitrified to N 2 . The proportion of newly nitrified NO 3 − on the shelf, evidenced by an inshore decrease in NO 3 − 18 O/ 16 O, 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 NO 3 − into the sediments. The isotopic impact of benthic N loss is further demonstrated by a correlation between the 15 N/ 14 N 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 NO 3 − isotope data indicating that the fixed N transported through Bering Strait has a 15 N/ 14 N 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.