Isotopic fingerprints of benthic nitrogen cycling in the Peruvian oxygen minimum zone.

Abstract Stable isotopes (15,14N, 18,16O) of dissolved inorganic nitrogen (N) were measured in sediment porewaters and benthic flux chambers across the Peruvian oxygen minimum zone (OMZ) from 74 to 1000 m water depth. Sediments at all locations were net consumers of bottom water NO 3 −. In waters shallower than 400 m, this sink was largely attributed to dissimilatory nitrate reduction to ammonium (DNRA) by filamentous nitrate-storing bacteria (Marithioploca and Beggiatoa) and to denitrification by foraminifera. The apparent N isotope effect of benthic NO 3 − loss (15ε app) was 7.4 ± 0.7‰ at microbial mat sites and 2.5 ± 0.9‰ at the lower fringe of the OMZ (400 m) where foraminifera were abundant. The OMZ sediments were a source of 15N-enriched NO 2 − (28.9 to 65.5‰) and NH 4 + (19.4–20.5‰) to the bottom water. Model simulations generally support a previous hypothesis attributing the 15NH 4 + enrichment to a coupling between DNRA and anammox (termed DAX) using biologically-stored NO 3 − from Marithioploca and NH 4 + from the porewater. The model predicts that 40% of NO 3 − that is actively transported into the sediment by Marithioploca is reduced to N 2 by this pathway. DAX enhances N 2 fluxes by a factor of 2–3 and accounts for 70% of fixed N loss to N 2. Moreover, because most of the ambient porewater NH 4 + is generated by DNRA, up to two-thirds of biologically-transported NO 3 − could end up being lost to N 2. This challenges the premise that Marithioploca -dominated sediments tend to conserve fixed N. By limiting the flux of 15NH 4 + back to the ocean, DAX also tends to decrease benthic N fractionation. Tracking the fate of NH 4 + once it leaves the sediment is critical for understanding how the benthos contributes to N isotope signals in the water column. [ABSTRACT FROM AUTHOR]