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

Stable isotopes (N-15,N-14, O-18,1(6)) 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 NO3-. 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 NO3- loss ((15)epsilon(app)) was 7.4 +/- 0.7% at microbial mat sites and 2.5 +/- 0.9 parts per thousand at the lower fringe of the OMZ (400 m) where foraminifera were abundant. The OMZ sediments were a source of N-15-enriched NO2- (28.9 to 65.5 parts per thousand) and NH4+(19.4-20.5 parts per thousand) to the bottom water. Model simulations generally support a previous hypothesis attributing the (NH4+)-N-15, enrichment to a coupling between DNRA and anammox (termed DAX) using biologically-stored NO3- from Marithioploca and NH(4)(+)from the porewater. The model predicts that 40% of NO3- 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 NH4+ is generated by DNRA, up to two-thirds of biologically-transported NO3- 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( 15)NH(4)(+), back to the ocean, DAX also tends to decrease benthic N fractionation. Tracking the fate of NH4+ once it leaves the sediment is critical for understanding how the benthos contributes to N isotope signals in the water column. (C) 2018 Elsevier Ltd. All rights reserved.