The ocean is an important source of nitrous oxide (N 2 O) to the atmosphere, yet the factors controlling N 2 O production and consumption in oceanic environments are still not understood nor constrained. We measured N 2 O concentrations and isotopomer ratios, as well as O 2 , nutrient and biogenic N 2 concentrations, and the isotopic compositions of nitrate and nitrite at several coastal stations during two cruises off the Peru coast (~5-16°S, 75-81°W) in December 2012 and January 2013. N 2 O concentrations varied from below equilibrium values in the oxygen deficient zone (ODZ) to up to 190 nmol L À1 in surface waters. We used a 3-D-reaction-advection-diffusion model to evaluate the rates and modes of N 2 O production in oxic waters and rates of N 2 O consumption versus production by denitrification in the ODZ. Intramolecular site preference in N 2 O isotopomer was relatively low in surface waters (generally À3 to 14‰) and together with modeling results, confirmed the dominance of nitrifier-denitrification or incomplete denitrifier-denitrification, corresponding to an efflux of up to 0.6 Tg N yr À1 off the Peru coast. Other evidence, e.g., the absence of a relationship between ΔN 2 O and apparent O 2 utilization and significant relationships between nitrate, a substrate during denitrification, and N 2 O isotopes, suggest that N 2 O production by incomplete denitrification or nitrifier-denitrification decoupled from aerobic organic matter remineralization are likely pathways for extreme N 2 O accumulation in newly upwelled surface waters. We observed imbalances between N 2 O production and consumption in the ODZ, with the modeled proportion of N 2 O consumption relative to production generally increasing with biogenic N 2 . However, N 2 O production appeared to occur even where there was high N loss at the shallowest stations.

Mesoscale eddies in Oxygen Minimum Zones (OMZs) have been identified as important fixed nitrogen (N) loss hotspots that may significantly impact both the global rate of N-loss as well as the ocean's N isotope budget. They also represent "natural tracer experiments" with intensified biogeochemical signals that can be exploited to understand the large-scale processes that control N-loss and associated isotope effects (ε; the ‰ deviation from 1 in the ratio of reaction rate constants for the light versus heavy isotopologues). We observed large ranges in the concentrations and N and O isotopic compositions of nitrate (NO 3 À ), nitrite (NO 2 À ), and biogenic N 2 associated with an anticyclonic mode-water eddy in the Peru OMZ during two cruises in November and December 2012. In the eddy's center where NO 3 À was nearly exhausted, we measured the highest δ 15 N values for both NO 3 À and NO 2 À (up to~70‰ and 50‰) ever reported for an OMZ.Correspondingly, N deficit and biogenic N 2 -N concentrations were also the highest near the eddy's center (up tõ 40 μmol L À1 ). δ 15 N-N 2 also varied with biogenic N 2 production, following kinetic isotopic fractionation during NO 2 À reduction to N 2 and, for the first time, provided an independent assessment of N isotope fractionation during OMZ N-loss. We found apparent variable ε for NO 3 À reduction (up to~30‰ in the presence of NO 2 À ).However, the overall ε for N-loss was calculated to be only~13-14‰ (as compared to canonical values of 20-30‰) assuming a closed system and only slightly higher assuming an open system (16-19‰). Our results were similar whether calculated from the disappearance of DIN (NO 3 À + NO 2 À ) or from the appearance of N 2 and changes in isotopic composition. Further, we calculated the separate ε values for NO 3 À reduction to NO 2 À and NO 2 À reduction to N 2 of~16-21‰ and~12‰, respectively, when the effect of NO 2 À oxidation could be removed. These results, together with the relationship between N and O of NO 3 À isotopes and the difference in δ 15 N between NO 3 À and NO 2 À , confirm a role for NO 2 À oxidation in increasing the apparent ε associated with NO 3 À reduction. The lower ε for N-loss calculated in this study could help reconcile the current imbalance in the global N budget if representative of global OMZ N-loss.

Abstract. O 2 deficient zones (ODZs) of the world's oceans are important locations for microbial dissimilatory nitrate (NO − 3 ) reduction and subsequent loss of combined nitrogen (N) to biogenic N 2 gas. ODZs are generally coupled to regions of high productivity leading to high rates of N-loss as found in the coastal upwelling region off Peru. Stable N and O isotope ratios can be used as natural tracers of ODZ Ncycling because of distinct kinetic isotope effects associated with microbially mediated N-cycle transformations. Here we present NO − 3 and nitrite (NO − 2 ) stable isotope data from the nearshore upwelling region off Callao, Peru. Subsurface oxygen was generally depleted below about 30 m depth with concentrations less than 10 µM, while NO − 2 concentrations were high, ranging from 6 to 10 µM, and NO − 3 was in places strongly depleted to near 0 µM. We observed for the first time a positive linear relationship between NO − 2 δ 15 N and δ 18 O at our coastal stations, analogous to that of NO − 3 N and O isotopes during NO − 3 uptake and dissimilatory reduction. This relationship is likely the result of rapid NO − 2 turnover due to higher organic matter flux in these coastal upwelling waters. No such relationship was observed at offshore stations where slower turnover of NO − 2 facilitates dominance of isotope exchange with water. We also evaluate the overall isotope fractionation effect for N-loss in this system using several approaches that vary in their underlying assumptions. While there are differences in apparent fractionation factor (ε) for N-loss as calculated from the δ 15 N of NO − 3 , dissolved inorganic N, or biogenic N 2 , values for ε are generally much lower than previously reported, reaching as low as 6.5 ‰. A possible explanation is the influence of sedimentary N-loss at our inshore stations which incurs highly suppressed isotope fractionation.

Nitrogen biogeochemistry during the beginning of a spring phytoplankton bloom in the Yellow Sea was investigated based on nutrient concentrations, benthic fluxes of nutrients, the nitrogen and oxygen isotope composition of NO3−, and the hydrological conditions. The δ15N and δ18O values for NO3− in the Yellow Sea were more variable in surface waters than in near‐bottom waters and were generally 6.3–8.2‰ for δ15N and 6.2–9.7‰ for δ18O, except in the Yellow Sea Coastal Current Water (YSCCW), where the maximum δ15N and δ18O values found were 13.2‰ and 18.8‰, respectively. Both the δ15N and δ18O values varied among the different water masses with higher values in the YSCCW than in the Yellow Sea Mixed Water and the Yellow Sea Warm Current Water. Higher δ18O relative to δ15N in NO3− in the Yellow Sea reflects contributions from multiple NO3− sources, including nitrification and atmospheric deposition with the later accounting for 71% of total external NO3− input. A simple nitrogen budget indicated the importance of nutrient regeneration from sediments as a source for water column NO3− representing 74% of total NO3− input in the YS.

Microbial dissimilatory nitrate reduction to nitrite, or nitrate respiration, was detected in association with copepods in the oxygenated water column of the North Atlantic subtropical waters. These unexpected rates correspond to up to 0.09 nmol N copepod−1 d−1 and demonstrate a previously unaccounted nitrogen transformation in the oceanic pelagic surface layers. Genes and transcripts for both the periplasmic and membrane associated dissimilatory nitrate reduction pathways (Nap and Nar, respectively) were detected. The napA genes and transcripts were closely related with sequences from several clades of Vibrio sp., while the closest relatives of the narG sequences were Pseudoalteromonas spp. and Alteromonas spp., many of them representing clades only distantly related to previously described cultivated bacteria. The discovered activity demonstrates a novel Gammaproteobacterial respiratory role in copepod association, presumably providing energy for these facultatively anaerobic bacteria, while supporting a reductive path of nitrogen in the oxygenated water column of the open ocean.