Vertical modeling of the nitrogen cycle in the eastern tropical South Pacific oxygen deficient zone using high‐resolution concentration and isotope measurements

Abstract

Marine oxygen deficient zones (ODZs) have long been identified as sites of fixed nitrogen (N) loss. However, the mechanisms and rates of N loss have been debated, and traditional methods for measuring these rates are labor‐intensive and may miss hot spots in spatially and temporally variable environments. Here we estimate rates of heterotrophic nitrate reduction, heterotrophic nitrite reduction (denitrification), nitrite oxidation, and anaerobic ammonium oxidation (anammox) at a coastal site in the eastern tropical South Pacific (ETSP) ODZ based on high‐resolution concentration and natural abundance stable isotope measurements of nitrate (NO3−) and nitrite (NO2−). These measurements were used to estimate process rates using a two‐step inverse modeling approach. The modeled rates were sensitive to assumed isotope effects for NO3− reduction and NO2− oxidation. Nevertheless, we addressed two questions surrounding the fates of NO2− in the ODZ: (1) Is NO2− being primarily reduced to N2 or oxidized to NO3− in the ODZ? and (2) what are the contributions of anammox and denitrification to NO2− removal? Depth‐integrated rates from the model suggest that 72–88% of the NO2− produced in the ODZ was oxidized back to NO3−, while 12–28% of NO2− was reduced to N2. Furthermore, our model suggested that 36–74% of NO2− loss was due to anammox, with the remainder due to denitrification. These model results generally agreed with previously measured rates, though with a large range of uncertainty, and they provide a long‐term integrated view that compliments incubation experiments to obtain a broader picture of N cycling in ODZs.

Multiple metabolisms constrain the anaerobic nitrite budget in the Eastern Tropical South Pacific

“…(ii) A role for denitrification is further corroborated by simultaneously measured nutrients and natural abundance isotope measurements from the pump cast, which predict contributions from both anammox and denitrification for this ODZ [ Peters et al , ].…”

“…This observation from direct rate measurements corroborates many of the ideas put forward by Casciotti et al [], who calculated that nitrite oxidation should be the major sink term of nitrite in the lower ODZ off of Peru and that nitrate reduction was larger than nitrite oxidation at the top of the ODZ. The absence of significant nitrite oxidation in the core of the ODZ is also corroborated by natural abundance isotope data collected simultaneously [ Peters et al , ]. Given the measured rates of anammox (<10 nmol N 2 L −1 d −1 ) and the stoichiometry of nitrite oxidation by anammox (NO 2 − oxidation = 0.3 × N 2 production) [ Strous et al , ], anammox can be responsible for only a small fraction of the observed nitrite oxidation rates.…”

Isotopic Constraints on Nitrogen Transformation Rates in the Deep Sedimentary Marine Biosphere

“…As introduced above, the initial model parameterization yields rates of nitrate reduction and nitrite oxidation that track each other over the vertical extent of this zone (6 to 10 mbsf), with the fastest rates predicted at 0.3 μM y −1 at a depth of 7 mbsf. These rates of nitrate reduction are much lower than values measured in sediment and the water column in Eastern Tropical Pacific waters (Buchwald et al, ; Peters et al, ; Ward et al, ) and other oligotrophic sediments from the North Atlantic at North Pond (Wankel et al, ). Nitrite oxidation rates here are also lower than values observed in ODZs (~3 μM y −1 ; (Buchwald et al, )), but closer to water column values than other sediment environments.…”

“…This reoxidation dynamic has been invoked in other zones where NO 2 − accumulation occurs, particularly in ODZs of the global ocean (Beman et al, ; Buchwald et al, ; Füssel et al, ; Martin & Casciotti, ; Peters et al, ). Indeed, reoxidation of nitrite under anoxic conditions, while still not well understood, may underpin a greater part of the natural nitrogen cycle than previously recognized (Babbin et al, ; Granger & Wankel, ; Sun et al, ).…”

Nitrogen assimilation in picocyanobacteria inhabiting the oxygen‐deficient waters of the eastern tropical North and South Pacific

“…The measurements of δ 15 N‐ reported in the literature for the ODZs range from 12.4‰ to 17.2‰, and from −16.0‰ to −17.8‰ for δ 15 N‐, showing a difference up to 35‰ between both nitrogen sources (Casciotti and McIlvin ). These differences are considered a consequence of isotope fractionation during the processes of denitrification (Casciotti and McIlvin ) and nitrite oxidation (Peters et al ). and urea exhibit very low (nanomolar) concentrations in the ocean (for our study sites, see Table ) and it has not been possible to measure their δ 15 N to date, as the most sensitive methods require concentrations of at least 0.5 μ mol L −1 (Zhang et al ).…”