Processes and microorganisms driving nitrous oxide production in the Benguela Upwelling System

Abstract

Upwelling systems and their associated oxygen deficient zones (ODZs) are hotspots of nitrous oxide (N 2 O) production in the ocean. The Benguela Upwelling System (BUS) is a highly productive region and an important, yet variable source of N 2 O to the atmosphere. This study examined underlying processes and microbial key players governing N 2 O production in the BUS during the austral winter. 15 N‐tracer incubation experiments were conducted to track N 2 O production from NH 4 + oxidation and denitrification. N 2 O production and consumption mechanisms over a longer temporal scale were determined through natural‐abundance isotope analyses. Metagenomics and 16S rRNA gene amplicon sequencing were used to identify potential key prokaryotes driving N 2 O production. Our results showed that, compared with permanent ODZs, the BUS is characterized by a higher oxidative and a lower reductive N 2 O production, both of which exhibit substantial spatial variability. N 2 O production peaked in low‐oxygen (O 2 ) waters, with nearly equal contributions of oxidative and reductive processes, suggesting their co‐occurrence across an O 2 concentration range broader than previously thought. However, the observed N 2 O isotope signatures implied a legacy of recent and extensive N 2 O reduction to N 2 . Metagenomic and 16S rRNA gene data identified denitrifiers belonging to Thioglobaceae and the archaeal ammonia‐oxidizers Nitrosopumilaceae among the potential key drivers of N 2 O production. Our study provides a comprehensive picture of N 2 O production in the BUS, revealing significant variability in the N‐cycling regime and underlying N 2 O production mechanisms, and demonstrating the value of combining direct rate measurements with more integrative approaches, such as molecular omics and natural‐abundance stable isotope tracers.

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