Denitrification, anammox, and DNRA in oligotrophic continental shelf sediments

Continental shelf sediments are considered hotspots for nitrogen (N) removal. While most investigations have quantified denitrification in shelves receiving large amounts of anthropogenic nutrient supply, we lack insight into the key drivers of N removal on oligotrophic shelves. Here, we measured rates of N removal through denitrification and anammox by the revised-isotope pairing technique (r-IPT) along the Northeastern New Zealand shelf. Denitrification dominated total N₂ production at depths between 30 and 128 m with average rates (± SE) ranging from 65 ± 28 to 284 ± 72 μmol N m⁻² d⁻¹. N₂ production by anammox ranged from 3 ± 1 to 28 ± 11 μmol N m⁻² d⁻¹ and accounted for 2–19% of total N₂ production. DNRA was negligible in these oligotrophic settings. Parallel microbial community analysis showed that both Proteobacteria and Planctomycetota were key taxa driving denitrification. Denitrification displayed a negative correlation with oxygen penetration depth, and a positive correlation with macrofauna abundance. Our denitrification rates were comparable to oligotrophic shelves from the Arctic, but were lower than those from nutrient-rich Pacific and Atlantic shelves. Based on our results and existing IPT measurements, the global shelf denitrification rate was reassessed to be 53.5 ± 8.1 Tg N yr⁻¹, equivalent to 20 ± 2% of marine N removal. We suggest that previous estimates of global shelf N loss might have been overestimated due to sampling bias toward areas with high N loads in the Northern Hemisphere.