Water samples were collected during cruises RR2104 and AT50-10 in the eastern tropical and subtropical Pacific Ocean. Discrete water samples were obtained using a CTD rosette sampler equipped with 24 x 10 L Niskin bottles. Different depths were sampled, ranging from 35 meters to 300 meters.

Nitrification and nitrite oxidation rates were determined using 15N isotope tracer methods. For oxygenated depths, incubations were conducted in 1L or 250 mL polycarbonate bottles (Nalgene) and for depths with low in situ O2 concentrations (≤ 20 µM), incubations were conducted in 500 mL Tedlar bags (Restek) equipped with a septum injection port and three-way stopcocks, following the procedures described previously (Santoro et al. 2021). Each bottle or bag was spiked with [15N]-tracer (99 atm% 15NH4Cl or 98 atm% Na15NO2-, Cambridge Isotope Laboratories) to a final label concentration of 70 to 200 nM (depending on the depth and productivity region the sample was collected from), and incubations were conducted as close to in situ temperature conditions as possible (±1.5°C). Phenylacetylene (10 µM) dissolved in dimethylsulfoxide (DMSO, 0.01%) was used to some bottles to test its efficiency in inhibiting ammonia oxidation activities. At timepoints of 0, 12, and 24 h, 50 mL samples were drawn from each bottle or bag, 0.2 µm syringe-filtered into a 20 mL HDPE bottle, and frozen at -20ºC. Frozen samples were transported to the laboratory, thawed, and prepared for δ15NNO2+NO3 analysis using the ‘denitrifier method’ (Sigman et al. 2001). For nitrite oxidation rate samples, the added 15NO2- tracer was removed using sulfamic acid addition and subsequent neutralization with NaOH prior to sample preparation (Granger et al. 2006). Samples were analyzed on a custom purge and trap system interfaced to a Thermo Delta Plus XP isotope ratio mass spectrometer (McIlvin and Casciotti 2011). δ15NNOx values were calibrated against NO3- isotope reference materials USGS 32, USGS 34, and USGS 35, analyzed in parallel.