Riverine nitrate and nitrification cannot be the only sources for nitrate assimilation in the Amazon River Plume margins

Abstract The Northern Amazon River Plume plays a crucial role in carbon sequestration and nitrogen cycling. This study quantified NO 3 − budgets along the plume during a period of high discharge, revealing NO 3 − uptake rates in excess of inputs from riverine sources and nitrification beyond 5°N. NO 3 − availability decreased from ~ 60 mmol m −2 near the river mouth to < 10 mmol m −2 in the northern plume, while nitrification rates dropped sharply. Since salinity gradients prevent diffusive NO 3 − inputs to the plume, this drop creates a NO 3 − deficit (23.6–50.3 × 10 8 mol d −1 ). NO 3 − uptake in the northern plume must rely on additional NO 3 − sources, for example, N 2 fixation, which produces organic N and ammonium. However, the lack of increased nitrification from NH 4 + suggests unexplored pathways or dynamics. These results are crucial for our understanding and as a baseline of the Amazon River as a NO 3 − ‐source supporting primary production in the northern plume.