2026 Articles

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

Choisnard, Noémie; Subramaniam, Ajit; Montoya, Joseph P.; Umbricht, Jacqueline; Voss, Maren

Nitrogen plays a key role in ecological and biogeochemical processes, influencing ecosystem productivity and health. However, the contributions of nitrogen transformations, such as nitrification and nitrate uptake on the nitrogen budget remain poorly constrained, particularly in ecosystems subject to high spatial and temporal variability, such as the Amazon River Plume. This study addresses this critical gap in understanding by exploring how these processes interact. By constructing a detailed nitrate budget during high discharge, this study reveals that the river alone does not provide enough nitrate to match nitrate uptake rates measured in the northern plume. In this region covering an area greater than 1 million km2, other processes, such as dinitrogen fixation or vertical transport of nitrate, must contribute to the process sustaining nitrate uptake in the Amazon River Plume.