Terrestrial nitrate and nitrification cannot be the only sources for nitrate assimilation in the Amazon River Plume margins (Version 1.0)

Choisnard, Noémie; Voss, Maren

Abstract. The Northern Amazon River Plume plays a crucial role in carbon sequestration and nitrogen cycling, influenced by NO₃^- uptake and dinitrogen fixation (N₂ fixation). Building on previous work, this study quantified NO₃^- budgets along the plume, revealing significant NO₃^- uptake exceeding inputs from riverine sources and nitrification beyond 5°N. NO₃^- 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 vertical salinity gradients prevent diffusive NO₃^- inputs, this drop creates a NO₃^- deficit (~27 Tg N y^-1). NO₃^- uptake in the northern plume thus must rely on additional NO₃^- sources, such as N₂ fixation, which produces organic N and ammonium. However, the lack of increased nitrification suggests unexplored pathways or dynamics. This comprehensive assessment provides a robust framework for future investigations into nitrogen dynamics and their broader ecological and environmental significance.

Scientific purpose: Data is generated for a publication in a scientific Journal evaluating the nutrient sources in the Amazon River plume. It shows budgets generated from field data from the M174 cruise. It includes nutrient concentrations and abiotic variables as well as integrated rates of nitrification, nitrate and inorganic carbon uptake through the mixed layer.

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