The deficit of nitrogen (N) over phosphate (PO₄) stimulates strong nitrogen fixation in the Amazon River plume water of the Western Tropical Atlantic (WTA), but it is not well established how the offshore-spreading plume water becomes N deficient over P. Sediment denitrification which removes bioavailable N along the southern North Brazil Shelf (sNBS) might play a crucial role in enhancing the N deficit. To examine the modification of the N:P ratios by sediment denitrification, we conduct factorial simulations with a high-resolution regional ocean model system (ROMS) coupled to a biogeochemical elemental cycling model (BEC). Modelling results show that sediment denitrification removes 52.3 Gmol N yr^–1 of fixed nitrogen over the sNBS region, lowering the nitrate (NO₃) concentration in bottom waters and its upward fluxes into the surface layer with an increase in the N deficit over PO₄ of by ~320%. Accordingly, the N deficit increases by 25% in the offshore-spreading plume to the WTA. The increased N deficit facilitates the growth of diazotrophs or their assemblages with diatoms (DDAs) on the sNBS and in the offshore plume region in the WTA, enhancing biological nitrogen fixation by 12%, especially on the outer sNBS region. Our study demonstrates that sediment denitrification on the sNBS is an important precursor process for marine nitrogen fixation via driving an enhanced N deficit over PO₄ both in the upwelled subsurface waters and offshore-spreading plume and therefore enhancing the niche for marine N-fixers in the area.

sediment denitrification,nitrate anomaly,nitrogen fixation,the Amazon plume