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. Nitrogen loads in coastal areas have increased dramatically, with detrimentalconsequences for coastal ecosystems. Shallow sediments and seagrass meadowsare hotspots for denitrification, favoring N loss. However, atmosphericdinitrogen (N2) fixation has been reported to support seagrassgrowth. Therefore, the role of coastal marine systems dominated by seagrassesin the net N2 flux remains unclear. Here, we measureddenitrification, anaerobic ammonium oxidation (anammox), and N2fixation in a tropical seagrass (Enhalus acoroides) meadow and theadjacent bare sediment in a coastal lagoon in the central Red Sea. Wedetected high annual mean rates of denitrification (34.9±10.3 and 31.6±8.9 mg N m−2 d−1) and anammox (12.4±3.4 and 19.8±4.4 mg N m−2 d−1) in vegetated and bare sediments. Theannual mean N loss was higher (between 8 and 63-fold) than the N2fixed (annual mean = 5.9±0.2 and 0.8±0.3 mg N m−2 d−1) in the meadow and bare sediment, leading toa net flux of N2 from sediments to the atmosphere. Despite theimportance of this coastal lagoon in removing N from the system, N2fixation can contribute substantially to seagrass growth since N2fixation rates found here could contribute up to 36 % of plant Nrequirements. In vegetated sediments, anammox rates decreased with increasingorganic matter (OM) content, while N2 fixation increased with OMcontent. Denitrification and anammox increased linearly with temperature,while N2 fixation showed a maximum at intermediate temperatures.Therefore, the forecasted warming could further increase the N2 fluxfrom sediments to the atmosphere, potentially impacting seagrass productivityand their capacity to mitigate climate change but also enhancing theirpotential N removal.

High denitrification and anaerobic ammonium oxidation contributes to net nitrogen loss in a seagrass ecosystem in the central Red Sea