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

Garcias-Bonet, Neus; Fusi, Marco; Ali, Muhammad; Shaw, Dario R.; Saikaly, Pascal E.; Daffonchio, Daniele; Duarte, Carlos M.

Nitrogen loads in coastal areas have increased dramatically, with detrimental consequences for coastal ecosystems. Shallow sediments and seagrass meadows are hotspots for denitrification, favoring N loss. However, atmospheric dinitrogen (inline-formulaN2) fixation has been reported to support seagrass growth. Therefore, the role of coastal marine systems dominated by seagrasses in the net inline-formulaN2 flux remains unclear. Here, we measured denitrification, anaerobic ammonium oxidation (anammox), and inline-formulaN2 fixation in a tropical seagrass (Enhalus acoroides) meadow and the adjacent bare sediment in a coastal lagoon in the central Red Sea. We detected high annual mean rates of denitrification (inline-formula34.9±10.3 and inline-formula31.6±8.9 mg N minline-formula−2 dinline-formula−1) and anammox (inline-formula12.4±3.4 and inline-formula19.8±4.4 mg N minline-formula−2 dinline-formula−1) in vegetated and bare sediments. The annual mean N loss was higher (between 8 and 63-fold) than the inline-formulaN2 fixed (annual mean inline-formula=inline-formula5.9±0.2 and inline-formula0.8±0.3 mg N minline-formula−2 dinline-formula−1) in the meadow and bare sediment, leading to a net flux of inline-formulaN2 from sediments to the atmosphere. Despite the importance of this coastal lagoon in removing N from the system, inline-formulaN2 fixation can contribute substantially to seagrass growth since inline-formulaN2 fixation rates found here could contribute up to 36 % of plant N requirements. In vegetated sediments, anammox rates decreased with increasing organic matter (OM) content, while inline-formulaN2 fixation increased with OM content. Denitrification and anammox increased linearly with temperature, while inline-formulaN2 fixation showed a maximum at intermediate temperatures. Therefore, the forecasted warming could further increase the inline-formulaN2 flux from sediments to the atmosphere, potentially impacting seagrass productivity and their capacity to mitigate climate change but also enhancing their potential N removal.

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Garcias-Bonet, Neus / Fusi, Marco / Ali, Muhammad / et al: High denitrification and anaerobic ammonium oxidation contributes to net nitrogen loss in a seagrass ecosystem in the central Red Sea. 2018. Copernicus Publications.

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