Abstract
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 (N-2) fixation has been reported to support seagrass growth. Therefore, the role of coastal marine systems dominated by seagrasses in the net N-2 flux remains unclear. Here, we measured denitrification, anaerobic ammonium oxidation (anammox), and N-2 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 (34:9 +/- 10:3 and 31:6-8:9 mgNm(-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. The annual mean N loss was higher (between 8 and 63fold) than the N-2 fixed (annual mean D 5:9-0:2 and 0:8 +/- 0:3 mgNm(-2) d(-1)) in the meadow and bare sediment, leading to a net flux of N-2 from sediments to the atmosphere. Despite the importance of this coastal lagoon in removing N from the system, N-2 fixation can contribute substantially to seagrass growth since N-2 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 N-2 fixation increased with OM content. Denitrification and anammox increased linearly with temperature, while N-2 fixation showed a maximum at intermediate temperatures. Therefore, the forecasted warming could further increase the N-2 flux from sediments to the atmosphere, potentially impacting seagrass productivity and their capacity to mitigate climate change but also enhancing their potential N removal.