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 (N2) fixation has been reported to support seagrass
growth. Therefore, the role of coastal marine systems dominated by seagrasses
in the net N2 flux remains unclear. Here, we measured
denitrification, anaerobic ammonium oxidation (anammox), and N2
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 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. The
annual mean N loss was higher (between 8 and 63-fold) than the N2
fixed (annual mean = 5.9±0.2 and 0.8±0.3 mg N m−2 d−1) in the meadow and bare sediment, leading to
a net flux of N2 from sediments to the atmosphere. Despite the
importance of this coastal lagoon in removing N from the system, N2
fixation can contribute substantially to seagrass growth since N2
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 N2 fixation increased with OM
content. 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 flux
from sediments to the atmosphere, potentially impacting seagrass productivity
and their capacity to mitigate climate change but also enhancing their
potential N removal.
Faculty Opinions recommendation of Massive nitrogen loss from the Benguela upwelling system through anaerobic ammonium oxidation.
