High denitrification and anaerobic ammonium oxidation contributes to net nitrogen loss in a seagrass ecosystem in the central Red Sea
<p>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 (<span class="inline-formula">N<sub...
| Main Authors: | , , , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2018-12-01
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| Series: | Biogeosciences |
| Online Access: | https://www.biogeosciences.net/15/7333/2018/bg-15-7333-2018.pdf |
| Summary: | <p>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 (<span class="inline-formula">N<sub>2</sub></span>) fixation has been reported to support seagrass
growth. Therefore, the role of coastal marine systems dominated by seagrasses
in the net <span class="inline-formula">N<sub>2</sub></span> flux remains unclear. Here, we measured
denitrification, anaerobic ammonium oxidation (anammox), and <span class="inline-formula">N<sub>2</sub></span>
fixation in a tropical seagrass (<i>Enhalus acoroides</i>) meadow and the
adjacent bare sediment in a coastal lagoon in the central Red Sea. We
detected high annual mean rates of denitrification (<span class="inline-formula">34.9±10.3</span> and <span class="inline-formula">31.6±8.9</span> mg N m<span class="inline-formula"><sup>−2</sup></span> d<span class="inline-formula"><sup>−1</sup></span>) and anammox (<span class="inline-formula">12.4±3.4</span> and <span class="inline-formula">19.8±4.4</span> mg N m<span class="inline-formula"><sup>−2</sup></span> d<span class="inline-formula"><sup>−1</sup></span>) in vegetated and bare sediments. The
annual mean N loss was higher (between 8 and 63-fold) than the <span class="inline-formula">N<sub>2</sub></span>
fixed (annual mean <span class="inline-formula">=</span> <span class="inline-formula">5.9±0.2</span> and <span class="inline-formula">0.8±0.3</span> mg N m<span class="inline-formula"><sup>−2</sup></span> d<span class="inline-formula"><sup>−1</sup></span>) in the meadow and bare sediment, leading to
a net flux of <span class="inline-formula">N<sub>2</sub></span> from sediments to the atmosphere. Despite the
importance of this coastal lagoon in removing N from the system, <span class="inline-formula">N<sub>2</sub></span>
fixation can contribute substantially to seagrass growth since <span class="inline-formula">N<sub>2</sub></span>
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 <span class="inline-formula">N<sub>2</sub></span> fixation increased with OM
content. Denitrification and anammox increased linearly with temperature,
while <span class="inline-formula">N<sub>2</sub></span> fixation showed a maximum at intermediate temperatures.
Therefore, the forecasted warming could further increase the <span class="inline-formula">N<sub>2</sub></span> flux
from sediments to the atmosphere, potentially impacting seagrass productivity
and their capacity to mitigate climate change but also enhancing their
potential N removal.</p> |
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| ISSN: | 1726-4170 1726-4189 |