Nitrogen cycling in the deep sedimentary biosphere: nitrate isotopes in porewaters underlying the oligotrophic North Atlantic
Nitrogen (N) is a key component of fundamental biomolecules. Hence, its cycling and availability are central factors governing the extent of ecosystems across the Earth. In the organic-lean sediment porewaters underlying the oligotrophic ocean, where low levels of microbial activity persist despite...
| Main Authors: | , , , , |
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| Format: | Article |
| Language: | English |
| Published: |
Copernicus Publications
2015-12-01
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| Series: | Biogeosciences |
| Online Access: | http://www.biogeosciences.net/12/7483/2015/bg-12-7483-2015.pdf |
| Summary: | Nitrogen (N) is a key component of fundamental biomolecules. Hence, its cycling and availability are central factors governing the extent of
ecosystems across the Earth. In the organic-lean sediment porewaters
underlying the oligotrophic ocean, where low levels of microbial activity
persist despite limited organic matter delivery from overlying water, the
extent and modes of nitrogen transformations have not been widely
investigated. Here we use the N and oxygen (O) isotopic composition of
porewater nitrate (NO<sub>3</sub><sup>−</sup>) from a site in the oligotrophic North
Atlantic (Integrated Ocean Drilling Program – IODP) to determine the extent and magnitude of microbial nitrate
production (via nitrification) and consumption (via denitrification). We find
that NO<sub>3</sub><sup>-</sup> accumulates far above bottom seawater concentrations
(~ 21 μM) throughout the sediment column (up to
~ 50 μM) down to the oceanic basement as deep as 90 m b.s.f. (below sea floor),
reflecting the predominance of aerobic nitrification/remineralization within
the deep marine sediments. Large changes in the δ<sup>15</sup>N and
δ<sup>18</sup>O of nitrate, however, reveal variable influence of nitrate
respiration across the three sites. We use an inverse porewater
diffusion–reaction model, constrained by the N and O isotope systematics of
nitrification and denitrification and the porewater NO<sub>3</sub><sup>-</sup> isotopic
composition, to estimate rates of nitrification and denitrification
throughout the sediment column. Results indicate variability of reaction
rates across and within the three boreholes that are generally consistent
with the differential distribution of dissolved oxygen at this site, though
not necessarily with the canonical view of how redox thresholds separate
nitrate regeneration from dissimilative consumption spatially. That is, we
provide stable isotopic evidence for expanded zones of co-occurring
nitrification and denitrification. The isotope biogeochemical modeling also
yielded estimates for the δ<sup>15</sup>N and δ<sup>18</sup>O of newly produced
nitrate (δ<sup>15</sup>N<sub>NTR</sub> (NTR, referring to nitrification) and δ<sup>18</sup>O<sub>NTR</sub>), as well as the
isotope effect for denitrification (<sup>15</sup>ϵ<sub>DNF</sub>) (DNF, referring to denitrification), parameters
with high relevance to global ocean models of N cycling. Estimated values of
δ<sup>15</sup>N<sub>NTR</sub> were generally lower than previously reported
δ<sup>15</sup>N values for sinking particulate organic nitrogen in this region. We suggest that these
values may be, in part, related to sedimentary N<sub>2</sub> fixation and
remineralization of the newly fixed organic N. Values of
δ<sup>18</sup>O<sub>NTR</sub> generally ranged between −2.8 and 0.0 ‰,
consistent with recent estimates based on lab cultures of nitrifying
bacteria. Notably, some δ<sup>18</sup>O<sub>NTR</sub> values were elevated,
suggesting incorporation of <sup>18</sup>O-enriched dissolved oxygen during
nitrification, and possibly indicating a tight coupling of NH<sub>4</sub><sup>+</sup> and
NO<sub>2</sub><sup>−</sup> oxidation in this metabolically sluggish environment. Our
findings indicate that the production of organic matter by in situ autotrophy
(e.g., nitrification, nitrogen fixation) supplies a large fraction of the
biomass and organic substrate for heterotrophy in these sediments,
supplementing the small organic-matter pool derived from the overlying
euphotic zone. This work sheds new light on an active nitrogen cycle
operating, despite exceedingly low carbon inputs, in the deep sedimentary
biosphere. |
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| ISSN: | 1726-4170 1726-4189 |