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, 9 (8), e104517
eCollection

The Fate of Nitrate in Intertidal Permeable Sediments

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The Fate of Nitrate in Intertidal Permeable Sediments

Hannah K Marchant et al. PLoS One.

Abstract

Coastal zones act as a sink for riverine and atmospheric nitrogen inputs and thereby buffer the open ocean from the effects of anthropogenic activity. Recently, microbial activity in sandy permeable sediments has been identified as a dominant source of N-loss in coastal zones, namely through denitrification. Some of the highest coastal denitrification rates measured so far occur within the intertidal permeable sediments of the eutrophied Wadden Sea. Still, denitrification alone can often account for only half of the substantial nitrate (NO3-) consumption. Therefore, to investigate alternative NO3- sinks such as dissimilatory nitrate reduction to ammonium (DNRA), intracellular nitrate storage by eukaryotes and isotope equilibration effects we carried out 15NO3- amendment experiments. By considering all of these sinks in combination, we could quantify the fate of the 15NO3- added to the sediment. Denitrification was the dominant nitrate sink (50-75%), while DNRA, which recycles N to the environment accounted for 10-20% of NO3- consumption. Intriguingly, we also observed that between 20 and 40% of 15NO3- added to the incubations entered an intracellular pool of NO3- and was subsequently respired when nitrate became limiting. Eukaryotes were responsible for a large proportion of intracellular nitrate storage, and it could be shown through inhibition experiments that at least a third of the stored nitrate was subsequently also respired by eukaryotes. The environmental significance of the intracellular nitrate pool was confirmed by in situ measurements which revealed that intracellular storage can accumulate nitrate at concentrations six fold higher than the surrounding porewater. This intracellular pool is so far not considered when modeling N-loss from intertidal permeable sediments; however it can act as a reservoir for nitrate during low tide. Consequently, nitrate respiration supported by intracellular nitrate storage can add an additional 20% to previous nitrate reduction estimates in intertidal sediments, further increasing their contribution to N-loss.

Conflict of interest statement

Competing Interests: The authors have declared that no competing interests exist.

Figures

Figure 1
Figure 1. Comparison of incubation methods.
Incubations were carried out using either membrane inlet mass spectrometry (MIMS) which allowed for continuous online measurements or by discrete porewater sampling into exetainers and subsequent measurement on a GC-IRMS. The example shown is taken from a replicate during the spring incubations directly after the addition of 15NO3 .
Figure 2
Figure 2. Rates of nitrogen cycling processes determined over 3 seasons.
Anaerobic dissimilatory nitrate reduction to ammonium (DNRA) and denitrification (Denit.) rates from the incubation amended with 15NO3 . Rates were determined from linear production slopes after oxygen had been consumed. Error bars are SD (n = 3)
Figure 3
Figure 3. Seasonal patterns in DNRA rates mediated by the prokaryotic and eukaryotic community.
A) Eukaryotic DNRA rates B) Prokaryotic DNRA rates C) Overall DNRA rates. Total rates were determined from the incubation amended with 15NO3 . Prokaryotic rates were determined after addition of the eukaryote inhibitor cycloheximide. Eukaryotic rates were calculated by subtracting prokaryotic rates from total rates. Error bars are SD (n = 3)
Figure 4
Figure 4. Exchange of 15NO3 and 14NO3 over time in summer incubations.
The dashed line indicates the initial amount of 15NO3 added to the incubation
Figure 5
Figure 5. Respiration of stored intracellular nitrate by eukaryotes and prokaryotes.
A) Example of the appearance of 15N labeled compounds stored in the sediment during the secondary incubation to which no label was added (example from spring incubations) B) Comparison of the overall fraction of 15N recovered as either N2 or NH4 + in both the 15NO3 incubation and the subsequent incubation with no additional 15NO3 and the partitioning between eukaryotes and prokaryotes C) Eukaryotic denitrification rates E) prokaryotic denitrification rates D) eukaryotic DNRA rates F) prokaryotic DNRA rates. Error bars are SD (n = 3).
Figure 6
Figure 6. Average percentage of 15N recovered in various pools.
Denitrification denotes 15N that was present as 29+30N2, DNRA denotes 15N that was present as 15NH4 + and intracellular storage represents 15N which was present as either 29+30N2 or 15NH4 at the termination of the secondary incubation to which no additional 15NO3 was added. The ‘missing label’ fraction refers to the discrepancy between added 15NO3 , measured 15NO3 and 15N2 production (shown in Fig. 4). Error bars are overall SD (n = 3)

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Grant support

This research was funded by the Max Planck Society. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript
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