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.
Highlights
Human activity has dramatically increased the amount of fixed nitrogen (N) in the environment, to the extent that anthropogenic sources contribute 156 Tg yr21, almost as much as biological nitrogen fixation in the ocean (177 Tg), and more than biological N2-fixation on land (107 Tg) [1,2]
We observed volumetric denitrification rates ranging from 8–23 mmol N m23sediment h21
Gao et al (2012) used whole core incubations percolated to 5 cm depth with 50 mmol 15NO32 L21 that were subsequently sacrificed at a given time point, and were restricted to at most 5 time points per incubation
Summary
Human activity has dramatically increased the amount of fixed nitrogen (N) in the environment, to the extent that anthropogenic sources contribute 156 Tg yr, almost as much as biological nitrogen fixation in the ocean (177 Tg), and more than biological N2-fixation on land (107 Tg) [1,2]. Nitrate storage offers an advantage to microorganisms in intertidal permeable systems, where oxygen and nitrate concentrations fluctuate frequently It can complicate stable isotope studies, as 15N-label additions to sediment incubations have been observed to cause nitrate release to the porewater [35,36], or rapid equilibrations between stored 14NO32 pools and added 15NO32 pools [22]. Diatoms, both from the water column and the microphytobenthic (MPB) layer, can contribute significantly to N-uptake and N-loss in permeable sediments [37]. Thereby we were able assess the fate of nitrate within the sediment by quantifying nitrate reduction to N2 and NH4+ by both eukaryotes and prokaryotes and determine the role that intracellular nitrate storage plays within permeable sediments
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
