The Fate of Nitrate in Intertidal Permeable Sediments

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.

Does denitrification occur within porous carbonate sand grains?

“…The denitrification rates measured in the present study spanned the range of flow-through reactor rates (∼< 1-32 µmol L −1 h −1 ) previously observed in silicate sands (Evrard et al, 2013;Kessler et al, 2012;Marchant et al, 2014;Rao et al, 2007). The availability, and composition, of organic matter is expected to be a key factor controlling potential denitrification rates (Eyre et al, 2013a;Seitzinger, 1988), and the importance of this in permeable sediments has also been recently underscored by Marchant et al (2016), who observed a strong relationship between potential denitrification rate and sediment oxygen consumption.…”

The drivers of biogeochemistry in beach ecosystems: A cross-shore transect from the dunes to the low-water line

“…O 2 consumption rates of surface sediments (upper 5 cm) from sites 3 and 2 were determined after Marchant et al (2014) using flow through columns of 10 cm diameter and 20 cm length. The flow through columns were filled with sand while completely submerged in seawater to prevent the trapping of air bubbles.…”

“…The latter were shown to be of great importance for carbon, nutrient, and trace metal cycles in the large tidal flat area called Wadden Sea stretching from the Netherlands to Denmark, and the frequent water exchange between tidal flat areas and the North Sea transport carbon, nutrient, and trace metals to coastal waters of the North Sea (e.g. Beck et al, 2008;Billerbeck et al, 2006;Grunwald et al, 2010;Marchant et al, 2014;Moore et al, 2011;Riedel et al, 2011;Røy et al, 2008;Santos et al, 2015). Pore water discharge rates calculated for nearby sandy Wadden Sea sediments vary depending on the method applied and the sediment depth considered Moore et al, 2011;Riedel et al, 2010), among which the method most comparable to the model approach applied in our study resulted in lower flux rates (0.97 m −3 per tide and meter of shoreline, which equals~2 m −3 per day and meter of shoreline; Riedel et al, 2010) compared to our study.…”

Seasonality of Organic Matter Degradation Regulates Nutrient and Metal Net Fluxes in a High Energy Sandy Beach

“…So far, the impact of pore water advection on biogeochemistry and pore water constituent effluxes from permeable sediments has mostly been studied in microtidal sheltered beaches (Beck et al, ; Gonneea & Charette, ; Liu et al, ; O'Connor et al, ; Santos et al, ), tidal flat areas (Beck et al, ; Billerbeck et al, ; Gao et al, ; Huettel & Rusch, ; Marchant et al, ; Riedel et al, ), or subtidal sediments (Ahmerkamp et al, ; Marchant et al, ; Shum & Sundby, ), which are exposed to a lower wave energy level than high energy beaches. In contrast, only few studies have been conducted at wave exposed mesotidal to macrotidal sites like the French Aquitanian coast (Anschutz et al, ; Charbonnier et al, ; Charbonnier et al, ) or the beaches of Spiekeroog Island, Germany (Beck et al, ; Reckhardt et al, ; Seidel et al, ; Waska et al, ).…”