Denitrification rates, net nitrate uptake rates, and nitrate penetration depths under advective conditions in sand from the Janssand, Germany intertidal flat

We collected sediment cores from the Janssand intertidal sand flat in Germany in April 2016, August 2016, and February 2017. All measurements were made in laboratory experiments using NOx (nitrate + nitrite) microbiosensors and oxygen microelectrodes. Cores were kept at in situ temperatures with a simulated day/night cycle and regular flushing with site seawater to simulate natural porewater advection during periods of inundation. We determined the depth of maximum microphytobenthos photosynthetic activity from steady-state oxygen profiles. We then flushed site seawater down through the core to measure net NOx and oxygen consumption rates at that depth. We measured potential denitrification rates at high spatial resolution by flushing acetylene- and nitrate-amended site seawater down through the core and measuring the accumulation of nitrous oxide using a microelectrode. Finally, we applied a range of realistic downward advective flows of seawater into sediment, in both the light and the dark, in both the spring and summer. We measured the depth to which NOx and oxygen penetrated over the course of several hours under these steady flow conditions using NOx microbiosensors and oxygen microelectrodes. from our study site, NOx always reached the depths of maximum denitrification potential, regardless of light availability or season