The seasonal response of in situ denitrification and DNRA rates to increasing nitrate availability

Abstract

Climate change may dramatically increase nitrate (NO3−) production and transport from watersheds to the coastal area due to short-term heavy precipitation events. However, the response of the coastal filter to such pulse NO3− delivery is poorly studied. We measured NO3− reduction rates via denitrification and DNRA in intact sediments collected from the Goro Lagoon (Po River Delta, Italy). We contrasted 2 seasons (spring and summer) and 2 sites (with lower and higher than average lagoon salinity). We performed 15NO3− additions along a concentration series (50–2000 μM) far exceeding the annual NO3− variations (20–150 μM) to quantify nitrate reduction saturation thresholds. In both seasons and sites denitrification largely outcompeted DNRA and increased along the NO3− gradient, following Michaelis-Menten kinetics. At the site characterized by low salinity, maximum potential denitrification rates decreased from spring to summer, following the decrease in the abundance of burrowing macrofauna. DNRA followed the same seasonal trend but was not affected by NO3− addition. At the site characterized by high salinity, denitrification rates slightly increased and DNRA decreased from spring to summer, despite the establishment of chemically reduced conditions and the increase in pore water free sulfides concentration. The Goro Lagoon sediments displayed elevated buffering capacity against NO3− pulse additions. Such buffer capacity was mainly supported by denitrification through spring bioturbation, decreased summer pathlength of NO3− to the anoxic zone, and unknown mechanisms preventing sulfide toxicity and the dominance of DNRA at the high salinity site.