Respiration and denitrification in permeable continental shelf deposits on the South Atlantic Bight: N2:Ar and isotope pairing measurements in sediment column experiments

Mechanisms of N removal in permeable sediments were examined using sediment columns packed with southeastern US continental shelf sands. While significant N2 production was observed in oxic SAB sands, the addition of 20 μM 15NO3− tracer in oxic sediment columns for periods of up to 12 days yielded a delayed production of 29N2 and 30N2 which, at the end of the incubation period, was still small relative to previously measured total N2 production rates. Possible explanations for these results include N removal via tightly coupled nitrification–denitrification in microenvironments isolated from the bulk porewater or denitrification mediated by aerobic denitrifiers. The addition of 15NO3− to porewater at levels above natural concentrations caused N assimilation and increases in sediment respiration rates in oxic columns, indicating N limitation. Delays in achieving steady-state denitrification rates following disturbances may be associated with deep penetration of O2 in advective regimes and amplified by isolation of respiration and denitrification in reactive microenvironments. Isolation of microzone denitrification, delays in achieving steady-state conditions, and the response of sediment community respiration and denitrification rates to increases in low background NO3− may interfere with the accuracy of denitrification rate measurements in sandy sediments on oligotrophic continental shelves with isotope pairing and traditional core incubation techniques. In columns with anoxic outflow, the immediate appearance of 29N2 and 30N2 following 15NO3− addition indicates that denitrification in suboxic or anoxic sands relies on external sources of NO3− rather than only on in situ nitrification, which is the main NO3− source for denitrification in oxic SAB sands.