N2, N2O and O2 profiles in a Tagus estuary salt marsh

Abstract

Vertical gas profiles of N2, N2O and O2 were obtained in intact sediment cores from a Tagus estuary salt marsh using membrane inlet mass spectrometry. This technique allows direct measurements of dissolved gas concentrations with minimal disturbance. O2 concentrations decreased sharply with depth, becoming undetectable below 14 mm. Denitrification products (N2 and N2O) occurred in the surface layer of the Sediment where O2 was present. Diffusion of N2 and N2O from the anaerobic zone, denitrification in anaerobic microsites and aerobic denitrification are possible explanations for this observation. N2 was the sole product of denitrification in control sediment cores probably because of the great demand for electron acceptors in this sediment. The addition of NO3- and CH3CO2- increased the concentrations of N2 and N2O in the sediment. Significantly higher concentrations in treated cores occurred between 1.5 and 2.0 cm for N2 and between 0.5 and 1.5 cm for N2O. The peak in N2 concentration occurred in the anaerobic zone of the sediment, close to the aerobic-anaerobic interface while the peak in N2O concentration occurred above this interface where concentrations of O2 were approximately 10 μM. This is indicative that, in this sediment, production of N2O is less sensitive to the presence of O2 than reduction of N2O to N2.