Factors controlling denitrification rates of tidal mudflats and fringing salt marshes in south-west England

Abstract

Denitrification rates were determined utilizing the acetylene blockage technique at three sites: upper mudflat, lower mudflat, and Halimione portulacoides marsh on the fringing wetlands of the Torridge River Estuary in South-west England. Denitrification rates were calculated from nitrous oxide (N 2 O) production each month for 1 year with intact sediment cores extracted at low tide (0–5 cm). In the lower and upper mudflat sites denitrification rates were low ranging from 0·52 to 5·78 μmol and 1·28 to 4·36 μmol N 2 m −2 h −1 , respectively. Denitrification rates in marsh sediments were consistently higher than those of the mudflat ranging from 2·51 to 59·00 μmol N 2 m −2 h −1 . Amending river water to sediment cores stimulated lower and upper mudflat denitrification rates approximately 10-fold up to 106·39 and 96·73 μmol N 2 m −2 h −1 , respectively. In marsh sediments, a two-fold increase in denitrification was found with river water amended resulting in a maximum rate of 114·80 μmol N 2 m −2 h −1 . During the winter months, when riverine NO 3 -N levels were at a maximum (2·47 to 2·93 mg l −1 ), denitrification rates were highest (75·24 to 114·99 μmol N 2 m −2 h −1 ) and conversely, during the summer both NO 3 -N concentrations (1·0 to 1·70 mg l −1 ) and denitrification (0·95 to 37·38 μmol N 2 m −2 h −1 ) rates were at a minimum. Mudflat sediment redox potentials (Eh), within the theoretical range of NO 3 −1 instability, were limited to the upper 5 mm, thus maximum denitrification rates may be restricted to the sediment surface. When calculating annual denitrification rates in tidal estuaries several factors should be considered including: seasonal NO 3 −1 concentrations in tidal water, tidal flooding duration and amplitude, and the depth of the aerobic/anaerobic zone of the sediment.