Influence of flooding on δ15N, δ18O, 1δ15N and 2δ15N signatures of N2O released from estuarine soils -: a laboratory experiment using tidal flooding chambers

The influence of flooding on N2O fluxes, denitrification rates, dual isotope (delta(18)O and delta(15)N) and isotoporner ((1)delta(15) N and (2)delta(15)N) ratios of emitted N2O from estuarine intertidal zones was examined in a laboratory study using tidal flooding incubation chambers. Five replicate soil cores were collected from two differently managed intertidal zones in the estuary of the River Torridge (North Devon, UK): (1) a natural salt marsh fringing the estuary, and (2) a managed retreat site, previous agricultural land to which flooding was restored in summer 2001. Gas samples from the incubated soil cores were collected from the tidal chamber headspaces over a range of flooding conditions, and analysed for the delta(18)O, delta(15)N, (1)delta(15)N and (2)delta(15)N values of the emitted N2O. Isotope signals did not differ between the two sites, and nitrate addition to the flooding water did not change the isotopic content of emitted N2O. Under non-flooded conditions, the isotopic composition of the emitted N2O displayed a moderate variability in delta(18)O and (2)delta(15)N delta values that was expected for microbial activity associated with denitrification. However, under flooded conditions, half of the samples showed strong and simultaneous depletions in (1)delta(15) N and delta(18)O values, but not in (2)delta(15)N. Such an isotope signal has not been reported in the literature, and it could point towards an unidentified N2O production pathway. Its signature differed from denitrification, which was generally the N2O production pathway in the salt marsh and the managed retreat site. Copyright (C) 2004 John Wiley Sons, Ltd.