Influence of flooding on δ 15 N, δ 18 O, 1 δ 15 N and 2 δ 15 N signatures of N 2 O released from estuarine soils—a laboratory experiment using tidal flooding chambers

The influence of flooding on N 2 O fluxes, denitrification rates, dual isotope ( δ 18 O and δ 15 N) and isotopomer ( 1 δ 15 N and 2 δ 15 N) ratios of emitted N 2 O 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 δ 18 O, δ 15 N, 1 δ 15 N and 2 δ 15 N values of the emitted N 2 O. Isotope signals did not differ between the two sites, and nitrate addition to the flooding water did not change the isotopic content of emitted N 2 O. Under non‐flooded conditions, the isotopic composition of the emitted N 2 O displayed a moderate variability in δ 18 O and 2 δ 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 δ 15 N and δ 18 O values, but not in 2 δ 15 N. Such an isotope signal has not been reported in the literature, and it could point towards an unidentified N 2 O production pathway. Its signature differed from denitrification, which was generally the N 2 O production pathway in the salt marsh and the managed retreat site. Copyright © 2004 John Wiley & Sons, Ltd.