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 (δ¹⁸O and δ¹⁵N) and isotopomer (¹δ¹⁵N and ²δ¹⁵N) 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 δ¹⁸O, δ¹⁵N, ¹δ¹⁵N and ²δ¹⁵N 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 N ₂O displayed a moderate variability in δ¹⁸O and ²δ¹⁵N 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 ¹δ¹⁵N and δ¹⁸O values, but not in ²δ¹⁵N. 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.