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The denitrification paradox: The role of O-2 in sediment N2O production

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We designed a novel laboratory sediment flux chamber in which we maintained the headspace O-2 partial pressure at preselected values, allowing us to experimentally regulate "in-situ" O-2 to evaluate its role in net N2O production by an intertidal estuarine sediment (Tyne, UK). In short-term (30 h) incubations with 10 L of overlying estuarine water (similar to 3 cm depth) and headspace O-2 regulation (headspace: sediment/ water ratio similar to 9:1), net N2O production was highest at 1.2 O-2 (sub-oxic; 32.3 nmol N2O m(-2) d(-1)), an order of magnitude higher than at either 0.0 (anoxic; 2.5 N2O nmol m(-2)) or 20.85 (ambient; 2.3 nmol N2O m(-2) d(-1)) O-2. In a longer-term sealed incubation (similar to 490 h) without O-2 control, time-dependent behaviour of N2O in the tank headspace was highly non-linear with time, showing distinct phases: (i) an initial period of no or little change in O-2 or N2O up to similar to 100 h; (ii) a quasi-linear, inverse correlation between O-2 and N2O to similar to 360 h, in which O-2 declined to similar to 2.1 and N2O rose to similar to 7800 natm; (iii) over the following 50 h a slower O-2 decline, to similar to 1.1, and a more rapid N2O increase, to similar to 12000 natm; (iv) over the next 24 h a slowed O-2 decline towards undetectable levels and a sharp fall in N2O to similar to 4600 natm; (iv) a continued N2O decrease at zero O-2, to similar to 3000 natm by similar to 490 h. These results show clearly that rapid N2O consumption (similar to 115 nmol m(-2) d(-1)), presumably via heterotrophic denitrification (HD), occurs under fully anoxic conditions and therefore that N2O production, which was optimal for sub-oxic O-2, results from other nitrogen transformation processes. In experiments in which we amended sediment overlying water to either 1 mM NH4+ or 1 mM NO3-, N2O production rates were 2-134 nmol N2O m(-2) d(-1) (NH4+ addition) and 0.4-2.2 nmol N2O m(-2) d(-1) (NO3- addition). We conclude that processes involving NH4+ oxidation (nitrifier nitrification; nitrifier denitrification; nitrification-coupled denitrification) are principally responsible for N2O production in Tyne sediments. Highest N2O production occurred under suboxic headspace (similar to 1.2 02) incubations. Anoxic sediments where HD was isolated acted as periodic N2O sinks or relatively small sources of N2O. Our experimental approach thus gives valuable insight into the O-2 effect on N2O fluxes from intertidal sediments. (C) 2017 Elsevier Ltd. All rights reserved.

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