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Abstract The creation and/or restoration of wetlands is an important strategy for controlling the release of reactive nitrogen (N) via denitrification, but there can be tradeoffs between enhanced denitrification and the production of nitrous oxide (N₂O), a potent greenhouse gas. A knowledge gap in current understanding of belowground wetland N dynamics is the role of gas transfer through the root aerenchyma system of wetland plants as a shortcut emission pathway for N₂O in denitrifying wetland soils. This investigation evaluates the significance of mass transfer into gas‐filled root aerenchyma for the N₂O budget in wetland mesocosms planted withSagittaria latifoliaWilld. andSchoenoplectus acutus(Muhl. ex Bigelow) Á. Löve & D. Löve. Dissolved gas tracer push–pull tests with N₂O and the nonreactive gas tracers helium, sulfur hexafluoride, and ethane were used to estimate first‐order rate constants for gas transfer into roots and microbial N₂O reduction and thereby disentangle the effects of root‐mediated gas transport from microbial metabolism on N₂O balances in saturated soils. Root‐mediated gas transport was estimated to account for up to 37% of overall N₂O removal from the wetland soils. Rates of microbial N₂O reduction varied widely based on the organic matter content of the soil media and served as a key control on the fraction of N₂O that transferred into roots. This research identifies transport through root aerenchyma as a potential shortcut pathway for N₂O emission from wetland soils and sediments and indicates that this process should be considered in both measurements and mechanistic modeling of belowground wetland N dynamics. 

Denitrification in woodchip bioreactors (WBRs) treating agricultural drainage and runoff is frequently carbon-limited due to the recalcitrance of carbon (C) in lignocellulosic woodchip biomass. Recent research has shown that redox fluctuations, achieved through periodic draining and re-flooding of WBRs, can increase nitrate removal rates by enhancing the release of labile C during oxic periods. While dying–rewetting (DRW) cycles appear to hold great promise for improving the performance of denitrifying WBRs, redox fluctuations in nitrogen-rich environments are commonly associated with enhanced emissions of the greenhouse gas nitrous oxide (N 2 O) due to inhibition of N 2 O reduction in microaerophilic conditions. Here, we evaluate the effects of oxic–anoxic cycling associated with DRW on the quantity and quality of C mobilized from woodchips, nitrate removal rates, and N 2 O accumulation in a complementary set of flow-through and batch laboratory bioreactors at 20 °C. Redox fluctuations significantly increased nitrate removal rates from 4.8–7.2 g N m −3 d −1 in a continuously saturated (CS) reactor to 9.8–11.2 g N m −3 d −1 24 h after a reactor is drained and re-saturated. Results support the theory that DRW conditions lead to faster NO 3 − removal rates by increasing mobilization of labile organic C from woodchips, with lower aromaticity in the dissolved C pool of oxic–anoxic reactors highlighting the importance of lignin breakdown to overall carbon release. There was no evidence for greater N 2 O accumulation, measured as N 2 O product yields, in the DRW reactors compared to continuously saturated reactors. We propose that greater organic C availability for N 2 O reducers following oxic periods outweighs the effect of microaerophilic inhibition of N 2 O reduction in controlling N 2 O dynamics. Implications of these findings for optimizing DRW cycling to enhance nitrate removal rates in denitrifying WBRs are discussed.