Search for: All records

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.