Bradley D. Eyre

Estuaries play an important role in regulating nitrous oxide (N 2 O) fluxes to the atmosphere, but little is known about how catchment land‐use changes influence benthic N 2 O cycling. We measured seasonal benthic N 2 O fluxes and constructed N 2 O budgets in three sub‐tropical estuaries draining catchments with contrasting levels of land‐use intensity. Benthic habitats were a net N 2 O sink in the minimally impacted Noosa River Estuary (−287 nmol m −2 h −1 ) and a net source of N 2 O in the highly impacted Brisbane River Estuary (126 nmol m −2 h −1 ). Vegetated habitats can act as an important sink of N 2 O with uptakes of −286 and −35 nmol m −2 h −1 in the Noosa and Maroochy River Estuaries, respectively. Benthic N 2 O fluxes were significantly correlated with benthic NO 3 − fluxes, suggesting NO 3 − availability was an important control on benthic N 2 O fluxes. Combining benthic flux data with surface water N 2 O emissions measurements showed that increased benthic N 2 O fluxes helped drive increasing water–air N 2 O emissions over the land‐use intensity gradient. Overall, our results show that land‐use driven changes to both the diversity and sediment composition of benthic habitats play an important role in regulating N 2 O dynamics in estuarine ecosystems. This highlights that both sediment quality and nitrogen loading need to be considered in order to reduce emissions of greenhouse gases in the coastal ecosystems.