Large aquatic nitrous oxide emissions downstream of intensive horticulture driven by rain events

Abstract Nitrogen (N) leaching from agricultural systems is a global threat to waterways. However, indirect aquatic emissions of the potent greenhouse gas nitrous oxide (N2O) from agricultural runoff remain poorly understood. Here, we assess how episodic rain events flushing an agricultural catchment drive N, radon (a natural groundwater tracer) and N2O emissions. We reveal significant nitrate + nitrite (NOX) loads and N2O emissions consistent with N leaching and runoff into Hearnes Lake, Australia. Aquatic N loads were equivalent to ∼15 % of the applied fertiliser and dominated by NOX (78 %). Aquatic N2O (up to 5655 % sat.) and atmospheric emissions (up to 2859 µmol m2 day-1) were amongst the highest reported from global waterways. N2O fluxes experienced large variations (∼800 fold) between stagnant and rain periods. Observed N2O emissions and IPCC EF5 N2O emissions were 17.2 g N2O-N day-1 and 72.6 g N2O-N day-1, respectively. Estimated N2O emissions ∼2 km downstream of farms were 0.79 % of dissolved inorganic nitrogen (DIN) loads and 0.004 % of applied fertiliser. Aquatic N2O emissions represented ∼10 % of the total (CO2 + CH4 + N2O) CO2 equivalent emissions which is ∼60 % higher than the IPCC global N2O average. Episodic rain events, rather than groundwater seepage traced by radon, were the major drivers of both NOX and N2O, highlighting the importance of detailed sampling approaches to capture extreme emission variabilities in heavily fertilised catchments.