Spatial nitrogen attenuation supports near‐neutral air‐water N 2 O exchange in the Doñana marshes ( SW Spain)

Abstract We present a 2‐year record (2016–2018) of dissolved N 2 O, CH 4 , and biogeochemical variables across the Doñana marshes (SW Spain). Nitrate concentrations declined by 91–98% from tidal creeks to flooded marshes, with the collapse of the nitrate/total dissolved nitrogen ratio across the salinity range being inconsistent with conservative dilution. Half the samples were N 2 O under‐saturated and within the marsh interior N 2 O saturation was unrelated to salinity but associated with dissolved inorganic nitrogen, indicating biological regulation. Areal weighting of the hydrological compartments yielded an estimated air‐water N 2 O budget of 0.8 t N yr −1 (95% CI −0.5 to 2.2), approximately 6‐ to 24‐fold lower than reference scenarios that did not account for this spatial heterogeneity, while concurrent CH 4 supersaturation dominated (≥ 96%) the CO 2 ‐equivalent emissions. The marshland thus acts as an active regulator of estuarine N 2 O, and budgets neglecting internal hydrological structure can severely overestimate wetland N 2 O emissions.