Widespread nitrous oxide undersaturation in farm waterbodies creates an unexpected greenhouse gas sink.

Nitrogen pollution and global eutrophication are predicted to increase nitrous oxide (NO) emissions from freshwater ecosystems. Surface waters within agricultural landscapes experience the full impact of these pressures and can contribute substantially to total landscape NO emissions. However, NO measurements to date have focused on flowing waters. Small artificial waterbodies remain greatly understudied in the context of agricultural NO emissions. This study provides a regional analysis of NO measurements in small (<0.01 km) artificial reservoirs, of which an estimated 16 million exist globally. We show that 67% of reservoirs were NO sinks (-12 to -2 μmol NO⋅m⋅d) in Canada's largest agricultural area, despite their highly eutrophic status [99 ± 289 µg⋅L chlorophyll-a (Chl-)]. Generalized additive models indicated that in situ NO concentrations were strongly and nonlinearly related to stratification strength and dissolved inorganic nitrogen content, with the lowest NO levels under conditions of strong water column stability and high algal biomass. Predicted fluxes from previously published models based on lakes, reservoirs, and agricultural waters overestimated measured fluxes on average by 7- to 33-fold, challenging the widely held view that eutrophic N-enriched waters are sources of NO.