Nitrous Oxide Fluxes from Agricultural Streams in East-Central Illinois

Indirect nitrous oxide (N₂O) emissions account for the majority of uncertainty associated with the global N₂O budget. Agricultural streams with subsurface (tile) drainage are potential hotspots of indirect N₂O emissions from streams and groundwater. However, there are only a limited number of studies with direct measurements from stream surfaces. Research presented here represents the first study of N₂O emissions from agricultural streams in Illinois, USA. We measured water chemistry data from 10 sites in three watersheds in east-central Illinois. Additionally, floating chambers and gas transfer velocity models were used to measure N₂O fluxes from the stream surface at 4 of the 10 sites. Dissolved N₂O concentrations ranged from < 0.1 to 7.46 μg N₂O-N L⁻¹. Floating chamber N₂O fluxes ranged from 0 to 13.84 μg N₂O-N m⁻² min⁻¹. We found strikingly different patterns of nitrate (NO₃−) concentrations at sites downstream of a wastewater treatment plant (WWTP) effluent. Data from sites not affected by the WWTP expressed seasonal variations of NO₃− with elevated concentrations in winter and spring months when subsurface tile drains were flowing. Floating chamber N₂O fluxes were strongly correlated (p value 0.001) with NO₃− at sites not affected by the WWTP. All sites were correlated with flow (p value 0.01) and dissolved N₂O (p value 0.02). Our data suggest flow and dissolved N₂O are stronger indicators of N₂O flux from stream surfaces than NO₃− concentrations in agricultural watersheds. Furthermore, this study supports growing concerns of estimating N₂O emissions using linear relationships between N₂O and NO₃−, such as those used in IPCC estimates.