Headwater streams as major interfaces for greenhouse gas emissions in agricultural landscapes

Accurate estimates of greenhouse gas (GHG) emissions from agricultural landscapes are essential to guide effective climate mitigation measures. While CO₂ dominates riverine GHG fluxes in terms of mass, N₂O is of particular concern due to its high global warming potential and its role as the most important ozone-depleting substance currently emitted. Although agricultural soils are recognized as major sources of both gases, the contribution of small streams draining agricultural landscapes remains poorly constrained. Indeed, headwater streams are still insufficiently characterized in terms of their biogeochemical functioning, leading some authors to describe them as an “Aqua Incognita”. Experimental studies indicate that gas exchange rates between headwater streams and the atmosphere are often underestimated, suggesting that GHG emissions from small agricultural catchments may be overlooked. Because headwater streams drain approximately 70% of the terrestrial surface, underestimating their contribution may lead to substantial biases in global assessments of GHG emissions from terrestrial and freshwater ecosystems.This study investigates the role of agricultural headwater streams in CO₂ and N₂O emissions. Dissolved CO₂ and N₂O concentrations were measured along multiple streams and across multiple spatial scales in Brittany (France), using gas chromatography with electron capture detection (GC-ECD). Measurements were combined with groundwater tracers such as radon (222Rn) and dissolved silica (DSi) to identify the origin of CO2 and N2O. In addition, in situ gas tracer experiments were conducted to quantify gas exchange rates with the atmosphere using a continuous-flow membrane inlet mass spectrometer (CF-MIMS) deployed in a mobile field laboratory.Results show that agricultural headwater streams are consistently supersaturated with both CO₂ and N₂O, with concentrations largely controlled by local groundwater discharge. Emissions of CO₂ and N₂O occur almost entirely within the first few hundred meters of the stream network due to rapid gas exchange, suggesting that downstream measurements tend to underestimate riverine GHG fluxes. By combining high-resolution field observations with regional scaling and a first-order global extrapolation, we estimate that headwater streams contribute a substantial fraction of lotic N₂O emissions. These findings identify the upper reaches of streams as critical interfaces between groundwater GHG and the atmosphere, and thus as overlooked hotspots of GHG release.