Impact of headwater streams on N2O emissions from agricultural catchments

Mineral and organic fertilization is estimated to be responsible for 70% of N₂O emissions worldwide, a greenhouse gas which is approximately 270 times more potent than CO₂. N₂O emissions occur during biogeochemical processes of the nitrogen cycle, which take place in the various compartments of the water cycle (soil, aquifer, hyporheic zone, streams, etc.). During the transport of nitrate in the aquifer, incomplete denitrification can produce N₂O and lead to groundwater concentration of N₂O higher than the atmospheric equilibrium concentration. As groundwater then discharges into streams, excess N₂O can be released to the atmosphere⁷. N₂O can also be produced through incomplete denitrification in the hyporheic zone.

The emission of N₂O from a stream depends on the denitrification occurring in the contributing compartments but also of the rate of gas exchanges between the stream and the atmosphere. Recent studies have shown that small-scale streambed heterogeneities are hot spots for gas exchanges. Yet, they are not considered in empirical equations to calculate gas exchange rates. Empirical equations only consider global parameters of the stream (ex: slope, water velocity, depth) and overlook local hot spots for gas exchanges. This suggests that N₂O emissions from headwaters could be underestimated. Since headwater streams drain about 70% of the land surface on Earth, underestimating their rule in N₂O emissions may lead to a significant bias in the global estimation of N₂O emissions from freshwater ecosystems.

Here we investigate the rule of headwaters in the global N₂O emissions, in order to better characterize the N-cycle in headwaters and the associated greenhouse gas emissions. We measure N₂O along various headwater streams in agricultural areas using gas chromatography coupled to electron capture detection (GC-ECD). We further perform in-situ monitoring of N₂O on a few representative sites using a continuous flow membrane inlet mass spectrometer (CF-MIMS) which is brought to the field in a mobile laboratory. To trace the origin of N₂O, measurements are coupled with other tracers (nitrate, nitrite, nitrogen isotopes, radon, dissolved silica, etc.). Results reveal a large oversaturation of N₂O in agricultural headwater streams and allow to track the production and emissions of N₂O along headwater streams. This research links the disruption of biogeochemical cycles to another largely crossed planetary boundary, global warming. It therefore addresses a crucial issue of ecological transition in rural areas, the use of fertilizers, from the global perspective of greenhouse gas emissions.