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.