Pathways and drivers of N2O, CH4, CO2along a soil–aquifer–stream–atmosphere continuum in the Orgeval headwater catchment, France

Globally, direct emissions from agricultural activities and land-use changes account for 23 % of the total greenhouse gas (GHG) emissions. In addition, dissolved GHGs leaching from soils via groundwater into streams and rivers represent important sources of indirect emissions also contributing to global warming. Over a 5-year period (2019–2023), we conducted monthly measurements of dissolved gas concentrations in water (N 2 O, CH 4 , CO 2 ) and ancillary variables (e.g., DOC, NO 3 − ) in piezometer transects and adjacent streams. This continuum was studied in a small catchment underlain by carbonate geology and subject to intensive agriculture. The hydrological model HYPE was used to determine two components of stream flow: subsurface runoff with less mineralized upslope water was considered fast flow; aquifer groundwater flow with more mineralized downslope water was considered slow flow. Using this approach for the 43 km 2 Avenelles sub-catchment, we calculated a mass balance for the three gases, leading to headwater emissions of 55 kg N 2 O-N y −1 , 41 kg CH 4 -C y −1 , and 145 t CO 2 -C y −1 . Due to rapid degassing from surface water, only ∼ 13 % of the N 2 O and CH 4 in groundwater were found in surface water, and ∼ 6 % for CO 2 . Indirect emissions from streams thus represented hardly 1 % of direct soil emissions in this headwater catchment subject to intensive cropping, where active riparian wetlands have been considerably reduced due to extensive hydraulic restructuring. Major factors controlling N 2 O and CH 4 emissions were seasonal (mainly temperature and hydrology) as well as chemical (e.g., NO 3 −, NO 2 − , DOC:NO 3 − , O 2 ) reflecting their biogenic origin, while geogenic (pH) and biogenic (DOC: NO 3 − ) processes appeared key drivers of CO 2 .