Role of nitrogen and iron biogeochemical cycles on the production and export of dissolved organic matter in agricultural headwater catchments

Abstract. To obtain better constraints on the control of the seasonal variations on dissolved organic carbon (DOC) export in lowland catchments, we combined monitoring of nitrates, iron, soluble phosphorus and dissolved organic matter concentration (as DOC) and composition (fluorescence) in soil and stream waters at a regular interval during one hydrological cycle. We installed 21 zero-tension lysimeters in in top soil organic-rich soil horizons (15 cm below the soil surface) in the riparian area of a well-monitored agricultural catchment in French Brittany and visited them with a weekly to biweekly frequency from October 2022 to June 2023. We observed a large increase in DOC concentrations in riparian soils during the high flow period, notably due to the establishment of Fe-reducing conditions and the subsequent release of organic molecules in soil waters. We also noted that the timing and the spatial variability of Fe(II) biodissolution in soils was regulated by nitrates from agricultural origin and the heterogeneity of water flow paths at the catchment scale. Contrary to our current understanding of DOC export in headwater catchment, our results lead us to consider the winter high flow period as an active phase of both DOC production and export in headwater catchments.