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

Dissolved organic carbon (DOC) in surface waters originates mainly from riparian soils, where several processes affect the mobilisation/immobilisation of organic compounds. Among these processes, iron reduction is thought to be of primary importance for DOC mobilisation and export to surface waters. However, this process can be inhibited by the presence of nitrate due to its higher redox potential than Fe(III), making microbial nitrate reduction thermodynamically favourable compared to iron reduction. In agricultural catchments, the groundwater is typically enriched in nitrate. Thus, rising water tables in riparian areas during the (winter) wet season may inhibit iron reduction and the subsequent DOC mobilisation in soil and surface water. In this study, we tested this hypothesis in a well-monitored agricultural catchment belonging to the OZCAR network, the so-called Kervidy-Naizin catchment (5 km²). We installed 21 zero-tension lysimeters in the riparian zone of the catchment along three transects to sample soil solution in organic-rich top soil horizons (15 cm below the soil surface), at weekly to fortnightly intervals (oct 2022 – jun 2023). We analysed DOC, nitrate, Fe(II) concentrations as well as dissolved organic matter (DOM) composition through its optical properties (3D fluorescence coupled with PARAFAC modelling) to obtain information about DOM sources and dynamics across the hydrological cycle. We found that DOC concentrations were positively correlated with Fe(II) concentrations both spatially and temporally. In contrast nitrate concentrations were negatively related to Fe(II) in the soil solutions during the winter period. These observations support the hypothesis that nitrate is an inhibitor of iron reduction and subsequent DOC mobilisation. Data on the optical properties of DOM show that the DOC mobilised by this process contains large proportions of organic molecules of microbial origin, probably derived from the processing of soil organic matter. In addition, the mobilisation of high amounts of DOC unrelated to iron reduction in some zero-tension lysimeters suggests that other controls, such as wet-dry cycles, may be equally important for sustaining organic compounds in soil solutions and surface waters.