Redox induced mobilisation of DOC from riparian wetlands (ReDOCs)

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Riparian wetlands are major sources of dissolved organic carbon (DOC) to streams. Increasing DOC concentrations were observed for many northern streams during the last decades, with potential implications for carbon (C) storages of wetland soils and streamwater quality. Drivers behind these trends, and particularly the significance of redox processes in wetland soils, are still incompletely understood. In soils, organic C is often associated with or bound to iron (oxy) hydroxides. These associations of iron (Fe) and organic C may immobilise and protect soil organic matter from mineralisation under oxic conditions. However, organic C can be remobilised if ferric Fe is reduced under anoxic conditions or if redox mediated changes in pH increase its solubility. Redox processes are therefore presumably important drivers of DOC dynamics in both wetland soils and the adjacent streams. While Fe-C-associations and their response to redox processes were investigated for mineral soils, we lack information about riparian organic soils and at catchment scales. To address this knowledge gap, experimental catchment studies are needed that trace these processes from the source areas of DOC to the stream network; and modelling techniques able to extract causal links from observational data need to be applied. The objective of this project is thus to mechanistically understand the interplay between DOC, Fe, and pH for boreal and temperate catchments featuring riparian wetlands. We hypothesise that in-stream DOC concentrations are mainly driven by redox conditions within riparian organic soils, where DOC mobilisation is controlled by reduction of DOC associated Fe, and by redox mediated changes in pH. We further propose that these DOC mobilising redox conditions are controlled by fluctuations of water tables and temperature. We propose to combine (A) the investigation of water along wetland-to-stream transects, looking at DOC molecular signatures that indicate specific mobilisation processes and (B) novel causal discovery methods to trace mobilisation and transport processes of DOC across scales. The proposed study site is the Krycklan catchment in northern Sweden, providing long data records and excellent field infrastructure for the proposed experimental and modelling work. Process understanding obtained at Krycklan is planned to be transferred to German catchments using Bayesian networks. At the latter sites, complementary research is conducted and a collaboration is intended. The 3-year project shall be conducted with a doctoral student in a cooperation of Uni Münster, BHT Berlin, UFZ Leipzig/Magdeburg, the Swedish Uni of Agricultural Sciences in Umeå, and Uni Bayreuth. While the student focuses on experimental work and data analysis in Krycklan and the labs at Münster/Leipzig, PI Selle (BHT) models monitoring and experimental data, and transfers models from Krycklan to German catchments. The entire team ultimately integrates knowledge and creates a synthesis.