Redox and pH driven mobilisation of dissolved organic carbon from boreal wetlands

Boreal and subarctic wetland soils accumulated at least 550 Gt of organic carbon (OC) over the last 10,000 years, a large part of which is associated with Fe and Al (hydr)oxides as coprecipitates and via adsorption processes. Mobilisation of some of these pools via dissolved organic carbon (DOC) from soils to streams could be enhanced by reduction of ferric iron - triggered by rising water tables and oxygen depletion - via two distinct processes. Fe reduction can (i) directly release coprecipitated OC if iron (hydr)oxides are reductively dissolved and (ii) release OC by desorption from mineral surfaces if pH increases with Fe reduction, which is referred as to indirect (redox driven) mobilisation here. Both redox driven direct and indirect mobilisation likely occur under relatively wet and warm conditions such as during rewetting in the vegetation period. However, the relative importance of reductive dissolution of Fe-OC associations versus desorption of OC and its controlling factors are still unclear under field conditions as they were only investigated in the lab so far. Therefore, the relative importance of direct versus indirect mobilisation of OC and its controlling factors was studied for twelve catchments of the Krycklan research site in boreal Sweden. From long term monitoring data on stream discharges, DOC and Fe, molar DOC/Fe ratios of riparian soil waters released into the stream during rewetting of catchments in summer were computed using Generalised Additive Models. From these ratios, the relative importance of desorption for total DOC mobilisation via Fe reduction was calculated assuming a constant DOC/Fe ratio for direct mobilisation, i.e. the ratio at which OC and Fe occur in coprecipitates. DOC/Fe ratios were found to be positively correlated with average DOC concentrations in streams (coefficient of linear correlation of ρ = 0.78), and with the fraction of forest covered by spruce (ρ = 0.81). Higher reactive Fe/Al contents and hence larger mineral surfaces may be linked to spruce forest promoting intense weathering of soil’s primary minerals. Both high DOC in porewater (DOC in the stream as a proxy) and large mineral surfaces (spruce cover as a proxy) are required for desorption (indicated by relatively high DOC/ Fe ratios) to happen. If direct release of DOC with Fe reduction was accompanied by additional indirect mobilisation via a pH dependent desorption, up to twelve times more DOC was released for the same amount of Fe (hydr)oxides being reduced - compared to direct mobilisation via dissolution of iron (hydr)oxides alone. Mobilisation processes driven by Fe reduction and subsequent pH increase may intensify with climate change by enhanced drying and wetting cycles in boreal systems such as Krycklan.