DOC-dynamics in a small headwater catchment as driven by redox fluctuations and hydrological flow paths – are DOC exports mediated by iron reduction/oxidation cycles?

Abstract

Dissolved organic carbon (DOC) exports from many catchments in Europe and North-America are steadily increasing. Several studies have sought to explain this observation. As possible causes, a decrease in acid rain or sulfate deposition, concomitant reductions in ionic strength and increasing temperatures were identified. DOC often originates from riparian wetlands; but here, despite higher DOC concentrations, ionic strength in pore waters usually exceeds that in surface waters. In the catchment under study, DOC concentrations were synchronous with dissolved iron concentrations in pore and stream water. This study aims at testing the hypothesis that DOC exports are mediated by iron reduction/oxidation cycles. Following the observed hydrographs, δ<sup>18</sup>O of water and DOC fluorescence, the wetlands were identified as the main source of DOC. Antecedent biogeochemical conditions, i.e., water table levels in the wetlands, influenced the discharge patterns of nitrate, iron and DOC during an event. The correlation of DOC with pH was positive in pore waters, but negative in surface waters; it was negative for DOC with sulfate in pore waters, but only weak in surface waters. Though, the positive correlation of DOC with iron was universal for pore and surface water. The decline of DOC and iron concentrations in transition from anoxic wetland pore water to oxic stream water suggests a flocculation of DOC with oxidising iron, leading to a drop in pH in the stream during high DOC fluxes. The pore water did not per se differ in pH. There is, thus, a need to consider processes more thoroughly of DOC mobilisation in wetlands when interpreting DOC exports from catchments. The coupling of DOC with iron fluxes suggested that increased DOC exports could at least, in part, be caused by increasing activities in iron reduction, possibly due to increases in temperature, increasing wetness of riparian wetlands, or by a shift from sulfate dominated to iron reduction dominated biogeochemical regimes

A generalized Damköhler number for classifying material processing in hydrological systems

“…Inamdar et al, 2004). Knorr (2013) observed that Fe and DOC were exported to streams during rainfall events only if the preceding period allowed a pool of Fe and DOC to be created (Regime I). Inversely, after a rainfall event, a rapid transfer of Fe to the stream would require the prevention of oxidation and retention at the site (Regime II).…”

From soil water to surface water – how the riparian zone controls element transport from a boreal forest to a stream

“…In the case of carbon the significance of the riparian zone is well established when it comes to both dissolved organic carbon (DOC) and CO 2 (e.g. Fiebig et al, 1990;Hinton et al, 1998;Oquist et al, 2009;Lyon et al, 2011;Winterdahl et al, 2011;Grabs et al, 2012;Knorr, 2013;Leith, 2015;Ledesma et al, 2015). However, the special biogeochemical conditions of the riparian zone also have consequences for the transport of widely different substances such as nitrogen (Hill, 1996;Cirmo and McDonnell, 1997;Sabater et al, 2003), phosphorus (Mulholland, 1992), base cations (Ledesma et al, 2013), aluminium (Cory et al, 2007), mercury , persistent organic pollutants (Bergknut et al, 2011) and pesticides (Vidon et al, 2010).…”

Can the heterogeneity in stream dissolved organic carbon be explained by contributing landscape elements?

“…Tank et al, 2012). While the accumulation of organic matter in unforested mires makes them the major source of DOC in the boreal landscape (Rantakari et al, 2010;Ågren et al, 2007), forested areas, which generally have the greatest areal extent in the boreal biome, also contribute large DOC concentrations because of the presence of organic-rich riparian soils Knorr, 2013).…”