Biogeochemical response to drying-rewetting in riparian soils influences carbon mobilization

Organic-rich riparian soils in northern boreal landscapes are often the primary source of organic and inorganic carbon (C) to headwater streams. During extreme hydro-climatic events, such as droughts, the production and mobilization of C in these soils may be sensitive to changes in groundwater levels. Yet, the biogeochemical effects of drying and rewetting have been under-investigated in boreal riparian zones, particularly when compared to peat soils in discrete landscape components (i.e., mires). Here, we experimentally assess the response of riparian soil cores to simulated drought and rewetting and test whether mobilization of dissolved organic matter (DOM), carbon dioxide (CO2), and methane (CH4) are altered by geochemical and biological drivers over a two-month rewetting period. Drought oxidized the soil profile, upregulated activities of oxidative enzymes, and replenished terminal electron acceptors (TEAs), most notably sulfate (SO42-), which likely suppressed DOM concentrations over the short term. However, over the longer term, soil DOM mobilization increased in response to rewetting, unrelated to the intensity of experimental drought. Enzyme activity during the rewetting phase indicates that the persistent increases in DOM may be linked to microbially-mediated decomposition of organic matter following drought. By contrast, CO2 production was sensitive to drought intensity, with concentrations suppressed in soils subjected to the most extreme drying treatment. Elevated SO42- concentrations also delayed the recovery of CH4 production in soils by creating a pool of more favorable TEAs. Our results collectively show that mobilization of different C forms in riparian soils is influenced by drying-rewetting events through multiple biogeochemical mechanisms operating at different time scales. These findings have broader implications for the lateral transfer of organic and inorganic C from riparian zones to streams in response to predicted increases in climate variability.