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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.

Increased demand for rare earth elements (REE) has resulted in their increased exploitation and the need to better understand their cycling in aquatic environments. Thus far much of the research in boreal areas focused on REE cycling in larger rivers, while relatively little is known regarding their mobilization in smaller headwater streams. Here we used the Krycklan Catchment Study in northern boreal Sweden to investigate how REE are mobilized from diverse boreal headwaters and how their catchment exports are influenced by processes such as weathering, hydrology, and complexation with other solutes. We found that wetland dominated headwaters were source limited and prone to dilution during high discharge, while forested headwaters had considerably higher total REE concentrations and were less affected by discharge fluctuations. Larger downstream catchments showed clear discharge driven seasonal patterns, with high concentrations during spring flood and low concentrations during base flow. While the proportion of mineral soils and water travel time in the catchment were important predictors of REE mobilization, complexation with organic matter appears to play a greater role in higher-order streams with higher pH and a larger contribution from deeper groundwater sources. Overall, we highlight the stark differences between REE cycling in the headwaters and in higher order streams, which can provide important information on the processes that mobilize REE from catchments.

Increasing iron (Fe) concentrations have been reported for freshwaters across northern Europe over the last decades. This increase, together with elevated concentrations of dissolved organic carbon (DOC), leads to browning of freshwaters, which affects aquatic organisms, ecosystem functioning, biogeochemical cycles, and brings challenges to drinking water production. However, how such increasing trends in stream Fe concentrations reflect the contribution of different catchment sources remains poorly resolved. Here, we explored how catchment characteristics, that is, mires and coniferous soils, regulate spatial and temporal patterns of Fe in a boreal stream network. For this, we determined Fe speciation in riparian and mire soils, and studied temporal Fe dynamics in soil-water and stream-water over a span of 18 years. Positive Fe trends were found in the solution of the riparian soil, while no long-term trend was observed in the mire. These differences were reflected in stream-water, where three headwater streams dominated by coniferous cover also displayed positive Fe trends, whereas the mire dominated stream showed no trend. Surprisingly, the majority of higher order streams showed declining Fe trends, despite long-term increases in DOC. In addition, we found that an extreme drought event led to a prolonged release of Fe and DOC from the riparian soils, that could have long-term effects on stream Fe concentrations. Our results show that riparian forest soils can be major contributors to ongoing increases in freshwater Fe concentrations and that drought can further promote the release of Fe from organic soils.