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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.

Microbial communities play a fundamental role in lake nutrient cycling, yet their composition and functional diversity in response to environmental gradients remain poorly understood. Specifically, little is known about how the supply of dissolved macronutrients, including inorganic and bioavailable organic fractions, shape microbial community structure, and functional diversity in lakes that are strongly subsidized by terrestrial inputs. Boreal lakes, with varying concentrations of total and bioavailable dissolved organic carbon (DOC), nitrogen (N) and phosphorus (P), provide an ideal setting to investigate these dynamics. Here, we hypothesize that microbial pathways related to N and P acquisition, as inferred from marker-gene data, are more represented under relative deficiency of available N and P resources, respectively. To test this, we analysed the rRNA-inferred microbial community composition and metabolic functional diversity across 34 south-Swedish lake outlets in relation to bioavailable nutrient supply. Results show that DOC and P were key drivers of microbial community structure, with bulk DOC concentrations being most relevant for bacteria (16S rRNA), while bioavailable fractions of DOC and P were relatively more influential for eukaryotic communities (18S rRNA). Predicted N- and P-related metabolic pathways correlated with nutrient ratio imbalances, supporting our hypothesis that microbial communities adjust their metabolic strategies in response to relative nutrient demand. These findings demonstrate that accounting for nutrient ratios and bioavailability, in addition to bulk concentrations, helps provide an improved mechanistical understanding of microbial functional potentials in lakes.

Carbon dioxide (CO₂) evasion and downstream export of carbon (C) from headwater streams represent important fluxes in the global C cycle. Yet, these fluxes are generally studied in isolation, leaving gaps in the understanding of the overall role of streams in the C cycle. In this study, we carried out high resolution measurements of dissolved inorganic and organic C to estimate CO₂ evasion and C export along a 400 m reach of a boreal headwater stream to assess the magnitude and control of the C evasion:export ratio. Higher downstream C export (3.1–74.0 kg C d⁻¹) compared to CO2 evasion rates (0.53–2.56 kg C d⁻¹) for the full stream network over the open water season resulted in an average C evasion:export ratio of 0.23, which corresponds to a 17% loss of C entering the stream through CO2 evasion. The temporal variation in C evasion:export ratios (0.03–0.60) was mainly driven by stream discharge, largely through its strong influence on downstream C export. Further, CO₂ evasion showed high spatial variability, and we demonstrate that using only data of a subset of the stream reach would lead to a wide range in the overall C evasion:export ratios upscaled to the whole stream network. Resolving the processes controlling spatial and temporal variation in C fluxes and understanding the importance of discharge for the fate of C routed through streams is crucial for predicting the terrestrial C sink capacity at high latitudes under a changing climate.

Decades of forest management in the boreal biome have involved digging drainage ditches and modifying streams to increase timber production, altering terrestrial ecosystems and expanding stream networks. Modified waterways and drainage ditches represent widespread novel aquatic and riparian ecosystems, with little known about their biodiversity. With approximately 68% of small waterways in Sweden human-made or modified, improved knowledge is needed to inform effective management of biodiversity of these often-disregarded habitats. Currently, ditches and modified waterways may undergo ditch network maintenance, involving the removal of debris and vegetation, with less emphasis placed on protecting them with vegetated buffers compared to natural waterways. In this study, we surveyed riparian vegetation across soil types (till and peat) along a drainage size gradient (0.5–60 ha) in a northern boreal catchment, assessing the riparian vegetation among ditches, modified waterways, and “natural” streams. We found that the vegetation in this catchment did not differ significantly between modified waterways and streams on till soils, highlighting an important role for modified waterways in maintaining riparian vegetation diversity. Till ditches exhibited similar community composition, although significantly lower diversity, compared to modified waterways and streams on till. By comparison, peat ditches harbored less vegetation diversity and exhibited lower species turnover and different community composition, driven by variation in soil conditions and catchment size. Our study demonstrates that plant species diversity and richness increase with catchment area along a ditch to stream gradient, highlighting a pattern well established in natural systems but underexplored in the context of artificial and modified waterways. To enhance vegetation diversity in boreal landscapes, conservation planning should include modified waterways on till soils. Management recommendations include avoiding ditch maintenance in self-eroding till systems and prioritizing the rewetting of sites with peat ditches and small catchments. Rather than a uniform restoration approach, our results advocate for catchment-scale planning and tailored ecological endpoints to maximize biodiversity and ecosystem resilience.

Climate-induced permafrost thaw unlocks large stores of organic carbon that are mineralized and emitted as carbon dioxide (CO2) from rivers to the atmosphere1. Concurrently, warming and permafrost thaw can increase mineral weathering rates, thus affecting the release and sequestration of inorganic carbon2, 3–4. Yet how these biological and geological carbon cycles interact and jointly affect CO2 dynamics (emission compared with drawdown) in permafrost rivers remains unknown5. Here we combine CO2 emissions, organic and inorganic solute concentrations, dual carbon isotopes (δ13C–Δ14C) and geochemical modelling to infer how permafrost thaw may affect river biogeochemistry over decades to centuries across the Qinghai–Tibet Plateau. Leveraging a gradient of thermal permafrost degradation, we find that river CO2 emissions decline, whereas solute fluxes from rock weathering increase with decreasing permafrost cover. Across this region, net CO2 drawdown fluxes from rock weathering are about 35% of river CO2 emissions, varying from around 15% in catchments with continuous permafrost to more than 100% in catchments with discontinuous or isolated permafrost. Thus, carbon fluxes from chemical weathering may become increasingly important with ongoing permafrost thaw, potentially even outpacing river CO2 emissions. Our findings disentangle the interplay between biological and geological carbon fluxes that are important for the cryosphere and the global carbon cycle.

Riparian zones are important ecological interfaces, acting as control points for biogeochemical cycling in landscapes. Yet we know relatively little about how the local hydrogeomorphic structure of riparian zones shapes the belowground microbial processes that underpin C and nutrient cycles. Here we assessed how topographically driven variation in riparian hydrogeomorphology along a boreal stream influences resource accumulation, microbial biomass and community composition, and extracellular enzyme activity in soils at the land-water interface. We found that riparian interfaces with lower average groundwater levels supported soils with greater organic matter content and capacity to generate solutes at the land-water interface. By contrast, microbial biomass in soils was elevated at interface sites with the greatest variability in groundwater level, whereas the fungal:bacterial ratio was lowest at sites with persistently high groundwater levels. Extracellular enzyme activities also varied with local hydrogeomorphology, but these responses were distinct among targeted enzymes. Specifically, patterns for some enzymes (beta-glucosidase and protease) were linked to soil properties (e.g., soil % loss on ignition, C:N), whereas others (cellulase and peroxidase) were more influenced by local hydrological variability. Collectively, our study shows how variation in the hydrogeomorphic template can drive heterogeneity in the capacity of riparian soils to store resources and support microbial activity along small streams. In combination with variation in local hydrologic connectivity, this heterogeneity adds complexity to the mechanisms regulating solute production, transformation, and exchange at the land-water interface.

Leaching – the release of elements from organic matter through dissolution in water – plays an important role in biogeochemical cycling and ecosystem processes. However, our limited understanding of the patterns and underlying drivers of element solubility in leaves hinders accurate predictions of leaching over space and time in terrestrial ecosystems. In this study, we quantify the solubility of carbon (C), nitrogen (N) and phosphorus (P) from leaves of Betula pubescens – a widespread boreal tree species – across a post-fire retrogressive chronosequence. We then relate solubility to variation in leaf-level traits and ecosystem properties (e.g. soil chemistry, tree density and productivity) across the chronosequence to quantify micro- and macro-scale determinants of leaching. We find that P is much more soluble than C and N and is released in solution mainly in readily accessible mineral form. Solubility patterns are strongly related to foliar chemical and structural traits, particularly for green leaves. Metrics related to ecosystem properties exert a stronger influence over solubility from senesced leaf litter. Overall, our results indicate that leaching could constitute an important flux of nutrients to the soil, particularly for P. The rate and spatio-temporal pattern of this leaching flux may be predicted from foliar traits and ecosystem properties. Further application of the method should allow for rapid integration of leaching-related foliar traits into broader plant trait frameworks and models of ecosystem biogeochemical cycling.

In boreal landscapes, forest management has the potential to become a major driver of surface water quality due to the large proportion of actively-used land areas and the intensity of forestry operations. In Fennoscandia, forest management is comprised of different operations during a single rotation, where final harvest by clear cutting and subsequent ditch cleaning to restore drainage capacity are among the most influential on water quality. Here, we analyzed the single and combined effect of these forest management operations on the concentrations and exports of dissolved organic carbon (DOC), dissolved organic nitrogen (DON), dissolved inorganic nitrogen (DIN) and dissolved inorganic phosphate (PO4) in boreal Sweden. We measured groundwater table level, stream discharge, and water chemistry data continuously following experimental clear cutting and ditch cleaning applied to a historically drained forest using a before-after-control-impact (BACI) design. We used linear mixed models to test whether DOC, DON, DIN and PO4 concentrations were affected after each individual forest management operation, and further analyzed the response of the cumulative operations. We found that after clear cutting, concentrations of organic and inorganic nutrients increased significantly. However, for catchments with ditch cleaning after clear cutting, concentrations of organic nutrients in surface water decreased to pre-disturbance levels; inorganic nutrient concentrations also decreased but less strongly than organic counterparts. Despite this effect, catchments with ditch cleaning after clear cutting still showed an increase in overall organic and inorganic nutrient exports when compared to the reference catchments and the pre-treatment period. Nevertheless, catchments without ditch cleaning showed an even higher increase in both concentration and exports of most solutes. Overall, our results suggest changes in C, N and P exports due to forest management, along with the large spatial extent of this activity, could promote biogeochemical shifts and trigger water quality deterioration in boreal streams.

Dissolved organic matter (DOM) is a major source of macronutrients to freshwaters, yet it has variable and poorly understood bioavailability. Because intrinsic variation in bioavailability is caused by chemical structures of organic nutrients, DOM composition data should improve predictions of bioavailable resource pool sizes. We hypothesized that bioavailable organic carbon (C) and nitrogen (N) fractions are made up of freshly produced humic- and protein-like DOM, respectively, whereas bioavailable phosphorus (P) is linked to microbially-derived DOM with potential organophosphate content and/or to humic-like structures associated with DOM-Fe-phosphate complexes. These hypotheses were tested from surface waters collected at eight, unproductive and organic matter-rich stream and lake sites, from which we performed C, N and P microbial bioassays with flow cytometry in combination with analyses of DOM composition using fluorescence excitation-emission matrix (EEM) analysis. Bioavailable C followed the predicted patterns, with strong links to fluorescent features indicating recently produced DOM. Surprisingly, bioavailable N was relatively poorly related to DOM composition, including protein-like fluorescence, and was instead driven mainly by the amount of inorganic N. The bioavailable P showed patterns in support of the hypothesized link to microbially-derived organic components, whereas its relationships to free or complex-bound forms of inorganic phosphate were inconclusive. Thus, the strength of the hypothesized patterns varied. Nonetheless, in addition to the variability in bioavailable nutrient concentrations explained by standard bulk nutrient variables, we show that DOM composition variables made significant and unique contributions to explaining the variance in bioavailable C (19%), N (13%) and P (18%). Therefore, improved regression models for bioavailable nutrient concentrations could be achieved by including DOM composition among the explanatory variables. Overall, DOM composition analysis is a promising tool to improve prediction and develop our understanding of bioavailable macronutrients in organic matter-rich freshwaters.

Coastal eutrophication results from increased riverine loads of inorganic nutrients, including phosphorus (P), which may co-occur with increased dissolved organic carbon (DOC) loading. These DOC molecules are often pigmented, causing water darkening, but they also contain dissolved organic P (DOP), which could exacerbate eutrophication. However, it is unclear how the bioavailable DOP (BDOP) pool responds to the individual and interactive effects of increased DOC, higher inorganic nutrient concentrations, and water darkening in coastal ecosystems. To explore these interactions, we conducted bioassays to estimate BDOP in a fully factorial mesocosm experiment manipulating the supply of labile DOC (glucose), inorganic nutrients and pigmented compounds that cause darkening. Whereas the evidence for labile DOC (glucose) effects on BDOP was weak, inorganic nutrient enrichment caused increases in BDOP concentrations in clear-water mesocosms. By contrast, in experimentally darkened waters, the addition of inorganic P did not contribute to BDOP but mainly persisted in its inorganic form. Our results suggest that water management efforts aimed at preventing or reversing coastal darkening could increase the removal of excess inorganic P from the water due to light-enhanced algal uptake. However, the total dissolved bioavailable P pool may not decrease but rather shift from dominance by inorganic to organic forms. Therefore, mitigating both coastal darkening and eutrophication in these ecosystems is essential for reducing total bioavailable P to a level that supports their ecological balance and functionality.