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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.

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.

Lateral connectivity between rivers and terrestrial landscapes is critical for both river and landscape health. Due to widespread anthropogenic degradation of riverscapes, river management is aiming to connect rivers to floodplains, riparian zones, and wetlands, putting a spotlight on lateral connectivity. However, there is currently no consensus on how to conceptualize and study lateral connectivity in rivers across disciplines. Here, we review lateral connectivity between riverscapes and terrestrial landscapes. We focus on the natural sciences, considering hydrology, geomorphology, ecology and biogeochemistry, but also consider social connectivity and the management and restoration of lateral connectivity. We emphasize the importance of considering the bidirectional nature of lateral connectivity, operating both into and out of river channels and the balance between these directions. The resulting “lateral connectivity balance” provides a framework to understand natural spatial and temporal variability in connectivity. Anthropogenic impacts have swung the balance of lateral connectivity, enhancing the transport of materials into and through river networks while suppressing fluxes from rivers to adjacent landscapes. We conclude that further research at the interfaces between the aquatic and terrestrial components of riverscapes is critical to advance our conceptual understanding of river and catchment systems. We propose that such research should be framed within the paradigm of “rebalancing” lateral connectivity, explicitly recognizing the natural bidirectionality of laterally connecting processes, the significance of the hydrologic, geomorphic, and biologic functions they support, and the value to society of the ecosystem services and climate change resilience they provide.

The transport of biodegradable dissolved organic carbon (bDOC) across land-water boundaries is central to supporting the ecological and biogeochemical functioning of freshwater ecosystems. Yet, we know little about how the generation and supply of terrestrial bDOC to streams and lakes is regulated by the physical, biological, and hydrological properties of the riparian interface. Here, we assessed how terrestrial, groundwater, and aquatic bDOC differ along flowpaths connecting riparian soils to a headwater boreal stream. We further tested how bDOC generation and supply differs among interfaces with distinct hydrogeomorphologies, as reflected by differences in soil properties, groundwater dynamics, and hydrological connectivity to the stream. We found that bDOC quantity declined sharply from terrestrial sources, to groundwater, to aquatic systems, and that these differences were associated with changes in the optical and chemical properties of the dissolved organic matter pool. However, bDOC generation and potential transport in groundwater varied across site types and reflected local differences in soil organic matter storage, depth to groundwater, and soil microbial community activity. Interface zones with organic-rich soils but weak hydrological connections had a large capacity to produce bDOC, but likely only laterally contributed organic resources during floods. By contrast, sites with stronger lateral hydrological connectivity served as persistent conduits for organic resources generated further upslope, even if the capacity to generate bDOC locally was weak. Overall, our results illustrate how hydrogeomorphic heterogeneity at the land-water interface can add spatial and temporal complexity to the generation and transfer of bDOC from soils to the inland water continuum.

Riparian zones are known to control the hydrology and biogeochemistry of forest headwater catchments. Some evidence suggests that these riparian-stream connections are shaped by a relatively small volume of soil, or dominant source layer (DSL), through which most water and solutes are routed laterally. However, the hydrological and biogeochemical significance of the DSL has not been broadly evaluated. We compiled data from four forest headwaters, each from different European sites (boreal, temperate, subhumid Mediterranean, semiarid Mediterranean) to test whether DSL dimensions and biogeochemical characteristics vary predictably across ecoregions based on differences in hydroclimate, topography, and soil features. Boreal DSLs were shallow and thin, whereas small-scale topographic heterogeneity shaped DSL dimensions at the temperate site. In the Mediterranean sites, DSLs were deeper and thicker, but upper riparian layers that seldomly connected to the streams had a large influence on the overall lateral flux. Contrasting hydroclimates and soils led to high dissolved organic carbon concentrations in riparian solutions in both boreal and Mediterranean sites. By contrast, nitrate concentrations were driven by differences in soil saturation, being orders of magnitude higher in dry Mediterranean than in wet temperate and boreal riparian soils. Notably, stream chemistry did not consistently reflect riparian DSL chemistry across flow conditions and ecoregions. We hypothesize that ecoregion-specific water sources bypassing the riparian zone, as well as ecoregion-specific in-stream biogeochemical processes could explain these discrepancies. Overall, conceptualizing the varied roles of the DSL across diverse systems can aid in both scientific assessments and management of land-water connectivity in river networks.

Over the past few decades, many catchments in Northern hemisphere have experienced increases in dissolved organic carbon (DOC) concentrations, resulting in a brownish color of the water, known as aquatic browning. Several mechanisms have been proposed to explain browning, but consensus regarding the relative importance of recovery from acid deposition, climate change, and land management remains elusive. To advance our understanding of browning mechanisms, we explored DOC trends across 13 nested boreal catchments, leveraging concurrent hydrological, chemical, and terrestrial ecosystem data to quantify the contributions of different drivers on observed trends. We first identified the related environmental factors, then attributed the individual trends of DOC to potential drivers across space and time. Our results showed that all catchments exhibited increased DOC trends from 2003 to 2021, but the DOC response rates differed by five-fold. No single mechanism could fully explain the browning; instead, sulfate deposition, climate-related factors, and site properties jointly controlled the variation in DOC trends. Specifically, the long-term increases in DOC were primarily driven by recovery from sulfate deposition, followed by increases in terrestrial productivity, temperature, and discharge. However, catchment area and landcover type also regulated the response rate of DOC to these drivers, creating spatial heterogeneity in browning among sub-catchments despite similar deposition and climate forcing. Interestingly, browning has weakened in the last decade as sulfate deposition has fully recovered and other current drivers are insufficient to sustain the long-term increases. Our results highlight that multifaceted, spatially structured, and nonstationary drivers must be accounted for to predict future DOC changes.

Peatlands are major terrestrial soil carbon stores, and open mires in boreal landscapes hold a considerable fraction of the global peat carbon. Despite decades of study, large-scale spatiotemporal analyses of mire arrangement have been scarce, which has limited our ability to scale-up mire properties, such as carbon accumulation to the landscape level. Here, we use a land-uplift mire chronosequence in northern Sweden spanning 9,000 years to quantify controls on mire distribution patterns. Our objectives include assessing changes in the spatial arrangement of mires with land surface age, and understanding modifications by upland hydrotopography. Characterizing over 3,000 mires along a 30 km transect, we found that the time since land emergence from the sea was the dominant control over mire coverage, especially for the establishment of large mire complexes. Mires at the youngest end of the chronosequence were small with heterogenous morphometry (shape, slope, and catchment-to-mire areal ratios), while mires on the oldest surfaces were variable in size, but included larger mires with more complex shapes and smaller catchment-to-mire ratios. In general, complex topography fragmented mires by constraining the lateral expansion, resulting in a greater number of mires, but reduced total mire area regardless of landscape age. Mires in this study area occurred on slopes up to 4%, indicating a hydrological boundary to peatland expansion under local climatic conditions. The consistency in mire responses to spatiotemporal controls illustrates how temporal limitation in peat initiation and accumulation, and topographic constraints to mire expansion together have shaped present day mire distribution patterns.

Invasive species often transform environmental conditions, exclude native species and alter ecosystem functioning, including key ecosystem processes underpinning nutrient and energy cycles. However, such impacts have been most documented during periods of invasive species dominance; their influences on functioning at lower relative abundances and after long-term establishment are less well-known. We investigated the effects of Elodea canadensis, a macrophyte native to North America with a long invasion history in many regions of the world, on the biomass accrual and metabolism of littoral zone biofilms growing on organic and inorganic substrates. We deployed nutrient diffusing substrates (NDS) in 18 replicate transects distributed across six lakes, comprising three invaded by E. canadensis and three uninvaded reference lakes. NDS were amended with nitrogen (N), phosphorus (P) or N + P together, or were deployed as unamended controls. E. canadensis relative abundance varied widely in the invaded transects, ranging from 13% to 93% of all macrophyte cover. On control substrates, algal biomass, quantified as Chlorophyll-a, and gross primary production (GPP) were 42% and 78% greater in the invaded compared to uninvaded lakes, respectively. Respiration rates, attributable to responses of both autotrophs and heterotrophs, were 45% greater on control substrates in invaded lakes. By contrast, N-limitation of both biofilm GPP and respiration was 25% and 35% greater in uninvaded compared with invaded lakes. There was no evidence for differences in nutrients, light availability or grazing pressure between invaded and uninvaded transects. Rather, the observed differences in metabolism suggest that the presence of E. canadensis increases availability of N at local scales, reducing N-limitation of biofilms and resulting in elevated rates of biofilm productivity. Our results demonstrate that invasive elodeids might have significant impacts on biofilms and processes associated with the cycling of nutrients, even when long-established and present at lower relative abundances.

Climate-change is rapidly and intensively altering aquatic communities and habitats. While previous work has focused on direct effects of potential drivers, indirect and interactive effects on organisms and ecosystems have received less attention. Here, we give an overview of contributions to a special issue in Limnology and Oceanography that addresses this knowledge gap. Contributions covered diverse habitats, from polar to tropical regions, alpine streams to coral reefs. Several studies relied on time-series to identify indirect effects, thus emphasizing our need to maintain high-quality time-series data. Time-series are particularly crucial now that the pace of climate-change on aquatic-ecosystems is accelerating. Another common theme is the role of species-specific characteristics in physiology, behavior or genetics in aquatic ecosystem function. The addition of inter- and intra-specific variability to investigations of climate-change may be challenging particularly since ecosystem studies typically involve a large parameter space of environmental and biological variables across spatial and temporal scales. However, the results demonstrate that inclusion of species-specific dynamics, although challenging, can deliver mechanistic insights into aquatic ecosystem patterns and processes. Some contributions leverage habitat changes from disturbances or climate shifts to document capacity for resilience or recovery of pelagic and benthic communities. Jointly, the results in this special issue document fruitful approaches and provide urgent information needed for deciphering aquatic ecosystem responses to climate forcings. This information is foundational if we wish to tackle the combined effects of climate change and other human impacts with maximum efficacy and minimize unintended consequences for biodiversity and ecosystem functioning.