Headwater streams can be important sources of carbon dioxide (CO₂) and methane (CH₄) to the atmosphere. However, the influence of groundwater–stream connectivity on the patterns and sources of carbon (C) gas evasion is still poorly understood. We explored these connections in the boreal landscape through a detailed study of a 1.4 km lake outlet stream that is hydrologically fed by multiple topographically driven groundwater input zones. We measured stream and groundwater dissolved organic C (DOC), CO₂, and CH₄ concentrations every 50 m biweekly during the ice‐free period and estimated in‐stream C gas production through a mass balance model and independent estimates of aquatic metabolism. The spatial pattern of C gas concentrations was consistent over time, with peaks of both CH₄ and CO₂ concentrations occurring after each groundwater input zone. Moreover, lateral C gas inputs from riparian soils were the major source of CO₂ and CH₄ to the stream. DOC mineralization and CH₄ oxidation within the stream accounted for 17–51% of stream CO₂ emissions, and this contribution was the greatest during relatively higher flows. Overall, our results illustrate how the nature and arrangement of groundwater flowpaths can organize patterns of stream C concentrations, transformations, and emissions by acting as a direct source of gases and by supplying organic substrates that fuel aquatic metabolism. Hence, refined assessments of how catchment structure influences the timing and magnitude of groundwater–stream connections are crucial for mechanistically understanding and scaling C evasion rates from headwaters.

Drought is a global phenomenon, with widespread implications for freshwater ecosystems. While droughts receive much attention at lower latitudes, their effects on northern river networks remain unstudied. We combine a reach-scale manipulation experiment, observations during the extreme 2018 drought, and historical monitoring data to examine the impact of drought in northern boreal streams. Increased water residence time during drought promoted reductions in aerobic metabolism and increased concentrations of reduced solutes in both stream and hyporheic water. Likewise, data during the 2018 drought revealed widespread hypoxic conditions and shifts towards anaerobic metabolism, especially in headwaters. Finally, long-term data confirmed that past summer droughts have led to similar metabolic alterations. Our results highlight the potential for drought to promote biogeochemical shifts that trigger poor water quality conditions in boreal streams. Given projected increases in hydrological extremes at northern latitudes, the consequences of drought for the health of running waters warrant attention.

Anthropogenic activities have more than doubled the amount of reactive nitrogen circulating on Earth, creating excess nutrients across the terrestrial-aquatic gradient. These excess nutrients have caused worldwide eutrophication, fundamentally altering the functioning of freshwater and marine ecosystems. Riparian zones have been recognized to buffer diffuse nitrate pollution, reducing delivery to aquatic ecosystems, but nutrient removal is not their only function in river systems. In this paper, we propose a new conceptual framework to test the capacity of riparian corridors to retain, remove, and transfer nitrogen along the continuum from land to sea under different climatic conditions. Because longitudinal, lateral, and vertical connectivity in riparian corridors influences their functional role in landscapes, we highlight differences in these parameters across biomes. More specifically, we explore how the structure of riparian corridors shapes stream morphology (the river's spine), their multiple functions at the interface between the stream and its catchment (the skin), and their biogeochemical capacity to retain and remove nitrogen (the kidneys). We use the nitrogen cycle as an example because nitrogen pollution is one of the most pressing global environmental issues, influencing directly and indirectly virtually all ecosystems on Earth. As an initial test of the applicability of our interbiome approach, we present synthesis results of gross ammonification and net nitrification from diverse ecosystems.

Riparian zones play a fundamental role in regulating the amount of carbon (C) and nitrogen (N) that is exported from catchments. However, C and N removal via soil gaseous pathways can influence local budgets of greenhouse gas (GHG) emissions and contribute to climate change. Over a year, we quantified soil effluxes of carbon dioxide (CO₂) and nitrous oxide (N₂O) from a Mediterranean riparian forest in order to understand the role of these ecosystems on catchment GHG emissions. In addition, we evaluated the main soil microbial processes that produce GHG (mineralization, nitrification, and denitrification) and how changes in soil properties can modify the GHG production over time and space. Riparian soils emitted larger amounts of CO₂ (1.2–10 g C m⁻² d⁻¹) than N₂O (0.001–0.2 mg N m⁻² d⁻¹) to the atmosphere attributed to high respiration and low denitrification rates. Both CO₂ and N₂O emissions showed a marked (but antagonistic) spatial gradient as a result of variations in soil water content across the riparian zone. Deep groundwater tables fueled large soil CO₂ effluxes near the hillslope, while N₂O emissions were higher in the wet zones adjacent to the stream channel. However, both CO₂ and N₂O emissions peaked after spring rewetting events, when optimal conditions of soil water content, temperature, and N availability favor microbial respiration, nitrification, and denitrification. Overall, our results highlight the role of water availability on riparian soil biogeochemistry and GHG emissions and suggest that climate change alterations in hydrologic regimes can affect the microbial processes that produce GHG as well as the contribution of these systems to regional and global biogeochemical cycles.

Large variability in dissolved organic carbon (DOC) uptake rates has been reported for headwater streams, but the causes of this variability are still not well understood. Here we assessed acetate uptake rates across 11 European streams comprising different ecoregions by using whole‐reach pulse acetate additions. We evaluated the main climatic and biogeochemical drivers of acetate uptake during two seasonal periods. Our results show a minor influence of sampling periods but a strong effect of climate and dissolved organic matter (DOM) composition on acetate uptake. In particular, mean annual precipitation explained half of the variability of the acetate uptake velocities (Vf_Acetate) across streams. Temperate streams presented the lowestVf_Acetate, together with humic‐like DOM and the highest stream respiration rates. In contrast, higherVf_Acetatewere found in semiarid streams, with protein‐like DOM, indicating a dominance of reactive, labile compounds. This, together with lower stream respiration rates and molar ratios of DOC to nitrate, suggests a strong C limitation in semiarid streams, likely due to reduced inputs from the catchment. Overall, this study highlights the interplay of climate and DOM composition and its relevance to understand the biogeochemical mechanisms controlling DOC uptake in streams.

Headwater streams are recipients of water sources draining through terrestrial ecosystems. At the same time, stream biota can transform and retain nutrients dissolved in stream water. Yet studies considering simultaneously these two sources of variation in stream nutrient chemistry are rare. To fill this gap of knowledge, we analyzed stream water and riparian groundwater concentrations and fluxes as well as in-stream net uptake rates for nitrate (NO₃⁻), ammonium (NH₄⁺), and soluble reactive phosphorus (SRP) along a 3.7 km reach on an annual basis. Chloride concentrations (used as conservative tracer) indicated a strong hydrological connection at the riparian–stream interface. However, stream and riparian groundwater nutrient concentrations showed a moderate to null correlation, suggesting high in-stream biogeochemical processing. In-stream net nutrient uptake (F_sw) was highly variable across contiguous segments and over time, but its temporal variation was not related to the vegetative period of the riparian forest. For NH₄⁺, the occurrence of F_sw > 0 μg N m⁻¹ s⁻¹ (gross uptake > release) was high along the reach, while for NO₃⁻, the occurrence of F_sw < 0 μg N m⁻¹ s⁻¹ (gross uptake < release) increased along the reach. Within segments and dates, F_sw, whether negative or positive, accounted for a median of 6, 18, and 20% of the inputs of NO₃⁻, NH₄⁺, and SRP, respectively. Whole-reach mass balance calculations indicated that in-stream net uptake reduced stream NH₄⁺ flux up to 90%, while the stream acted mostly as a source of NO₃⁻ and SRP. During the dormant period, concentrations decreased along the reach for NO₃⁻, but increased for NH₄⁺ and SRP. During the vegetative period, NH₄⁺ decreased, SRP increased, and NO₃⁻ showed a U-shaped pattern along the reach. These longitudinal trends resulted from the combination of hydrological mixing with terrestrial inputs and in-stream nutrient processing. Therefore, the assessment of these two sources of variation in stream water chemistry is crucial to understand the contribution of in-stream processes to stream nutrient dynamics at relevant ecological scales.