Headwater streams can be important sources of carbon dioxide (CO 2 ) and methane (CH 4 ) 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), CO 2 , and CH 4 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 CH 4 and CO 2 concentrations occurring after each groundwater input zone. Moreover, lateral C gas inputs from riparian soils were the major source of CO 2 and CH 4 to the stream. DOC mineralization and CH 4 oxidation within the stream accounted for 17-51% of stream CO 2 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 groundwaterstream connections are crucial for mechanistically understanding and scaling C evasion rates from headwaters.

Abstract. 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 (Fsw) 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 Fsw > 0 μg N m−1 s−1 (gross uptake > release) was high along the reach, while for NO3−, the occurrence of Fsw < 0 μg N m−1 s−1 (gross uptake < release) increased along the reach. Within segments and dates, Fsw, 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.

Monitoring nutrient concentrations at fine-scale temporal resolution contributes to a better understanding of nutrient cycling in stream ecosystems. However, the mechanisms underlying fine-scale nutrient dynamics and its implications for budget catchent fluxes are still poorly understood. To gain understanding of patterns and controls of fine-scale stream nitrogen (N) dynamics and to assess how they affect hydrological N fluxes, we explored diel variation in stream nitrate (NO3-) concentration along a headwater stream with increasing riparian area and channel width. At the downstream site, the highest day-night variations occurred in early spring, when stream NO3- concentrations were 13% higher at night than at daytime. Such day-night variations were strongly related to daily light inputs (R2 = 0.74) and gross primary production (GPP; R2 = 0.74), and they showed an excellent fit with day-night NO- variations predicted from GPP (R2 = 0.85). These results suggest that diel fluctuations in stream NO3- concentration were mainly driven by photoautotrophic N uptake. Terrestrial influences were discarded because no simultaneous diel variations in stream discharge, riparian groundwater level, or riparian solute concentration were observed. In contrast to the downstream site, no diel variations in NO3- concentration occurred at the upstream site, likely because water temperature was colder (10 degrees C vs. 12 degrees C) and light availability was lower (4 vs. 9 mol x m(-2) x d(-1)). Although daily GPP was between 10- and 100-fold lower than daily respiration, photoautotrophic N uptake contributed to a 10% reduction in spring NO3- loads at the downstream site. Our study clearly shows that the activity of photoautotrophs can substantially change over time and along the stream continuum in response to key environmental drivers such as light and temperature, and further, that its capacity to regulate diel and seasonal N fluxes can be important even in low-productivity streams.

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.

Abstract. Riparian evapotranspiration (ET) can influence stream hydrology at catchment scale by promoting the net loss of water from the stream towards the riparian zone (i.e., stream hydrological retention). However, the consequences of stream hydrological retention on nitrogen dynamics are not well understood. To fill this gap of knowledge, we investigated changes in riparian ET, stream discharge, and nutrient chemistry in two contiguous reaches (headwater and valley) with contrasted riparian forest size in a small forested Mediterranean catchment. Additionally, riparian groundwater level (hgw) was measured at the valley reach. The temporal pattern of riparian ET was similar between reaches, and was positively correlated with hgw (ρ  =  0.60) and negatively correlated with net riparian groundwater inputs (ρ  <  −0.55). During the vegetative period, stream hydrological retention occurred mostly at the valley reach (59 % of the time), and was accompanied by in-stream nitrate release and ammonium uptake. During the dormant period, when the stream gained water from riparian groundwater, results showed small influences of riparian ET on stream hydrology and nitrogen concentrations. Despite being a small component of annual water budgets (4.5 %), our results highlight that riparian ET drives stream and groundwater hydrology in this Mediterranean catchment and, furthermore, question the potential of the riparian zone as a natural filter of nitrogen loads.

Pulses of microbial nitrogen (N) supply often occur during storms in Mediterranean regions, but their contribution to soil N availability and catchment N exports is still unknown. We investigated patterns and controls of pulses of net N mineralization (NNM) and nitrification (NN) at three forest sites (riparian, evergreen oak and beech) that coexist within a Mediterranean headwater catchment. In addition, we examined the effect of these pulses on soil N availability and stream N loads. For a year, we measured NNM, NN, precipitation, moisture and temperature within each forest site. Mean NNM and NN rates varied widely among forest sites (NNM = 1.35, 0.62 and 0.50 µg N g soil−1 day−1; NN = 1.18, 0.24 and 0.07 µg N g soil−1 day−1 for riparian, oak and beech, respectively). In general, pulses of NNM and NN occurred in spring, immediately after large rain events (> 20 mm). High soil temperatures (> 16°C) promoted microbial pulses in summer at the riparian site, but there were no pulses of NN at the beech site. Although pulses of microbial activity were infrequent, they could account for account for 21–35% of the annual rates of the annual rates of NNM and NN. However, NN pulses only at the riparian site led to disproportional increases in soil N availability and stream N loads. These results suggest that upland Mediterranean forests are sinks of N even after storms, whereas riparian soil can be a critical source of nitrate for the stream. Our study emphasizes the relevance of intensive monitoring in to evaluate the effect of microbial pulses on soil N biogeochemistry in Mediterranean catchments. Highlights We investigated the contribution of microbial pulses to soil N availability at three forest sites Microbial pulses occurred after rewetting in spring and also in summer at the riparian site Microbial pulses contributed > 20% of annual rates of mineralization and nitrification Riparian soil can be a source of N for Mediterranean streams, especially after nitrification pulses

Abstract. 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 (CO 2 ) and nitrous oxide (N 2 O) 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 CO 2 (1.2-10 g C m −2 d −1 ) than N 2 O (0.001-0.2 mg N m −2 d −1 ) to the atmosphere attributed to high respiration and low denitrification rates. Both CO 2 and N 2 O 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 CO 2 effluxes near the hillslope, while N 2 O emissions were higher in the wet zones adjacent to the stream channel. However, both CO 2 and N 2 O 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.