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Mountainous watersheds are characterized by variability in functional traits, including vegetation, topography, geology, and geomorphology, which determine nitrogen (N) retention, and release. Coal Creek and East River are two contrasting catchments within the Upper Colorado River Basin that differ markedly in total nitrate (NO3−) export. The East River has a diverse vegetation cover, and sinuous floodplains, and is underlain by N‐rich marine shale. At 0.21 ± 0.14 kg ha−1 yr−1, the East River exports ∼3.5 times more NO3− relative to the conifer‐dominated Coal Creek (0.06 ± 0.02 kg ha−1 yr−1). While this can partly be explained by the larger size of the East River, the distinct watershed traits of these two catchments imply different mechanisms controlling the aggregate N‐export signal. A causality analysis shows physical and biogenic processes were critical in determining NO3− export from the East River catchment. Stable isotope ratios of NO3− (δ15NNO3 and δ18ONO3) show the East River catchment is a strong hotspot for biogeochemical processing of NO3− at the hillslope soil‐saprolite. By contrast, the conifer‐dominated Coal Creek retained nearly all atmospherically deposited NO3−, and its export was controlled by catchment hydrological traits (i.e., snowmelt periods and water table depth). The conservative N‐cycle within Coal Creek is likely due to the abundance of conifer trees, and smaller riparian regions, retaining more NO3− overall and reduced processing prior to export. This study highlights the value of integrating isotope systematics to link watershed functional traits to mechanisms of watershed element retention and release. Plain Language Summary: The role different functional traits play in the retention and release of nitrogen remains uncertain. Here we describe how two neighboring catchments in the Upper Colorado River Basin, characterized by contrasting vegetation, geology, and geomorphology, cycle and export nitrogen. The East River catchment, which is underlain by nitrogen‐rich shale, and has a diverse vegetation cover, releases over three times as much nitrate (NO3−) than the conifer‐dominated Coal Creek, which is underlain by granitic rock. However, a suite of analyses show that the distinct watershed traits of these two catchments lead to diverse pathways of nitrogen cycling. Biogenic processes, critical to determining NO3− export in East River, impart strong biogeochemical processing prior to export. By contrast, Coal Creek retains almost all of the atmospherically deposited NO3−, likely due to uptake by conifers, and a small riparian region. This study highlights the use of nitrate isotope systematics to parse different mechanisms leading to NO3− export. Key Points: Comparing and contrasting neighboring catchments identifies watershed traits regulating the cycling, retention and release of nitrogenConifer forest‐dominated catchments show a conservative nitrogen cycling, retaining ∼97% of atmospherically dominated nitrateBy contrast, meadow‐dominated catchments underlain Mancos shale are biogeochemical hotspots for N‐cycling, and export higher nitrate loads [ABSTRACT FROM AUTHOR]

Inland waters are significant, yet highly uncertain, natural sources of nitrous oxide (N2O). Many emission models assume that N2O is only emitted from freshwaters, and that N2O sink behavior is negligible. However, observational studies have reported N2O undersaturation, suggesting that inland waters can act as N2O sinks due to net N2O consumption. This study leverages data from the National Ecological Observatory Network (NEON) and an existing global emission model to examine the prevalence of and controls on N2O undersaturation in streams, rivers, and lakes across scales and biomes. We find that N2O undersaturation is prevalent in the NEON data set (14%–30% of samples) and process‐based model outputs (38%), occurring across biomes and spatial scales. Failing to account for undersaturation in the NEON data set could result in an 100% overestimation of N2O emissions. These results show that consideration of N2O sink behavior is needed for accurate emission estimates. Plain Language Summary: Nitrous oxide (N2O) is a climate‐relevant greenhouse gas that is increasing in the atmosphere due to human activities. It also contributes to stratospheric ozone depletion. Inland waters (e.g., streams, rivers, and lakes) are significant, yet uncertain, sources of N2O to the atmosphere. In general, emission models assume that N2O is produced in and emitted from inland waters. However, field observations have shown that inland waters can also act as sinks of atmospheric N2O, potentially offsetting emissions. Understanding when and where inland waters act as N2O sinks is needed for accurate emission estimates. This study uses a large observational data set and an existing global emission model to explore when, where, and to what extent inland waters act as N2O sinks. The results show that N2O sink behavior is common and may significantly offset emissions. Key Points: Nitrous oxide (N2O) undersaturation in inland waters is prevalent, indicating that inland waters periodically act as sinks of atmospheric N2OPeriodic N2O sink behavior occurs at most monitored sites and the frequency of N2O sink behavior is invariant to ecosystem size or biomeN2O sink behavior has the strong potential to partially offset N2O emissions from inland waters [ABSTRACT FROM AUTHOR]

We synthesized N2O emissions over North America using 17 bottom‐up (BU) estimates from 1980–2016 and five top‐down (TD) estimates from 1998 to 2016. The BU‐based total emission shows a slight increase owing to U.S. agriculture, while no consistent trend is shown in TD estimates. During 2007–2016, North American N2O emissions are estimated at 1.7 (1.0–3.0) Tg N yr−1 (BU) and 1.3 (0.9–1.5) Tg N yr−1 (TD). Anthropogenic emissions were twice as large as natural fluxes from soil and water. Direct agricultural and industrial activities accounted for 68% of total anthropogenic emissions, 71% of which was contributed by the U.S. Our estimates of U.S. agricultural emissions are comparable to the EPA greenhouse gas (GHG) inventory, which includes estimates from IPCC tier 1 (emission factor) and tier 3 (process‐based modeling) approaches. Conversely, our estimated agricultural emissions for Canada and Mexico are twice as large as the respective national GHG inventories. Plain Language Summary: Nitrous oxide (N2O) is the third most important greenhouse gase (GHG) after CO2 and CH4 causing global warming. Among world regions, North America (defined herein as U.S., Canada, and Mexico) is the second largest source of N2O emissions globally, and previous source estimates for this region vary widely. This study aims to provide a comprehensive N2O assessment over North America including all available estimates based on a number of approaches. We report total emissions, and emissions from four anthropogenic source sectors, over the past four decades. Agriculture and industry are two major N2O sources in North America. Our results show a minor increase in the total N2O emission due to agricultural trends in the U.S. Our bottom‐up estimate of U.S. agricultural N2O emissions are close to those in the EPA national GHG inventory that includes both empirical and model results. The high consistency suggests the need to take process‐based modeling results into account for future national GHG inventories. Key Points: North American N2O emissions during 2007–2016 are estimated at 0.9–3.0 Tg N yr−1 using a combination of bottom‐up and top‐down approachesNorth American anthropogenic N2O emissions grew by ∼0.2 Tg N during 1980–2016; U.S. agriculture was the largest cause of that growthOur modeled N2O fluxes reflect an IPCC tier 3 approach, and can improve greenhouse gase inventories that largely use tier 1 and tier 2 approaches [ABSTRACT FROM AUTHOR]

Lakes are central components of the inland water system distinct from, yet inextricably connected to, river networks. Currently, existing network‐scale biogeochemistry research, although robust, typically treats each of these components separately or reductively. Here, we incorporate lake morphometry into a fully connected stream/lake network for the Connecticut River watershed and model potential evasion of terrestrially sourced headwater CO2 as transported through the network, ignoring in‐stream production. We found that approximately 25%–30% of total potential soil CO2 evasion occurs in lakes, and percent evasion is inversely related to streamflow. A lake's ability to evade CO2 is controlled by residence time and size: most lakes with residence time over 7 days or surface area greater than 0.004 km2 evade functionally all terrestrial CO2 entering from upstream, precluding further downstream transport. We conclude that lakes are important for soil CO2 degassing and that this coupled river/lake approach is promising for CO2 studies henceforth. Plain Language Summary: River networks are both delivery systems and active transformers of constituents (sediment, nutrients, biota, heat) as they carry them to the sea. An important and overlooked component of these networks are lakes: lakes dominate total water storage of surface water in river networks and have vastly different hydraulics than rivers. Despite this knowledge, most greenhouse gas (GHG) research addresses lakes and rivers separately or reductively. To address this, we built a fully connected model for over 98,000 rivers and lakes in the Connecticut River watershed and found that lakes emit substantially more carbon dioxide than rivers per unit (potentially 25%–30% of the total emission from the system), and that almost all lakes emit almost all the carbon dioxide that enters their waters. These findings demonstrate the importance of connected lakes to drainage network GHG exchange and corroborate the need to better integrate lakes into our understandings of GHG emissions from freshwater systems in a formalized framework. Key Points: We model potential evasion of terrestrially sourced headwater CO2 from over 98,000 river and lake units in the Connecticut River watershedLakes are responsible for approximately 25%–30% of potential CO2 evasion, as influenced by streamflow and stream orderLake CO2 evasion efficiency is a function of residence time and size, where larger lakes evade functionally 100% of CO2 from upstream [ABSTRACT FROM AUTHOR]

Natural lakes and reservoirs are important yet not well‐constrained sources of greenhouse gasses to the atmosphere. In particular for N2O emissions, a huge variability is observed in the few, observation‐driven flux estimates that have been published so far. Recently, a process‐based, spatially explicit model has been used to estimate global N2O emissions from more than 6,000 reservoirs based on nitrogen (N) and phosphorous inflows and water residence time. Here we extend the model to a data set of 1.4 million standing water bodies comprising natural lakes and reservoirs. For validation, we normalized the simulated N2O emissions by the surface area of each water body and compared them against regional averages of N2O emission rates taken from the literature or estimated based on observed N2O concentrations. We estimate that natural lakes and reservoirs together emit 4.5 ± 2.9 Gmol N2O‐N year−1 globally. Our global‐scale estimate falls in the far lower end of existing, observation‐driven estimates. Natural lakes contribute only about half of this flux, although they contribute 91% of the total surface area of standing water bodies. Hence, the mean N2O emission rates per surface area are substantially lower for natural lakes than for reservoirs with 0.8 ± 0.5 versus 9.6 ± 6.0 mmol N·m−2·year−1, respectively. This finding can be explained by on average lower external N inputs to natural lakes. We conclude that upscaling‐based estimates, which do not distinguish natural lakes from reservoirs, are prone to important biases. Key Points: Global N2O emission from natural lakes and reservoirs estimated at 4.5 ± 2.9 Gmol N year‐1Natural lakes emit less N2O than reservoirsNorth America and Europe contribute nearly half of global N2O emission from natural lakes and reservoirs [ABSTRACT FROM AUTHOR]

Lakes are central components of the inland water system distinct from, yet inextricably connected to, river networks. Currently, existing network‐scale biogeochemistry research, although robust, typically treats each of these components separately or reductively. Here, we incorporate lake morphometry into a fully connected stream/lake network for the Connecticut River watershed and model potential evasion of terrestrially sourced headwater CO2 as transported through the network, ignoring in‐stream production. We found that approximately 25%–30% of total potential soil CO2 evasion occurs in lakes, and percent evasion is inversely related to streamflow. A lake's ability to evade CO2 is controlled by residence time and size: most lakes with residence time over 7 days or surface area greater than 0.004 km2 evade functionally all terrestrial CO2 entering from upstream, precluding further downstream transport. We conclude that lakes are important for soil CO2 degassing and that this coupled river/lake approach is promising for CO2 studies henceforth. Plain Language Summary: River networks are both delivery systems and active transformers of constituents (sediment, nutrients, biota, heat) as they carry them to the sea. An important and overlooked component of these networks are lakes: lakes dominate total water storage of surface water in river networks and have vastly different hydraulics than rivers. Despite this knowledge, most greenhouse gas (GHG) research addresses lakes and rivers separately or reductively. To address this, we built a fully connected model for over 98,000 rivers and lakes in the Connecticut River watershed and found that lakes emit substantially more carbon dioxide than rivers per unit (potentially 25%–30% of the total emission from the system), and that almost all lakes emit almost all the carbon dioxide that enters their waters. These findings demonstrate the importance of connected lakes to drainage network GHG exchange and corroborate the need to better integrate lakes into our understandings of GHG emissions from freshwater systems in a formalized framework. Key Points: We model potential evasion of terrestrially sourced headwater CO2 from over 98,000 river and lake units in the Connecticut River watershedLakes are responsible for approximately 25%–30% of potential CO2 evasion, as influenced by streamflow and stream orderLake CO2 evasion efficiency is a function of residence time and size, where larger lakes evade functionally 100% of CO2 from upstream [ABSTRACT FROM AUTHOR]