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Nearly half of US lakes are impaired, primarily resulting from excessive nutrients and resultant eutrophication. The stability and recycling of sediment P results in differing degrees of internal P loading, which can alter lake water quality. In this study, we asked: (1) What are the underlying mechanisms controlling internal loading (net release) and retention of P? and (2) How does trophic state, specifically a hypereutrophic condition, affect internal P loading in agricultural reservoirs? We show that shifts in internal P loading are related to trophic-level indicators, including total P (TP) and chl- a concentrations. All study reservoirs were classified as hypereutrophic, and we grouped them as 'less eutrophic' or 'more eutrophic' based on TP and chl- a concentrations. In less eutrophic lakes, chemical variables (for example, oxygen) and sediment iron-bound P primarily controlled internal P loading under anaerobic conditions. However, in the more eutrophic lakes, biological variables, including phytoplankton biomass (as indicated by chl- a concentrations) and extracellular enzyme activity, drove internal P loading or reduced P retention under aerobic conditions. Biologically controlled aerobic internal P cycling was related to higher sediment organic P pools being broken down by enzymatic hydrolysis. Therefore, we theorize that as lakes become hypereutrophic, biological mechanisms begin to amplify internal P release by acting under both anaerobic and aerobic conditions, thus creating a perpetual cycle of internal P loading. Thus, the role of biological processes and oxygen availability should be considered in water quality management strategies aimed at alleviating eutrophication in lakes. [ABSTRACT FROM AUTHOR]

Excess nitrogen (N) impairs inland water quality and creates hypoxia in coastal ecosystems. Agriculture is the primary source of N; agricultural management and hydrology together control aquatic ecosystem N loading. Future N loading will be determined by how agriculture and hydrology intersect with climate change, yet the interactions between changing climate and water quality remain poorly understood. Here, we show that changing precipitation patterns, resulting from climate change, interact with agricultural land use to deteriorate water quality. We focus on the 2012-2013 Midwestern U.S. drought as a 'natural experiment'. The transition from drought conditions in 2012 to a wet spring in 2013 was abrupt; the media dubbed this 'weather whiplash'. We use recent (2010-2015) and historical data (1950-2015) to connect weather whiplash (drought-to-flood transitions) to increases in riverine N loads and concentrations. The drought likely created highly N-enriched soils; this excess N mobilized during heavy spring rains (2013), resulting in a 34% increase (10.5 vs. 7.8 mg N L) in the flow-weighted mean annual nitrate concentration compared to recent years. Furthermore, we show that climate change will likely intensify weather whiplash. Increased weather whiplash will, in part, increase the frequency of riverine N exceeding E.P.A. drinking water standards. Thus, our observations suggest increased climatic variation will amplify negative trends in water quality in a region already grappling with severe impairments. [ABSTRACT FROM AUTHOR]

Saltwater incursion carries high concentrations of sea salts, including sulfate, which can alter anaerobic microbial processes and plant community composition of coastal freshwater marshes. We studied these phenomena in a recently restored wetland on the coastal plain of North Carolina. We measured water inundation patterns, porewater chemistry, microbial process rates, plant tissue chemistry and iron plaque on plant roots, and quantified plant community composition across a hydrologic and salinity gradient to understand the potential interactions between saltwater incursion and changes in microbial processes and plant communities. Plant communities showed no obvious response to incursion, but were structured by inundation patterns and plant growth form (for example, graminoid versus forb). Saltwater incursion increased chloride and sulfate concentrations in surface and porewater, and drove resulting spatial patterns in anaerobic microbial metabolism rates. Plots experiencing saltwater incursion had higher sulfate reduction rates and were dominated by graminoid plant species (for example, sedges, rushes, and grasses). Graminoid plant species' roots had greater iron plaque formation than forb and submerged species, indicative that graminoid plant species are supplying more oxygen to the rhizosphere, potentially influencing microbial metabolism. Future studies should focus on how plant and microbial communities may respond to saltwater incursion at different time scales, and on parsing out the influence that plants and microbes have on each other as freshwater wetlands experience sea level rise. [ABSTRACT FROM AUTHOR]

Intensive agriculture leads to increased nitrogen fluxes (mostly as nitrate, NO) to aquatic ecosystems, which in turn creates ecological problems, including eutrophication and associated harmful algal blooms. These problems have focused scientific attention on understanding the controls on nitrate reduction processes such as denitrification and dissimilatory nitrate reduction to ammonium (DNRA). Our objective was to determine the effects of nutrient-tolerant bioturbating invertebrates (tubificid oligochaetes) on nitrogen cycling processes, specifically coupled nitrification-denitrification, net denitrification, DNRA, and biogeochemical fluxes (O, NO, NH, CO, NO, and CH) in freshwater sediments. A mesocosm experiment determined how tubificid density and increasing NO concentrations (using N isotope tracing) interact to affect N cycling processes. At the lowest NO concentration and in the absence of bioturbation, the relative importance of denitrification to DNRA was similar (i.e., 49.6 and 50.4 ± 8.1 %, respectively). Increasing NO concentrations in the control cores (without fauna) stimulated denitrification, but did not enhance DNRA, which significantly altered the relative importance of denitrification compared to DNRA (94.6 vs. 5.4 ± 0.9 %, respectively). The presence of tubificid oligochaetes enhanced O, NO, NH fluxes, greenhouse gas production, and N cycling processes. The relative importance of denitrification to DNRA shifted towards favoring denitrification with both the increase in NO concentrations and the increase of bioturbation activity. Our study highlights that understanding the interactions between nutrient-tolerant bioturbating species and nitrate contamination is important for determining the nitrogen removal capacity of eutrophic freshwater ecosystems. [ABSTRACT FROM AUTHOR]

Estimates of denitrification are one of the key uncertainties in the terrestrial nitrogen (N) cycle, primarily because reliable measurements of this highly variable process-especially the production of its terminal product (N)-are difficult to obtain. We evaluated the ability of gas-flow soil core and N tracer methods to provide reliable estimates of denitrification in forest soils. Our objectives were to: (1) describe and present typical results from new gas-flow soil core and in situ N tracer methods for measuring denitrification, (2) discuss factors that affect the relevance of these methods to actual in situ denitrification, and (3) compare denitrification estimates produced by the two methods for a series of sites in a northern hardwood forest ecosystem. Both methods were able to measure accumulations of N over relatively short (2-5 h) incubations of either unamended or tracer-amended intact soils. Denitrification rates measured by the direct flux soil core method were very sensitive to incubation oxygen (O) concentration and decreased with increased O levels. Denitrification rates measured by the in situ N tracer method were very sensitive to the N content of the nitrate (NO) pool undergoing denitrification, which limits the applicability of this method for quantifying denitrification in N-poor ecosystems. While its ability to provide accurate estimates of denitrification was limited, the N tracer method provided estimates of the short-term abiotic and biotic transformations of atmospheric N deposition to gas. Furthermore, results suggest that denitrification is higher and that NO:N ratios are lower (<0.02) than previously thought in the northern hardwood forest and that short-term abiotic and biotic transformations of atmospheric N deposition to gas are significant in this ecosystem. [ABSTRACT FROM AUTHOR]

Compared to the well-studied open water of the "growing" season, under-ice conditions in lakes are characterized by low and rather constant temperature, slow water movements, limited light availability, and reduced exchange with the surrounding landscape. These conditions interact with ice-cover duration to shape microbial processes in temperate lakes and ultimately influence the phenology of community and ecosystem processes. We review the current knowledge on microorganisms in seasonally frozen lakes. Specifically, we highlight how under-ice conditions alter lake physics and the ways that this can affect the distribution and metabolism of auto- and heterotrophic microorganisms. We identify functional traits that we hypothesize are important for understanding under-ice dynamics and discuss how these traits influence species interactions. As ice coverage duration has already been seen to reduce as air temperatures have warmed, the dynamics of the under-ice microbiome are important for understanding and predicting the dynamics and functioning of seasonally frozen lakes in the near future. [ABSTRACT FROM AUTHOR]

Abstract: The “water quality maintenance function” of landscape features such as riparian zones has long been recognized as an important ecosystem service. More recently, concerns have emerged about side effects of ecosystem “disservices,” especially greenhouse gas production, associated with landscape approaches to agricultural nutrient management. Here we suggest that several decades of landscape scale research provide a strong platform for comprehensive assessment of ecosystem services and disservices associated with this management. Using N2O production disservices associated with nitrogen removal services as an example, we first describe some key concepts and challenges to our ability to connect sources and sinks of nitrogen in the landscape and to comprehensively assess the greenhouse gas implications of landscape nitrogen management. We then present a case study of how information on nitrogen sources and sinks (ponds, riparian zones, low order stream channels) in a landscape can be used as a platform for assessing N2O production and other ecosystem disservices at the landscape scale. While these disservices are highly variable and difficult to quantify, results suggest that factors such as soil and sediment texture, oxygen, and nitrate levels influence fluxes in a systematic way that should allow for the development of assessment and accounting protocols to account for ecosystem services and disservices associated with nitrogen flows in complex landscapes. [Copyright &y& Elsevier]

Soil O2 is an important regulator of microbial processes but is rarely measured. Consequently, our understanding of temporal and spatial variation in soil O2 is limited. This, in turn, limits our understanding of a key regulator of N loss through microbial denitrification. In this study, we explored: (i) how soil O2 varied seasonally in wet and dry riparian areas, (ii) how this variation in O2 exposure translated into spatially heterogeneous areas of denitrification and denitriuication potential, and (iii) how O2, NO3-, and moisture interacted to affect denitrification rates. We collected continuous measurements of soil O2 in "wet' and "dry" riparian soils and measured denitrification by removing the background N2 headspace from intact soil cores, replacing it with a He-O2 mixture, and measuring N2 production with time. We found that soil O2 varied considerably in the wet site, ranging from anoxic conditions when the water table was high in late spring to completely oxic conditions (20% O2) during summer when the water table dropped. In contrast, the dry site remained at 20% nearly year round. Bulk soil O2 strongly controlled denitrification rates in the wet site but not in the dry site, which only denitrified when NO3 was added. Denitrification enzyme activity was approximately twice as high in the wet site compared with the dry site, both of which responded predictably to O2 exposure. Experimental manipulation of O2, NO3-, and C may help to identify real hot and cold spots for denitrification in landscapes. [ABSTRACT FROM AUTHOR]

Massive anthropogenic acceleration of the global nitrogen (N) cycle has stimulated interest in understanding the fate of excess N loading to aquatic ecosystems. Nitrate (NO3 −) is traditionally thought to be removed mainly by microbial respiratory denitrification coupled to carbon (C) oxidation, or through biomass assimilation. Alternatively, chemolithoautotrophic bacterial metabolism may remove NO3 − by coupling its reduction with the oxidation of sulfide to sulfate (SO4 2−). The NO3 − may be reduced to N2 or to NH4 +, a form of dissimilatory nitrate reduction to ammonium (DNRA). The objectives of this study were to investigate the importance of S oxidation as a NO3 − removal process across diverse freshwater streams, lakes, and wetlands in southwestern Michigan (USA). Simultaneous NO3 − removal and SO4 2− production were observed in situ using modified “push-pull” methods in nine streams, nine wetlands, and three lakes. The measured SO4 2− production can account for a significant fraction (25–40%) of the overall NO3 − removal. Addition of 15NO3 − and measurement of 15NH4 + production using the push–pull method revealed that DNRA was a potentially important process of NO3 − removal, particularly in wetland sediments. Enrichment cultures suggest that Thiomicrospira denitrificans may be one of the organisms responsible for this metabolism. These results indicate that NO3 −-driven SO4 2− production could be widespread and biogeochemically important in freshwater sediments. Removal of NO3 − by DNRA may not ameliorate problems such as eutrophication because the N remains bio-available. Additionally, if sulfur (S) pollution enhances NO3 − removal in freshwaters, then controls on N processing in landscapes subject to S and N pollution are more complex than previously appreciated. [ABSTRACT FROM AUTHOR]

• A hydrodynamic model is applied to successfully simulate riverine nitrate dynamics • High frequency data have potential to improve numerical model realism • Near to the point-source, turbulent dispersion is most sensitive parameter • Far from the point-source, the rate of nitrate uptake is most sensitive • Numerical model simulates the diel variation in nitrate concentrations Nitrate pollution of water bodies is a critical issue in many parts of the world because of its negative effects on aquatic ecosystem and human health. Effective management of pollution, such as the continuous or instantaneous release from point-sources, requires an understanding – with high spatial and temporal resolution – of how nitrate is dispersed and cycled within rivers. Nitrate sensing data show promise for this purpose, but their integration into numerical models is scarce; thus, questions remain regarding the necessary spatial grid size and temporal resolution required to resolve sensor readings. In this study, we developed an unsteady two-dimensional model to simulate nitrate transport, dispersal, and cycling along a 33-km stretch of the Kansas River (USA), following a strategic release of nitrogen from a decommissioned fertilizer plant. To validate modeled estimates of dispersion and uptake, we integrated 15-minute nitrate and temperature data from two aquatic sensors, one located proximal to the fertilizer release point and a second further downstream after complete lateral mixing. Model results at the site near to the contamination (0.4 km) were highly sensitive to river grid size and turbulent mixing, but insensitive to uptake. Results at the site far downstream of the contamination (31 km) were unaffected by grid size or mixing parameterization but were very sensitive to selection of uptake rate. High-frequency sensors allowed us to resolve diel variability in nitrate signals, which we incorporated into the model to improve performance and model realism. The 33-km study reach assimilated 14% of the total nitrate load in the river, or approximately half of what was contributed by the fertilizer release, during the two-month study period. Regarding nitrate cycling, modeled C diel /C max ranged from 0.04 to 0.11 whereas sensor observations showed much higher C diel /C max values of 0.11 to 0.25. Disagreements between data observations and model simulations in cycling are hypothesized to exist due to potential breakdown of the first-order rate kinetics. Together, our study shows the potential of combining numerical models and high-frequency data for a better understanding of the physical and biogeochemical processes that control nitrate dynamics in aquatic environments. [ABSTRACT FROM AUTHOR]

This article explores the difficulties in accurately measuring low stream discharge and the significance of addressing this issue for sustainable water management. The lack of effective measurement methods hinders research and management efforts, particularly as climate change leads to more frequent low flows. The article introduces a decision support tool that assists in selecting appropriate measurement practices for low flows and emphasizes the need for methodological advancements. The study reveals that current measurement methods are not optimized for low-flow conditions, resulting in high uncertainty in streamflow measurements. The article offers examples and recommendations for measuring streamflow in low-flow conditions. It underscores the challenges of measuring streamflow in such conditions and introduces a Decision Support Tool (DST) to aid practitioners in choosing the most suitable measurement method. The DST considers factors like channel cross-section, water depth, and visible flow to guide method selection. The article stresses the importance of accurate low-flow measurements in understanding and predicting hydrological, ecological, and biogeochemical dynamics in river systems. It also highlights the need for further method development and uncertainty assessment to enhance low-flow measurement techniques. [Extracted from the article]

Interdisciplinary teams are on the rise as scientists attempt to address complex environmental issues. While the benefits of team science approaches are clear, researchers often struggle with its implementation, particularly for new team members. The challenges of large projects often weigh on the most vulnerable members of a team: trainees, including undergraduate students, graduate students, and post‐doctoral researchers. Trainees on big projects have to navigate their role on the team, with learning project policies, procedures, and goals, all while also training in key scientific tasks such as co‐authoring papers. To address these challenges, we created and participated in a project‐specific, graduate‐level team science course. The purposes of this course were to: (1) introduce students to the goals of the project, (2) build trainees' understanding of how big projects operate, and (3) allow trainees to explore how their research interests dovetailed with the overall project. Additionally, trainees received training regarding: (1) diversity, equity & inclusion, (2) giving and receiving feedback, and (3) effective communication. Onboarding through the team science course cultivated psychological safety and a collaborative student community across disciplines and institutions. Thus, we recommend a team science course for onboarding students to big projects to help students establish the skills necessary for collaborative research. Project‐based team science classes can benefit student advancement, enhance the productivity of the project, and accelerate the discovery of solutions to ecological issues by building community, establishing a shared project vocabulary, and building a workforce with collaborative skills to better answer ecological research questions. [ABSTRACT FROM AUTHOR]

Internal phosphorus (P) loading from the sediment poses a high risk of being an additional P source to deteriorate water quality. Previous studies hypothesized that internal P loads can be as high as external P inputs, especially in P enriched landscapes such as agricultural areas. However, internal P loadings in eutrophic conditions are rarely quantified or compared with external P loads. In this study, we aimed to answer these three questions; 1) how much P is internally released from the sediment of hypereutrophic lakes? 2) how much do internal P loads contribute to lake water quality compared to external loads? and 3) what factors regulate the release and retention of P in the sediment? We selected four hypereutrophic lakes located in Eastern Nebraska. In the study lakes and watersheds, internal and external P loads were quantified in 2014. Total P concentrations of inflow water collected from primary water channels feeding the study lakes and daily inflow water discharge rates were used to calculate external P loads. Internal P loads were quantified from flow-through soil core incubation experiments. External TP loads varied temporarily depending on the changes in discharge, and were highest during spring storm events. The majority of internal P loading (i.e. P release from sediment) occurred in the summer when lakes experience strong stratification (i.e. anaerobic conditions). This is likely associated with oxygen availability in the sediments and chemical dissolution of P. By comparing the annual-scale of external and internal P inputs, external P loadings were still the dominant P source to the lakes, contributing up to 98% of the total P input whereas internal P loadings accounted for 4–12% of the total P input. Although internal P loads were relatively minor on an annual time scale, we found that summer internal loadings in some of study lakes exceeded their external loadings. Our results confirmed the dominant influence of external P loadings on water quality in the reservoirs. This suggests that non-point source controls and watershed management strategies to reduce external loadings should be implemented prior to internal P loading controls. Internal P loadings can be a significant P source, even if just temporarily, worsening water quality of agricultural reservoirs and downstream ecosystems in the summer. [ABSTRACT FROM AUTHOR]

Understanding linked hydrologic and biogeochemical processes such as nitrate loading to agricultural streams requires that the sampling bias and precision of monitoring strategies be known. An existing spatially distributed, high-frequency nitrate monitoring network covering ~40% of Iowa provided direct observations of in situ nitrate concentrations at a temporal resolution of 15 min. Systematic subsampling of nitrate records allowed for quantification of uncertainties (bias and precision) associated with estimates of various nitrate parameters, including: mean nitrate concentration, proportion of samples exceeding the nitrate drinking water standard (DWS), peak (>90th quantile) nitrate concentration, and nitrate flux. We subsampled continuous records for 47 site-year combinations mimicking common, but labor-intensive, water-sampling regimes (e.g., time-interval, stage-triggered, and dynamic-discharge storm sampling). Our results suggest that time-interval sampling most efficiently characterized all nitrate parameters, except at coarse frequencies for nitrate flux. Stage-triggered storm sampling most precisely captured nitrate flux when less than 0.19% of possible 15 min observations for a site-year were used. The time-interval strategy had the greatest return on sampling investment by most precisely and accurately quantifying nitrate parameters per sampling effort. These uncertainty estimates can aid in designing sampling strategies focused on nitrate monitoring in the tile-drained Midwest or similar agricultural regions. [ABSTRACT FROM AUTHOR]

Hot spots and hot moments of greenhouse gas (GHG) fluxes can contribute significantly to overall GHG budgets. Hot spots and hot moments occur when dynamic soil hydrology triggers important shifts in soil biogeochemical and physical processes that control GHG emissions. Soil oxygen (O 2 ), a direct control on biogenic GHG production (i.e., nitrous oxide-N 2 O, carbon dioxide-CO 2 and methane-CH 4 ), may serve as both an important proxy for determining sudden shifts in subsurface biogenic GHG production, as well as the physical transport of soil GHG to the atmosphere. Recent technological advancements offer opportunities to link in-situ , near-continuous measurements of soil O 2 concentration to soil biogeochemical processes and soil gas transport. Using high frequency data, this study asked: Do soil O 2 dynamics following short-term (<8 days) soil saturation correspond to changes in soil GHG concentrations and GHG surface fluxes? We addressed this question in a restored riparian wetland in Ohio, USA. Changes in subsurface (10 and 20 cm) CO 2 and N 2 O concentrations were inversely related to short-term (<48 h) changes in soil O 2 concentrations. Subsurface CH 4 concentrations, however, did not change in response to soil O 2 dynamics. Changing subsurface GHG concentrations did not necessarily translate into altered surface (soil to atmosphere) GHG fluxes; soil O 2 dynamics at 10 cm did not correspond with changes in surface N 2 O and CH 4 fluxes. However, changes in soil O 2 concentration at 10 cm had a significant positive linear relationship with change in surface CO 2 flux. Our study suggests that monitoring near-continuous soil O 2 concentration under dynamic soil hydrology may lead to greater understanding of hot spots and hot moments of GHG emissions. This understanding is increasingly important for estimating the contribution of soil processes to atmospheric GHG concentrations. [ABSTRACT FROM AUTHOR]

As we build a more diverse, equitable, and inclusive culture in the ecological research community, we must work to support new ecologists by empowering them with the knowledge, tools, validation, and sense of belonging in ecology to succeed. Undergraduate research experiences (UREs) are critical for a student's professional and interpersonal skill development and key for recruiting and retaining students from diverse groups to ecology. However, few resources exist that speak directly to an undergraduate researcher on the diversity, equity, and inclusion (DEI) dimensions of embarking on a first research experience. Here, we write primarily for undergraduate readers, though a broader audience of readers, especially URE mentors, will also find this useful. We explain many of the ways a URE benefits undergraduate researchers and describe how URE students from different positionalities can contribute to an inclusive research culture. We address three common sources of anxiety for URE students through a DEI lens: imposter syndrome, communicating with mentors, and safety in fieldwork. We discuss the benefits as well as the unique vulnerabilities and risks associated with fieldwork, including the potential for harassment and assault. Imposter syndrome and toxic field experiences are known to drive students, including students from underrepresented minority groups, out of STEM. Our goal is to encourage all students, including those from underrepresented groups, to apply for UREs, build awareness of their contributions to inclusion in ecology research, and provide strategies for overcoming known barriers. [ABSTRACT FROM AUTHOR]

The issues facing academic mothers have been discussed for decades. Coronavirus Disease 2019 (COVID-19) is further exposing these inequalities as womxn scientists who are parenting while also engaging in a combination of academic related duties are falling behind. These inequities can be solved by investing strategically in solutions. Here we describe strategies that would ensure a more equitable academy for working mothers now and in the future. While the data are clear that mothers are being disproportionately impacted by COVID-19, many groups could benefit from these strategies. Rather than rebuilding what we once knew, let us be the architects of a new world. The COVID-19 pandemic is highlighting the many long-standing inequalities that academic mothers face. This Essay describes solutions for a more equitable academia, now and in the future, maintaining that rather than rebuilding what we once knew, we should be the architects of a new world. [ABSTRACT FROM AUTHOR]

Wetlands are the largest natural source of atmospheric methane (CH4), a potent greenhouse gas. Hydrology and species composition are important controls on wetland CH4 emissions. Few studies target interactive effects on CH4 fluxes, but rather study variables in isolation. Therefore, we asked: How do hydrology and plant species interact to affect CH4 fluxes from wetland soils? We measured CH4 fluxes under stable water tables in mesocosms planted with Asclepias incarnata L. and mesocosms planted with Alisma triviale Pursh. We then tested the interactive effects in saturated and unsaturated restored field locations by measuring CH4 fluxes from the plants and the surrounding soil. In mesocosms, CH4 fluxes from A. incarnata were 8-fold greater than fluxes from control (no plant) or A. triviale mesocosms. Alisma triviale mesocosms had higher CO2 to CH4 ratio (less methanogenic dominance) than control mesocosms but did not differ significantly from A. incarnata mesocosms. In the field, hydrology was the dominant control of CH4 flux; both plant species produced approximately 10-fold more CH4 in saturated plots than in unsaturated plots. Incorporating hydrology and species composition into modeling will better predict CH4 fluxes from wetland soils, which in turn could aid in designing restored wetlands that offset greenhouse gas emissions. [ABSTRACT FROM AUTHOR]

Abstract To Understanding the mechanisms that lead to greenhouse gas (GHG) emissions requires knowledge of short-term fluctuations in drivers of fluxes. To better understand how variation and lags in abiotic conditions affect soil GHG fluxes, we coupled weekly methane (CH 4) and nitrous oxide (N 2 O) flux measurements with time-series soil sensor data for oxygen (O 2), moisture and temperature. Including time-series data improved models for soil CH 4 and N 2 O fluxes compared to models which did not contain time series data. Fluctuations in soil O 2 drove CH 4 fluxes, where rapid changes in redox conditions led to high fluxes. N 2 O fluxes occurred when soils were warm and dry. Soil O 2 was the best predictor and thus sensor for understanding CH 4 fluxes, whereas soil moisture best predicted N 2 O fluxes. Combining data from multiple sensors improved models for both gases, underscoring the relative importance of interactions among drivers. Overall, our top models explained 15% and 30% of the variance in N 2 O and CH 4 fluxes, respectively. Fluxes predicted using linear interpolation between measured fluxes were lower than time-series model predictions for both N 2 O and CH 4 fluxes. This suggests linear interpolation may fail to capture "hot moments" or episodic events which lead to high fluxes. Long-term, continuous data from sensors, which accounts for short-term variation in abiotic drivers, may improve our ability to predict the timing and intensity of GHG emissions from wetland soils. Highlights • Time-lags and variance in drivers improved GHG emissions models. • Soil O 2 data predicted CH 4 fluxes; soil moisture data predicted N 2 O fluxes. • Sensor-informed models captured GHG hot moments missed by linear-interpolation. [ABSTRACT FROM AUTHOR]

In the Anthropocene, watershed chemical transport is increasingly dominated by novel combinations of elements, which are hydrologically linked together as ‘chemical cocktails.’ Chemical cocktails are novel because human activities greatly enhance elemental concentrations and their probability for biogeochemical interactions and shared transport along hydrologic flowpaths. A new chemical cocktail approach advances our ability to: trace contaminant mixtures in watersheds, develop chemical proxies with high-resolution sensor data, and manage multiple water quality problems. We explore the following questions: (1) Can we classify elemental transport in watersheds as chemical cocktails using a new approach? (2) What is the role of climate and land use in enhancing the formation and transport of chemical cocktails in watersheds? To address these questions, we first analyze trends in concentrations of carbon, nutrients, metals, and salts in fresh waters over 100 years. Next, we explore how climate and land use enhance the probability of formation of chemical cocktails of carbon, nutrients, metals, and salts. Ultimately, we classify transport of chemical cocktails based on solubility, mobility, reactivity, and dominant phases: (1) sieved chemical cocktails (e.g., particulate forms of nutrients, metals and organic matter); (2) filtered chemical cocktails (e.g., dissolved organic matter and associated metal complexes); (3) chromatographic chemical cocktails (e.g., ions eluted from soil exchange sites); and (4) reactive chemical cocktails (e.g., limiting nutrients and redox sensitive elements). Typically, contaminants are regulated and managed one element at a time, even though combinations of elements interact to influence many water quality problems such as toxicity to life, eutrophication, infrastructure corrosion, and water treatment. A chemical cocktail approach significantly expands evaluations of water quality signatures and impacts beyond single elements to mixtures. High-frequency sensor data (pH, specific conductance, turbidity, etc.) can serve as proxies for chemical cocktails and improve real-time analyses of water quality violations, identify regulatory needs, and track water quality recovery following storms and extreme climate events. Ultimately, a watershed chemical cocktail approach is necessary for effectively co-managing groups of contaminants and provides a more holistic approach for studying, monitoring, and managing water quality in the Anthropocene. [ABSTRACT FROM AUTHOR]

Nitrous oxide (N2O) is a potent greenhouse gas that contributes to climate change and stratospheric ozone destruction. Anthropogenic nitrogen (N) loading to river networks is a potentially important source of N2O via microbial denitrification that converts N to N2O and dinitrogen (N2). The fraction of denitrified N that escapes as N2O rather than N2 (i.e., the N2O yield) is an important determinant of how much N2O is produced by river networks, but little is known about the N2O yield in flowing waters. Here, we present the results of whole-stream 15N-tracer additions conducted in 72 headwater streams draining multiple land-use types across the United States. We found that stream denitrification produces N2O at rates that increase with stream water nitrate (NO3-) concentrations. but that <1% of denitrified N is converted to N2O. Unlike some previous studies, we found no relationship between the N2O yield and stream water NO3. We suggest that increased stream NO3 loading stimulates denitrification and concomitant N2O production, but does not increase the N2O yield. In our study, most streams were sources of N2O to the atmosphere and the highest emission rates were observed in streams draining urban basins. Using a global river network model, we estimate that microbial N transformations (e.g.. denitrification and nitrification) convert at least 0.68 Tg'y' of anthropogenic N inputs to N2O in river networks, equivalent to 10% of the global anthropogenic N2O emission rate. This estimate of stream and river N2O emissions is three times greater than estimated by the Intergovernmental Panel on Climate Change. [ABSTRACT FROM AUTHOR]