In agricultural landscapes, increasing groundwater nitrate concentrations are common and reflect leaching from cultivated soils, often into adjacent riparian zones within stream corridors. High nitrate concentrations may be attenuated in riparian groundwater, where abundant organic matter and saturated anoxic soils and sediments (collectively "substrate") support denitrifying activity. Variable substrate and the resulting residence time distribution in shallow groundwater can drive redox status and net nitrate removal, yet can be challenging to simulate in detail. This study explores how spatial variation in the texture of riparian aquifer substrate may influence groundwater residence times and biogeochemical behaviour of a riparian aquifer subject to chronic nitrate loading from non-irrigated wheat production in the semiarid Northern Great Plains. This is addressed using a novel combination of physicochemical measurements, geophysical observation of a groundwater tracer injection, and a simplified groundwater mixing model analysis. Higher and more variable nitrate concentrations were documented in wells completed in coarser substrates, suggesting generally shorter residence times compared to finer substrates, which exhibited lower nitrate concentrations suggestive of longer residence times. Therefore, we hypothesised that net nitrate consumption could be captured with a simple simulation approach using (a) the proportion of finer-textured riparian aquifer substrate to quantify redoximorphic processes that result in net nitrate consumption and (b) the proportion of coarser riparian substrate to quantify the groundwater residence time distribution. We tested this hypothesis by first exploring spatial patterns in groundwater chemistry and hydraulic characteristics at 16 shallow (< 1.5 m) wells and then by directly observing residence time of solutes in groundwater flow, using high frequency monitoring of groundwater specific conductivity and time-lapse electrical resistivity tomography imaging of a cross-section of a riparian groundwater flow path. Mixing models informed by geophysical imaging and tracer breakthrough constrain the potential influence of exchanges between fine and coarse substrates on the net nitrate transformation occurring along riparian groundwater flow paths. Dual-textured groundwater mixing model simulations illustrate how the proportion of coarse textured material may dictate the total amount of flow through riparian substrate while the proportion of flow through the finer-textured material that mixes with flow through coarse material may dictate the extent of net nitrate consumption processes. This work leverages novel geophysical observations to contribute a simple bimodal approach exploring how hydrologic complexity in riparian subsurface flow systems may influence the overall potential of riparian groundwaters to process nutrients before watershed export.

Understanding how ecological and social constraints interacted to shape bison hunting systems during the late Holocene reveals the dynamic ways bison hunting strategies adapted to changing conditions. At the Bergstrom site in central Montana, bison were hunted intermittently for roughly seven centuries before archaeologically visible use ceased near 1100 cal yr BP. To explain why hunting stopped despite continued regional bison presence, we integrate archaeological excavation, radiocarbon chronology, and multiproxy riparian paleoecology (pollen, charcoal, coprophilous fungal spores) with regional drought reconstructions and analysis of radiocarbon-dated bison occurrences. Local environmental proxies show stable vegetation, low fire activity, and persistent large-herbivore indicators after abandonment, providing little support for ecological transformation as a cause. Regional synthesis reveals that archaeological bison frequencies increased 5.5-fold through the Holocene while paleontological frequencies remained stable, with peak hunting intensity coinciding with severe, multi-decadal droughts. These findings contradict models of population tracking and indicate that hunting reorganization, not prey scarcity, led to site abandonment. The most parsimonious explanation is convergence of constraints: drought reduced processing water at hydrologically marginal sites while rising organizational demands favored larger, infrastructure-intensive communal operations. The abandonment of the small Bergstrom site likely reflects an adaptive reorganization of bison hunting efforts toward larger, topographically advantageous sites better suited to increasingly coordinated communal hunting systems. This case study illustrates how historical hunting systems maintained regional persistence through episodic site use and localized abandonment, providing empirical guidance for contemporary managers seeking to restore the spatial heterogeneity and adaptive capacity that supported bison-human systems under climatic variability.

Environmental research software applications are foundational tools for modeling and predicting Earth system dynamics. Among these, reactive transport models (RTMs) are particularly important because they simulate critical system processes that affect contaminant transport and water quality. Many RTMs are developed by domain scientists rather than software engineers, and have quality-in-use (QIU) issues that frustrate end users. Addressing these frustrations requires a clear understanding of the problems and challenges that users experience. We characterize the QIU frustrations most concerning to RTM users using a systematic approach. Specifically, we analyze RTM user forums and codify user-reported challenges by operationalizing the QIU characteristics and subcharacteristics defined in the ISO/IEC 25019:2023 standard. We extracted 3,941 forum threads from four widely used RTM user forums. Terms and phrases related to QIU barriers were identified using natural language processing (NLP) and mapped to QIU subcharacteristics to calculate their frequencies. The QIU characteristic of Beneficialness challenged users most frequently. Within Beneficialness, Usability caused the most frustration across the four forums, suggesting users struggle to achieve their goals effectively, efficiently, and with satisfaction. These findings highlight the importance of providing clear, accurate user resources, such as documentation and tutorials, to support research goals. By operationalizing the full scope of the QIU standard, we offer a transferable method for identifying end-user challenges across domains. By treating QIU as a first-class citizen, developers can improve user satisfaction and develop user-centric, trustworthy software systems across software domains.

Headwater streams gain groundwater carrying dissolved inorganic carbon (DIC) from terrestrial environments and are hotspots for biogeochemical processing. The Judith River Watershed is located in a semi-arid region of the northern Great Plains in central Montana, where stream corridors have spatial variation in hydrological connectivity among upland landscapes, riparian aquifers and stream channels, allowing for assessment of the role of riparian zone in corridor DIC processing. Upland groundwaters draining into stream corridors either contribute directly to the stream channel as surface flow (i.e., springs) or recharge biogeochemically active subsurface flow paths through riparian substrates. Here, we quantified DIC lateral inputs via springs versus riparian groundwater, carbon dioxide () emissions from channel waters, and estimated stream metabolism based on continuous two-station monitoring of dissolved oxygen (DO) and at three corridor reaches across different seasons and years. We found that the three stream reaches received different proportions of riparian groundwater and springs. Reaches dominated by riparian groundwater inflows had and DIC concentrations that were typically two times greater compared to the reach dominated by spring inflows, and displayed larger seasonal variations in discharge (15-100 L ) and gas transfer velocities ( from 2 to 8 m ). values dictate fates (emission vs. export) of DIC generated not only from lateral inputs, but in-stream metabolic processes with 0%-75% of Net Ecosystem Production exported downstream. This study demonstrates that DIC cycling in headwater streams is tightly linked to landscape patterns of stream corridors.

Stream corridors play a critical role in reducing contaminant export, yet limited understanding of controls on riparian biogeochemical processes hinders effective water quality management. To infer outcomes from riparian biogeochemical processes, we analyzed nitrate and sulfate abundance and isotopic composition in water samples from upland groundwater, riparian groundwater, and stream water across three ca. 0.7-km reaches draining an extensively cultivated terrace landform. Nitrate showed net loss from upland groundwaters to stream water, with stream samples having nitrate-δ15N and δ18O values up to ca. + 12‰ and + 2‰, respectively, and lower nitrate concentrations (ca. 3 mg L−1) than terrace groundwater inflows (ca. 20 mg N L−1). Riparian groundwater samples had nitrate-δ15N and δ18O values up to + 40‰ and + 15‰, respectively, with low concentrations near 1 mg N L−1, indicating loss along riparian flow paths. Sulfate showed net gains in concentration, with stream water having low sulfate-δ34S values (ca. - 18‰) compared with terrace groundwater (ca. - 10‰), and high sulfate-δ18O values (up to + 6‰) compared to ambient riparian groundwater (water-δ18O: - 20 to - 14‰). These results suggest that sulfide oxidation during marine shale weathering is cycled through redox transformations under fluctuating saturation conditions in riparian systems. We use relationships in abundance and isotopic composition from uplands to streams to constrain the potential magnitude of gross gains and losses influencing observed net sulfate gains and nitrate losses. Our findings highlight how losses, gains, and mixing processes influence water quality through solute loss to gaseous phases, solute production in the riparian system, and redox cycling in stream corridors.

Rangelands play a crucial role in providing various ecosystem services and have significant potential for carbon sequestration. However, monitoring soil organic carbon (SOC) stocks in rangelands is challenging due to the large size of ranches and the high spatial variability influenced by climate and management factors. To address these challenges, we have developed the Rangeland Carbon Tracking and Management (RCTM) system, which integrates remote sensing inputs, survey data sources, and both empirical and process-based SOC models. In this work, we will introduce the structure of RCTM v1.0, its data input requirements, data processing pipelines, and the resulting data outputs. Additionally, we will discuss the high-resolution soil moisture data layers, baseline SOC maps, and the targeted field sampling plan generated through an empirical digital soil mapping approach. The Bayesian calibration and validation scheme for obtaining grassland plant functional type (PFT)-specific parameters using flux tower network data will also be explained. After calibration, the RCTM system generated estimates of rangeland carbon fluxes across PFTs (R2 between 0.6 and 0.7) and surface depth SOC stocks (R2 = 0.6) with moderate accuracy at the regional scale. The visualization of modeling results associated with long-term rangeland C dynamics at different scales will be demonstrated using the Google Earth Engine platform to inform management decisions and policymaking.

Deep exposures of soil profiles on Miocene or Mio-Pliocene alluvial deposits were studied along a 500 km N-S transect in the Atacama Desert. These ancient deposits, with excellent surface preservation, now stand many meters above a broad incised PlioPleistocene alluvial terrain. Total geochemical analyses and mass balance calculations allowed the establishment of elemental gains, losses, and redistribution in the soils. From north to south (presently hyperarid to arid), the ancient soils reveal an increase in losses of rock-forming elements (Si, Al, Fe, K, Mg). Additionally, rare earth elements (REE) show losses with increasing southerly latitude and systematic patterns with soil depth. Some REEs appear to be unique chemical tracers of exogenous dust and aerosol additions to the soils. The removal of major elements and REEs is impossible in the present climate (one of salt and dust accumulation), revealing that for a significant period following the deposition of the alluvium, soils were exposed to rainfall, chemical weathering, and mass loss-with a geographical pattern that mirrors the present rainfall gradient in the region. Following the cessation of weathering, the pre-weathered soils have undergone enormous dust and salt accumulations, with the rates and types of salt accumulation consistent with latitude: (1) carbonate in the south and (2) sulfate, chlorides, and nitrates to the north. The quantity, and apparent rates, of salt accumulation have a strong latitudinal trend. Isotopes of sulfate have predictable depth patterns based on isotope fractionation via vertical reaction and transport. The relict hyperarid soils are geochemically similar to buried Miocene soils (ca. 10-9 Ma) in the region, but they differ from older Miocene soils, which formed in more humid conditions. The overall soil record for the Atacama Desert appears to be the product of changes in Pacific Ocean sea surface temperatures over time, and resulting changes in rainfall. The mid-Miocene was relatively humid based on buried soil chemistry and evidence of fluvial activity. The mid to late Miocene cooling (ca. 10-5.5 Ma) appears to have aridified the region based on paleosol soil chemistry. Pliocene to earliest Pleistocene conditions caused weathering of the relict soils examined here, and regional fluvial activity. Since the earliest Pleistocene, the region has largely experienced the accumulation of salts and, except for smaller scale oscillations (glacial-interglacial), has experienced protracted hyperaridity.

Water quality is an emergent property of solute interactions with process domains, such as stream corridors, aquifers, and soils. Here, we explore the utility of a "storage-exchange reference frame" that partitions a process domain as an "environmental reactor" having both a nonreactive transport compartment and a reactive storage zone. We developed a simulation model of stream corridors with a storage-exchange reference frame and varied the exchange of water and solutes between the channel (transport compartment) and hyporheic zone (reactive storage), and the first-order reaction rate constant governing reaction-rate kinetics in the reactive storage. We subsequently calculated Damkohler indices to quantitatively describe controls on environmental reactive transport. Whole stream Damkohler indices were strongly governed by the interaction of the biogeochemical reaction-rate constant with the hydraulic exchange of water and advective exchange of solutes between the channel and hyporheic zone. Thus, the storage-exchange reference frame is useful for incorporating the effects of linked channel-hyporheic processes in stream corridors and shows promise for extension to other process domains.

Rangelands provide significant environmental benefits through many ecosystem services, which may include soil organic carbon (SOC) sequestration. However, quantifying SOC stocks and monitoring carbon (C) fluxes in rangelands are challenging due to the considerable spatial and temporal variability tied to rangeland C dynamics as well as limited data availability. We developed the Rangeland Carbon Tracking and Management (RCTM) system to track long‐term changes in SOC and ecosystem C fluxes by leveraging remote sensing inputs and environmental variable data sets with algorithms representing terrestrial C‐cycle processes. Bayesian calibration was conducted using quality‐controlled C flux data sets obtained from 61 Ameriflux and NEON flux tower sites from Western and Midwestern US rangelands to parameterize the model according to dominant vegetation classes (perennial and/or annual grass, grass‐shrub mixture, and grass‐tree mixture). The resulting RCTM system produced higher model accuracy for estimating annual cumulative gross primary productivity (GPP) (R2 > 0.6, RMSE <390 g C m−2) relative to net ecosystem exchange of CO2 (NEE) (R2 > 0.4, RMSE <180 g C m−2). Model performance in estimating rangeland C fluxes varied by season and vegetation type. The RCTM captured the spatial variability of SOC stocks with R2 = 0.6 when validated against SOC measurements across 13 NEON sites. Model simulations indicated slightly enhanced SOC stocks for the flux tower sites during the past decade, which is mainly driven by an increase in precipitation. Future efforts to refine the RCTM system will benefit from long‐term network‐based monitoring of vegetation biomass, C fluxes, and SOC stocks.

Coal mining and reclamation can have a profound influence on hydrogeologic systems, with clear consequences for groundwater quality, yet their long-term influence on downgradient water quality over time following reclamation is less well documented. Geochemical trends were evaluated in water quality downgradient of a fully reclaimed landscape at the former Big Sky Mine in the Rosebud Creek watershed (southeastern Montana, USA), over a 3-year period (2020–2022), including bond release in 2022. Within 6 km downgradient from the reclaimed area, sulfate concentrations decreased from approximately 3500 to 1800 mg l−1 within the Miller Coulee alluvial aquifer. Major ions, δ34SSO4 values, and residence time tracers suggest that the observed decreases in sulfate concentration result from a combination of dilution by mixed-age inflows and incomplete transit of the high salinity plume from the mine boundary. Both bedrock and alluvial aquifers of the Rosebud Creek corridor contained contributions of millennia-old regional groundwater, which may serve to mitigate mine-derived high salinity waters. Rosebud Creek, which traverses the outflow zone of Miller Coulee in the study area, exhibited high sulfate concentrations during low flows and consistent downgradient increases in sulfate concentration. The possibility of plume dynamics in Miller Coulee suggests that the greatest water quality impacts may not yet have reached Rosebud Creek.

Relative to their limited areal extent, riparian ecosystems are disproportionately important in regulating inorganic solute export from agricultural landscapes. We investigated spatial patterns of solute concentrations in surface and ground waters of stream corridors to infer the dominant hydrologic transport and biogeochemical pathways that influence riparian nitrate and sulfate processing from uplands to streams. We selected three reaches of stream corridors draining an agricultural landscape that vary in hydrologic connection with upland aquifers. Non-irrigated crop production dominates land use in the study area and influences the quality of upland groundwater draining to the stream corridors. We interpret patterns in solute concentrations of riparian groundwater and stream water relative to upland groundwater to infer the influences of biogeochemical processing and hydrologic connectivity. Excess nitrate from cultivated soils is evident in upland groundwater concentrations that consistently exceed the U.S. Environmental Protection Agency public drinking water standard. Nitrate and oxygen concentrations in riparian groundwaters were consistently lower than in terrace groundwater and adjacent stream waters, suggesting rapid consumption of oxygen and influence of anaerobic metabolic reduction processes in subsurface flow. Sulfate concentrations in streams were higher than in terrace groundwater, likely due to weathering of shale-derived substrate in riparian aquifers. The degree of solute mitigation or augmentation by riparian biogeochemical processes depended on the geomorphic context that controlled the fraction of upland water passing through the riparian substrate. Observed net nitrate losses with net sulfate gains from uplands to stream channels reflect flow paths through a complex distribution of redox conditions throughout the riparian areas, emphasizing the importance of considering riparian area heterogeneity in predicting solute export in streams. This research contributes to understanding how stream corridor substrate and geomorphic context controls the biogeochemical and hydrologic processes influencing the quality of water exported from agricultural landscapes.

In recent years, the National Fish and Wildlife Foundation (NFWF) has invested heavily in improving grazing management across the United States, with particular emphasis on the Great Plains (Figure EX1). This work advances the goals of many partners (government, business and nonprofit) who share interests in the conservation of grasslands through management and restoration actions that sequester carbon (C) and reduce atmospheric greenhouse gases (GHG). Alongside wildlife benefits, NFWF and many other organizations currently estimate the GHG benefits of improved grazing management using simplistic emission factor-based methods; however, our collective confidence in those estimates has been constrained by the short duration and limited sampling (number of samples, spatial extent, controls) of previous studies.

Accurately quantifying high-resolution field-scale soil organic carbon (SOC) stocks is challenging yet crucial for improving site-specific land management and carbon accounting. This challenge is even greater when the study units are large heterogenous ranches. This study utilizes a digital soil mapping (DSM) approach and a U.S. legacy soil dataset, combined with soil, climate, biotic, and topographic covariate datasets, to design a targeted soil sampling plan for acquiring local samples. The resulting local samples were then used in combination with the legacy data to build optimal ranch-scale SOC stock models. We provide an example of this approach using ranch units in the western U.S. as a case study. In our approach we first applied a clustering analysis to generate spatial clusters. This was followed by adopting a conditioned Latin hypercube sampling scheme within each cluster, to generate sets of strategically selected local sampling points. The local samples required for improved estimates of SOC stocks were determined to have a sample size of 15 and 40 soil cores, within the respective 13 and 36 km(2) parcels. While our modeling results for SOC concentrations at a relatively homogeneous site in eastern Montana showed a significant two-fold improvement in model fit when using individually selected calibration datasets for each point, as opposed to selecting the calibration dataset as a whole at the ranch level, the disparity between the pixel- and ranch-based models was inconsequential for the other two ranch sites in Colorado that were more spatially diverse in terms of land management and vegetation cover. Compared to SOC concentration models (R-2 between 0.3 and 0.7), the performance of models for bulk density (BD) (R-2 < 0.4) and SOC stocks (R-2 < 0.2) were relatively poor. Strategies including utilizing a subset of covariates, incorporating broader-scale national calibration datasets, and modeling by depths did not further improve BD and SOC stock models. Future work should explore whether the addition of temporally dynamic environmental covariates can improve SOC stock estimates, and whether the DSM-supported targeted field sampling strategy and high-resolution mapping approach can be successfully applied elsewhere.

Climate forecasts for semi-arid landscapes suggest changes in seasonality and form of precipitation. These shifts are expected to alter the structure and function of grassland and steppe ecosystems and present challenges for land management and crop production in regions like the Northern Great Plains, North America. Precipitation in lower-elevation, semi-arid areas provides a local supply of soil water that drives biogeochemical cycling, agricultural production, and groundwater recharge. However, studies of the fate of precipitation are far less common in lower-elevation areas compared to studies of the fate of seasonal snowpack and runoff in alpine areas. This research uses isotopic composition of water (δ O and δ H) to explore the sources and fate of soil water in lower elevation areas of the Judith River watershed, in the headwaters of the Missouri River in Montana, USA. Extensive non-irrigated crop production in this area occurs on well-drained soils and depends on careful water management. Agricultural fertilization and organic matter mineralization have resulted in excessive nitrate leaching from cultivated soils into shallow aquifers and streams. Our observations indicate that colder precipitation contributes isotopically distinct water to cultivated terrace soils relative to downgradient groundwaters and streams. Riparian waters also exhibit isotopically distinct contributions from colder precipitation. Apparent contributions from colder precipitation in terrace and riparian soil waters suggest that snowmelt is an important component of water supply to these systems. In riparian waters, influence of evaporation is also evident, suggesting sufficient residence times and atmospheric exposure for local processing to occur. The evolution of isotopic composition from soils to shallow aquifers to stream corridors indicates source water partitioning as precipitation moves through a semi-arid agricultural landscape. Mixing processes apparent in landscape water isotopic compositions reveal source water dynamics that facilitate plant uptake, solute processing, and contaminant leaching.

The thickness or depth of fine-textured soil (zf) dominates water storage capacity and exerts a control on nutrient leaching in semi-arid agroecosystems. At small pixel sizes (< 1 m; ‘fine resolution’), the normalized difference vegetation index (NDVI) of cereal crops during senescence (Zadoks Growth Stages [ZGS] 90–93) offers a promising alternative to destructive sampling of zf using soil pits. However, it is unclear whether correlations between zf and NDVI exist (a) at larger pixel sizes (1–10 m; ‘intermediate resolution’) and (b) across field boundaries. The relationship of zf to NDVI of wheat (Triticum aestivum L.) was tested using images from a combination of multispectral sensors and fields in central Montana. NDVI was derived for one field using sensors of fine and intermediate spatial resolution and for three fields using intermediate resolution sensors only. Among images acquired during crop senescence, zf was correlated with NDVI (p < 0.05) independent of sensor (p = 0.22) and field (p = 0.94). The zf relationship to NDVI was highly dependent on acquisition day (p < 0.05), but only when pre-senescence (ZGS ≤ 89) images were included in the analysis. Results indicate that cereal crop NDVI of intermediate resolution can be used to characterize zf across field boundaries if image acquisition occurs during crop senescence. Based on these findings, an empirical index was derived from multi-temporal Sentinel-2 imagery to estimate zf on fields in and beyond the study area.

Low nitrogen use efficiency (NUE) is ubiquitous in agricultural systems, with mounting global scale consequences for both atmospheric aspects of climate and downstream ecosystems. Since NUE-related soil characteristics such as water holding capacity and organic matter are likely to vary at small scales (< 1 ha), understanding the influence of soil characteristics on NUE at the subfield scale (< 32 ha) could increase fertilizer NUE. Here, we quantify NUE in four conventionally managed dryland winter-wheat fields in Montana following multiple years of sub-field scale variation in experimental N fertilizer applications. To inform farmer decisions that incorporates NUE, we developed a generalizable model to predict subfield scale NUE by comparing six candidate models, using ecological and biogeochemical data gathered from open-source data repositories and from normal farm operations, including yield and protein monitoring data. While NUE varied across fields and years, efficiency was highest in areas of fields with low N availability from both fertilizer and estimated mineralization of soil organic N (SON). At low levels of applied N, distinct responses among fields suggest distinct capacities to supply non-fertilizer plant-available N, suggesting that mineralization supplies more available N in locations with higher total N, reducing efficiency for any applied rate. Comparing modelling approaches, a random forest regression model of NUE provided predictions with the least error relative to observed NUE. Subfield scale predictive models of NUE can help to optimize efficiency in agronomic systems, maximizing both economic net return and NUE, which provides a valuable approach for optimization of nitrogen fertilizer use.