Browsing by Author "Ewing, Stephanie A."

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Connections among soil, ground, and surface water chemistries characterize nitrogen loss from an agricultural landscape in the upper Missouri River basin
(2018-01) Sigler, W. Adam; Ewing, Stephanie A.; Jones, Clain A.; Payn, Robert A.; Brookshire, E. N. Jack; Klassen, Jane K.; Jackson-Smith, Douglas; Weissmann, Gary S.
Elevated nitrate in shallow aquifers is common in agricultural areas and remediation requires an understanding of nitrogen (N) leaching at a variety of spatial scales. Characterization of the drivers of nitrate leaching at the mesoscale level is needed to bridge from field-scale observations to the landscape-scale context, allowing informed water resource management decisions. Here we explore patterns in nitrate leaching rates across a depositional landform in the northern Great Plains within the Upper Missouri Basin, where the predominant land use is non-irrigated small grain production, and nitrate-N concentrations above 10 mg L1 are common. The shallow Moccasin terrace (260 km2) aquifer is bounded in vertical extent by underlying shale and is isolated from mountain front stream recharge, such that aquifer recharge is dominated by infiltration of precipitation through agricultural soils. This configuration presents a simple landform-scale water balance that we leveraged to estimate leaching rates using groundwater nitrate concentrations and surface water discharge, and quantify uncertainty using a Monte Carlo approach based on spatial variation in observations of groundwater nitrate concentrations. A participatory research approach allowed local farmer knowledge of the landscape to be incorporated into the study design, improved selection of and access to sample sites, and enhanced prospects for addressing nitrate leaching through collaborative understanding of system hydrology. Mean landform-scale nitrate-N leaching rates were 11 and 18 kg during the 2012-2014 study for the two largest catchments draining the terrace. Over a standard three-year crop rotation, these leaching rates represent 19 to 31% of typical fertilizer N application rates; however, leaching losses are likely derived not only from fertilizer but also from soil organic N mineralization, and are apparently higher during the post-fallow phase of the crop rotation. Groundwater apparent age is relatively young (0-5 yr) based on tritium-helium analysis, but whole-aquifer turnover time calculations are an order of magnitude longer (20-23 yr), suggesting changes in groundwater may lag behind changes in land management by years to decades.
Data from: An exploration for truth in soil parent materials in Hyalite Canyon, Montana [dataset]
(2015-08) Sugden, John Charles; Ewing, Stephanie A.; Challender, Stuart; Hartshorn, Anthony
The raw data set contains geochemical data from a soil "lithosequence" in Hyalite Canyon, MT near the Langohr Campground and includes data for underlying rock. Data for many major and minor (including rare earth and high field strength) elements and oxides are included. Atmospheric deposition samples were collected at the Arthur Post Farm and from the rooftops of Bobcat Stadium. Duplicates included in the data set are marked. The three calculation spreadsheets convey methods to calculate the fractional rock influence on soil materials <2mm; to determine chemical depletion from rock only and from rock and atmospheric deposition; and to calculate mass balance using rock only, atmospheric deposition only, and mass balance using both rock and atmospheric deposition scaled using the fraction rock calculations mentioned above.
Fallow replacement and nitrogen management for reducing nitrate leaching in a semi-arid region
(2017-05) John, Andrew A.; Jones, Clain A.; Ewing, Stephanie A.; Sigler, W. Adam; Bekkerman, Anton; Miller, Perry R.
Nitrate (NO3 −) leaching into groundwater is a growing global concern for health, environmental, and economic reasons, yet little is known about the effects of agricultural management practices on the magnitude of leaching, especially in dryland semiarid regions. Groundwater nitrate–nitrogen (nitrate–N) concentrations above the drinking water standard of 10 mg L−1 are common in the Judith River Watershed (JRW) of semiarid central Montana. A 2-year study conducted on commercial farms in the JRW compared nitrate leaching rates across three alternative management practices (AMP: pea, controlled release urea, split application of N) and three grower standard practices (GSP: summer fallow, conventional urea, single application of urea). Crop biomass and soil were collected at ten sampling locations on each side of a management interface separating each AMP from its corresponding GSP. A nitrogen (N) mass balance approach was used to estimate the amount of nitrate leached annually. In 2013, less nitrate leached the year after the pea AMP (18 ± 2.5 kg N ha−1) than the year after the fallow GSP (54 ± 3.6 kg N ha−1), whereas the two AMP fertilizer treatments had no effect on nitrate leaching compared to GSPs. In 2014, leaching rates did not differ between each AMP and its corresponding GSP. The results suggest that replacing fallow with pea has the greatest potential to reduce nitrate leaching. Future leaching research should likely focus on practices that decrease deep percolation, such as fallow replacement with annual or perennial crops, more than on N fertilizer practices.
Long-term anoxia and release of ancient, labile carbon upon thaw of Pleistocene permafrost
(2015-12) Ewing, Stephanie A.; O'Donnell, Jonathan A.; Aiken, George R.; Butler, Kenna; Butman, David; Windham-Myers, Lisamarie; Kanevskiy, Mikhail Z.
The fate of permafrost carbon upon thaw will drive feedbacks to climate warming. Here we consider the character and context of dissolved organic carbon (DOC) in yedoma permafrost cores from up to 20 m depth in central Alaska. We observed high DOC concentrations (4 to 129 mM) and consistent low molecular weight organic acid concentrations in three cores. We estimate a DOC production rate of 12 µmol DOC m−2 yr−1 based on model ages of up to ~200 kyr derived from uranium isotopes. Acetate C accounted for 24 ± 1% of DOC in all samples. This proportion suggests long-term anaerobiosis and is likely to influence thaw outcomes due to biolability of acetate upon release in many environments. The combination of uranium isotopes, ammonium concentrations, and calcium concentrations explained 86% of the variation in thaw water DOC concentrations, suggesting that DOC production may be related to both reducing conditions and mineral dissolution over time.
A satellite-driven hydro-economic model to support agricultural water resources management
(2020-12) Maneta, Marco P.; Coburn, K.; Kimball, John S.; He, Mingzhu; Silverman, N. L.; Chaffin, Brian C.; Ewing, Stephanie A.; Ji, X.; Maxwell, Bruce D.
The management of water resources among competing uses presents a complex technical and policy challenge. Integrated hydro-economic models capable of simulating the hydrologic system in irrigated and non-irrigated regions including the response of farmers to hydrologic constraints and economic and policy incentives, provide a framework to understand biophysical and socioeconomic implications of changing water availability. We present a transformative hydro-economic model of agricultural production driven by multi-sensor satellite observations, outputs from regional climate models, and socioeconomic data. Our approach overcomes the limitations of current decision support systems for agricultural water management and provides policymakers and natural resource managers with satellite data-driven, state-wide, operational models capable of anticipating how farmers allocate water, land, and other resources when confronted with new climate patterns, policy rules, or market signals. The model can also quantify how farming decisions affect agricultural water supplies. We demonstrate the model through an application in the state of Montana.
Temperature, heat flux, and reflectance of common subarctic mosses and lichens under field conditions: might changes to community composition impact climate-relevant surface fluxes?
(2012-06) Stoy, Paul C.; Street, Lorna E.; Johnson, Aiden V.; Prieto-Blanco, Ana; Ewing, Stephanie A.
Bryophytes and lichens are ubiquitous in subarctic ecosystems, but their roles in controlling energy fluxes are rarely studied at the species level despite large, recent observed shifts in subarctic vegetation. We quantified the surface and subsurface temperatures and spectral reflectance of common moss and lichen species at field sites in Alaska and Sweden. We also used MODIS observations to determine if the removal of Cladonia spp. by reindeer overgrazing impacts land surface albedo and temperature. Radiometric surface temperature of a feather moss (Pleurozium schreberi) exceeded 50 °C on occasion when dry, up to 20 °C higher than co-located Sphagnum fuscum or C. rangiferina. Spectral reflectance of S. fuscum was on average higher than Polytrichum piliferum across the 350–1400 nm range, with substantial within-species variability. MODIS albedo was significantly higher on the Norwegian (relatively undisturbed) side versus the Finnish (disturbed) side of a border reindeer fence by an average of 1% during periods without snow cover. MODIS nighttime land surface temperatures were often significantly higher on the Norwegian side of the fence by an average of 0.7 °C despite higher albedo, likely due to poor conductance of heat to the subsurface as observed in C. rangiferina in the field. Changes to bryophyte and lichen community composition alter the surface energy balance, and future work must determine how to best incorporate these effects into Earth system models.
Uranium Isotopes and Dissolved Organic Carbon in Loess Permafrost: Modeling the Age of Ancient Ice
(2015-03) Ewing, Stephanie A.; Paces, J. B.; O'Donnell, Jonathan A.; Jorgenson, M. T.; Kanevskiy, Mikhail Z.; Aiken, George R.; Shur, Y.; Harden, J. W.; Striegl, R.
The residence time of ice in permafrost is an indicator of past climate history, and of the resilience and vulnerability of high-latitude ecosystems to global change. Development of geochemical indicators of ground-ice residence times in permafrost will advance understanding of the circumstances and evidence of permafrost formation, preservation, and thaw in response to climate warming and other disturbance. We used uranium isotopes to evaluate the residence time of segregated ground ice from ice-rich loess permafrost cores in central Alaska. Activity ratios of 234U vs. 238U (234U/238U) in water from thawed core sections ranged between 1.163 and 1.904 due to contact of ice and associated liquid water with mineral surfaces over time. Measured (234U/238U) values in ground ice showed an overall increase with depth in a series of five neighboring cores up to 21 m deep. This is consistent with increasing residence time of ice with depth as a result of accumulation of loess over time, as well as characteristic ice morphologies, high segregated ice content, and wedge ice, all of which support an interpretation of syngenetic permafrost formation associated with loess deposition. At the same time, stratigraphic evidence indicates some past sediment redistribution and possibly shallow thaw among cores, with local mixing of aged thaw waters. Using measures of surface area and a leaching experiment to determine U distribution, a geometric model of (234U/238U) evolution suggests mean ages of up to ∼200 ky BP in the deepest core, with estimated uncertainties of up to an order of magnitude. Evidence of secondary coatings on loess grains with elevated (234U/238U) values and U concentrations suggests that refinement of the geometric model to account for weathering processes is needed to reduce uncertainty. We suggest that in this area of deep ice-rich loess permafrost, ice bodies have been preserved from the last glacial period (10–100 ky BP), despite subsequent fluctuations in climate, fire disturbance and vegetation. Radiocarbon (14C) analysis of dissolved organic carbon (DOC) in thaw waters supports ages greater than ∼40 ky BP below 10 m. DOC concentrations in thaw waters increased with depth to maxima of >1000 ppm, despite little change in ice content or cryostructures. These relations suggest time-dependent production of old DOC that will be released upon permafrost thaw at a rate that is mediated by sediment transport, among other factors.
Using spatially variable nitrogen application and crop responses to evaluate crop nitrogen use efficiency
(Springer Science and Business Media LLC, 2023-05) Hegedus, Paul B.; Ewing, Stephanie A.; Jones, Claim; Maxwell, Bruce D.
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.
Water and nitrate loss from dryland agricultural soils is controlled by management, soils, and weather
(2020-12) Sigler, W. Adam; Ewing, Stephanie A.; Jones, Clain A.; Payn, Robert A.; Miller, Perry R.; Maneta, Marco P.
The vast majority (82 %) of the earth’s cultivated area is not irrigated, and half is in semi-arid regions where water tends to limit crop growth. In dryland semi-arid agroecosystems, any precipitation not transpired indicates crop yield that is below potential. Precipitation that is partitioned to deep percolation can transport nitrate out of the root zone, reducing nitrogen use efficiency and potentially contaminating groundwater. To mitigate loss of crop yield to drought, the practice of chemical summer-fallow (suppressing plant growth for a full growing season with herbicide) has been common in semi-arid regions to store water for the following growing season. However, precipitation losses during fallow tend to exceed the amount of precipitation stored, and fallow tends to increase nitrate leaching. We present model simulations informed by field observations that explore the interaction of crop rotation, weather, and soils as controls on precipitation partitioning and nitrate leaching. Simulations reveal that high intensity precipitation periods produce hot moments of deep percolation and nitrate leaching such that 54 % of deep percolation and 56 % of leaching occurs in two of 14 model years. Simulations indicate that thin soils (having limited water storage capacity) produce hot spots for deep percolation and nitrate leaching such that thinner soils (<25 cm) experience water and nitrate loss rates five to 16 times higher than thicker soils (>100 cm). The practice of fallow facilitates mineralization of soil organic nitrogen to nitrate and increases deep percolation, magnifying the interaction of hot moments and hot spots. Simulations suggest that a field with fallow in rotation once every three years experiences 55 % of its deep percolation and 43 % of its leaching losses during fallow years.