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Item Dam removals: an agricultural analysis(Montana State University - Bozeman, College of Agriculture, 2024) Bush, Nathan Alexander; Chairperson, Graduate Committee: Melissa C. LoPaloDam removals are occurring with increased frequency throughout the United States. 77% of all dam removal projects in U.S. history have occurred in the 21st century and the number of dams being removed each year is rising. Dams often play a key role in agricultural production, making it important for agricultural producers and policymakers to understand the effects of these removals as they become more common. This paper explores the causal effects of dam removal on agricultural productivity in the United States using a two-way fixed effects event study and an instrumental variable framework. Primary results of the analysis are mixed and differ based on exact specifications used but show initial evidence of per acre crop productivity increases and cash receipt declines following a removal. Further research is needed to explore the fine-scale effects of dam removals on individual agricultural producers and to expand on the preliminary causal relationships observed in this paper.Item When and where does irrigation water originate? Leveraging stable water isotopes and synthetic aperture radar to assess the complex hydrology of a snow-dominated catchment in southwestern Montana(Montana State University - Bozeman, College of Letters & Science, 2023) Rickenbaugh, Eliza Apple; Chairperson, Graduate Committee: Eric A. Sproles; This is a manuscript style paper that includes co-authored chapters.Many agricultural regions around the world rely on water stored in mountainous snowpacks for irrigation supply. Consequently, our current and future ability to produce food is threatened by more frequent, severe, and extended snow droughts. As these snow droughts intensify, water resource managers will need more efficient and accurate methods to characterize the snowmelt cycle and forecast water availability. Focusing on a montane headwater catchment in Southwestern Montana (423 km 2 in area, between 1465 m to 3270 m in elevation), we integrate in-situ and remotely sensed data to assess the relative contributions of groundwater and the current season's snowmelt to irrigation supply for water year (WY, Oct 1 - Sep 30) 2023. To understand the period over which snow contributes to stream water in this catchment, we analyze backscatter data from Sentinel-1 Synthetic Aperture Radar (SAR). This provides approximate dates of snowmelt runoff onset at 10 m resolution every twelve days. We find that the median date of snowmelt runoff onset in WY 2023 in this catchment was April 20, six days later than the 7-year median date of snowmelt runoff onset. To assess relative contributions to streamflow we compare stable water isotope ratios (deltaH2 and deltaO18) from biweekly samples of stream water at low elevations against monthly samples of snow and groundwater. Samples range in elevation from 1,475 m to 2,555 m. We find that stream water below the highest diversion point is predominantly composed of groundwater. Results demonstrate alignment between two disparate approaches for estimating temporal trends in snowpack contribution to stream flow. While our work focuses on a catchment in Montana, the efforts and approaches used are potentially applicable globally for agricultural regions that rely on snowmelt for irrigation.Item Fatal attraction for an imperiled songbird: is cropland in the northern Great Plains an ecological trap for breeding thick-billed longspur?(Montana State University - Bozeman, College of Agriculture, 2022) Swicegood, Amber Elizabeth; Chairperson, Graduate Committee: Lance McNewThick-billed longspur (Rhynchophanes mccownii) populations have declined 4% annually during the past 50 years. This species nests in recently disturbed or sparsely vegetated patches within native mixed-grass prairie and is also known to occur in crop fields in northeastern Montana during the breeding season. Maladaptive habitat selection may result in crop fields operating as ecological traps, but information on thick-billed longspur use of and demography in crop fields are lacking. We hypothesized that crop fields provide cues for territory selection, but frequent human disturbance and increased exposure to weather and predators should result in reduced reproductive success relative to native grassland habitats. To address this hypothesis, we 1) used dynamic occupancy models to compare arrival times of territorial male longspurs using data collected with autonomous acoustic recorders, 2) used open population distance sampling models to compare trends in longspur abundance over the breeding season, 3) compared indices of nest density and number of young fledged, and 4) used nest survival models to compare survival rates of nests between crop and native sites. Arrival times were similar in both site types and occupancy ranged from 0.52 + or - 0.17SE on April 7 to 0.99 + or - 0.01 on April 30. Bird abundances appeared to be mediated by vegetation biomass and drought conditions. Standardized nest densities were 0.15 + or - 0.22SD and 0.23 + or - 0.32 nests/hour/plot in crop and native sites, respectively; the number of young fledged per successful nest was similar in crop and native sites and averaged 2.9 + or - 0.18SE. Nest survival was similar in crop and native sites and averaged 0.24 + or - 0.03 SE (n=222 nests). The data did not support our ecological trap hypothesis: longspurs did not exhibit a clear preference for crop sites and reproductive output was not significantly reduced. Our results suggest that crop fields may expand nesting opportunities for thick-billed longspur in a region where native habitat may be limited.Item Water quality response to water and nitrogen movement through a semi-arid dryland agroecosystem in Montana, USA(Montana State University - Bozeman, College of Agriculture, 2020) Sigler, William Adam; Chairperson, Graduate Committee: Stephanie A. Ewing; Stephanie A. Ewing, Clain A. Jones, Robert A. Payn, E.N. Jack Brookshire, Jane K. Klassen, Douglas Jackson-Smith and Gary S. Weissmann were co-authors of the article, 'Connections among soil, ground, and surface water chemistries characterize nitrogen loss from an agricultural landscape in the upper Missouri River Basin' in the journal 'Journal of hydrology' which is contained within this dissertation.; Stephanie A. Ewing, Clain A. Jones, Robert A. Payn, Perry Miller and Marco Maneta were co-authors of the article, 'Water and nitrate loss from dryland agricultural soils is controlled by management, soils, and weather' submitted to the journal 'Agricultural ecosystems & environment' which is contained within this dissertation.; Stephanie A. Ewing, Scott D. Wankel, Clain A. Jones, Sam Leuthold, E.N. Jack Brookshire and Robert A. Payn were co-authors of the article, 'Drivers of denitrification across a semiarid agroecosystem revealed by nitrate isotopic patterns' which is contained within this dissertation.Humans have increased reactive nitrogen (N) on the planet by an order of magnitude over the past 150 years. Most of this reactive N is used for fertilizer to feed a growing population, but loss of N from cultivated soils threatens agricultural and environmental sustainability. Nitrate accumulated in soil from fertilization or decomposition of soil organic N (SON) may be lost via leaching, which can reduce soil fertility and compromise water quality. Nitrate concentrations commonly exceed human drinking standards in groundwater resources around the globe. In the Judith River Watershed (JRW) in central Montana, nitrate has been detected above the standard since the 1960s. This dissertation contributes to a more holistic understanding of the fate and transport of N in the JRW. An interdisciplinary team engaged with farmers in a participatory research project, making observations in soils, groundwater, and streams to characterize water and N movement. At the landform scale, 5 to 9 cm yr -1 of the 38 cm yr -1 mean annual precipitation moves through soil to recharge groundwater and leaches 11 to 18 kg ha -1 yr -1 of nitrate-N from soil. These leaching rates are approximately 20-30% of fertilizer rates but likely reflect inmixing of nitrate from SON decomposition. Soil modeling analyses suggested that water and N losses were dominated by intense precipitation periods on wetter soils, such that more than half of simulated deep percolation and leaching occurred in two of 14 model years. Simulations further suggest that thinner soils (<25 cm fine-textured materials) experience water and nitrate loss rates five to 16 times higher than thicker soils (>100 cm). Soil sampling demonstrated that increased soil water during fallow periods facilitates conversion of SON to nitrate. Soils are then primed for water and N loss with subsequent precipitation, resulting in disproportionately high leaching rates during and following fallow periods. Isotopic evidence from fallow periods further suggests that nitrate is lost to the atmosphere via denitrification, a gas phase loss combining with leaching losses to compromise the goal of delivering N to crops. These findings suggest that reduction of fallow increases N use efficiency and reduces nitrate loss to groundwater.Item Soil legacy effects alter plant volatile emissions in response to diversified cropping systems(Montana State University - Bozeman, College of Agriculture, 2020) Malone, Shealyn Chelsea; Chairperson, Graduate Committee: David K. Weaver and Amy Trowbridge (co-chair); David K. Weaver, Fabian Menalled, Tim Seipel, Justin B. Runyon, Lila Hamburg, Megan L. Hofland and Amy M. Trowbridge were co-authors of the article, 'Cropping systems alter crop volatile cues important for insect pests through soil legacy effects' which is contained within this thesis.; David K. Weaver, Tim F. Seipel, Fabian D. Menalled, Megan L. Hofland, Justin B. Runyon and Amy M. Trowbridge were co-authors of the article, 'Soil microbes alter herbivore-induced volatile emissions in response to cereal cropping systems' submitted to the journal 'Plant and soil' which is contained within this thesis.Soil microbes can influence the emissions of plant volatile organic compounds (VOCs) that serve as host-location cues for insects and their natural enemies. The influence of the soil microbial community on the plasticity of plant VOC synthesis and emissions is particularly important in agricultural settings where crop rotations and management practices cause significant shifts in the soil microbiome. Studies have shown agricultural soils to influence plant-insect interactions through changes in foliar chemistry, but their potential to alter VOC emissions is unknown. To determine the effect of diversified agricultural practices on crop VOC emissions through microbe-mediated soil legacy effects, I measured VOCs from wheat (Triticum aestivum L.) in a series of field and greenhouse experiments. In Chapter II, I determined the effect of the soil microbiome on VOCs in the greenhouse by first measuring VOCs from wheat plants grown in sterilized soil or soil with added inoculum from an agricultural field. Next, to determine the effect of diversified agricultural practices on VOC phenotypes, I measured VOCs from wheat plants in the field in rotation with either fallow or a mixture of cover crops that was terminated by grazing cows. Finally, in Chapter III, I explored the interactive effect of herbivory and the soil microbiome on VOC emissions in a full factorial experiment in which wheat grown in soil inoculum from wheat-fallow or wheat-cover crop rotation that was subjected to larval feeding by the wheat stem sawfly (WSS; Cephus cinctus Norton), a major pest of wheat. Across all studies I found that soils associated with a higher microbial diversity--cover crop soils and inoculated soils--tended to emit more total VOCs and blends that would likely increase pest resistance to the WSS through 1) shifts in key bioactive compounds and 2) enhanced herbivore-induced VOC emissions. Results also suggest that soil microbes may be more likely to alter plant VOCs when plants experience abiotic or biotic stressors. Together, these results suggest that agricultural practices may indirectly influence plant resistance through microbe-altered VOCs, and these effects are more likely to occur when plants experience additional stressors, such as herbivory or drought.Item Investigating the impacts of agricultural land use change on regional climate processes in the northern North American Great Plains(Montana State University - Bozeman, College of Agriculture, 2021) Bromley, Gabriel Trees; Chairperson, Graduate Committee: Paul C. Stoy; Jack Brookshire (co-chair); Tobias Gerken, Andreas F. Prein and Paul C. Stoy were co-authors of the article, 'Recent trends in the near-surface climatology of the northern North American Great Plains' in the journal 'Journal of climate' which is contained within this dissertation.; Andreas F. Prein, Shannon Albeke and Paul C. Stoy were co-authors of the article, 'Simulating the impacts of agricultural land use change on the climate of the northern North American Great Plains: validating a convection-permitting climate model' submitted to the journal 'Climate dynamics' which is contained within this dissertation.; Andreas F. Prein, Shannon Albeke and Paul C. Stoy were co-authors of the article, 'The decline in summer fallow in the northern plains cooled near-surface climate but had minimal impacts on precipitation' submitted to the journal ' ' which is contained within this dissertation.; Andreas Prein and Paul C. Stoy were co-authors of the article, 'Recent enhancement of thermodynamic environments in the northern North American Great Plains' submitted to the journal 'Geophysical research letters' which is contained within this dissertation.The northern North American Great Plains (NNAGP) is the area defined by the Upper Missouri River Basin and the Canadian Prairies. It is a semi-arid region categorized by large stretches of grassland, pasture, and crops. During the last century and extending to the present day, a standard agricultural practice was to utilize a wheat-summer fallow rotation schedule, where the fields were left unplatted and an herbicide was often applied to keep weeds at bay. Concerns over soil health and profitability have led to the systematic decline of summer fallow, and nearly 116,000 km 2 that used to be fallow during the summer in the 1970s are now planted. An observational analysis discovered that from 1970-2015, during the early warm season, the NNAGP have cooled at -0.18 °C decade -1, nearly the same magnitude as the annual global warming rate. The near-surface atmosphere also moistened, evidenced by a decreasing vapor pressure deficit (VPD) trend, and monthly mean precipitation increased in excess of 8 mm per decade. Monthly mean convective available potential energy (CAPE) increased by 80% at Glasgow, MT and by 35% at Bismarck, ND based on atmospheric sounding observations. To test whether a reduction in summer fallow is responsible for these observed changes, a set of convection-permitting model experiments were performed over the NNAGP. Two sets (4 total) of three-year simulations were driven by ERA5 data with the vegetative fraction adjusted using satellite estimated fallow amounts for 2011 and 1984. The control simulations were extensively validated against an ensemble of observations with large temperature biases in Winter by ~ -3 °C and Summer by ~3°C. The areas where fallow area declined from 1984-2011 were cooler by about 1.5 °C and had a lower VPD by 0.15 kPa compared to where it did not. CAPE increased where fallow declined from 1984-2011 but so did convective inhibition (CIN). These findings insinuate that the observed change to monthly mean precipitation cannot be explained by summer fallow reduction alone. Trends in observed low level moisture transport show that the Great Plains Low Level Jet has been intensifying, bringing increased moisture to the NNAGP and partially responsible for the precipitation increase.