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Browsing by Author "Paustian, Keith, committee member"

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    Aboveground woody biomass estimation of green ash trees (Fraxinus pennsylvanica Marsh.) along Colorado's Northern Front Range in response to the invasive emerald ash borer (Agrilus plannipenis Fairmaire)
    (Colorado State University. Libraries, 2018) Truslove, Micaela, author; Mackes, Kurt, advisor; Nagel, Linda, advisor; Davis, Seth, committee member; Paustian, Keith, committee member; Wood, Keith, committee member
    The invasive emerald ash borer (Agrilus planipennis Fairmaire) has killed hundreds of millions of ash trees (Fraxinus spp.) in forests and urban areas across the United States. Green ash (Fraxinus pennsylvanica Marsh.) is the most widely planted street tree in the greater Denver Metro Area, comprising 15% of the urban tree population on a per-stem basis, and up to 33% of the canopy cover in some cities. EAB is currently established in Boulder, Colorado and as the infestation progresses along the Colorado Northern Front Range, municipalities will need to predict and budget for woody debris disposal from EAB-killed trees. Though existing green ash biomass predictive equations exist, most were developed for areas outside the arid West and generally represent only trees in natural forests, with full, healthy crowns. This study aimed to test whether these equations can accurately predict aboveground woody biomass of green ash trees removed as part of emerald ash borer mitigation efforts in urban areas of Colorado's Northern Front Range. Data from 42 destructively sampled ash trees removed from 11 sites as part of emerald ash borer mitigation efforts were used to evaluate the predictive capability of 12 forest-derived and five urban green ash biomass equations. The published urban equations underpredicted total sampled biomass by as much as 38% and overpredicted by as much as 47%. Forest-derived equations underpredicted by as much as 57% and overpredicted up to 52%. A local, published equation developed in the Northern Front Range overpredicted biomass by 47%. This local urban equation was developed using only open-grown trees with full, healthy crowns while the trees sampled for this study exhibited a broad spectrum of crown conditions, better representing trees that will routinely be removed as part of emerald ash borer management strategies. Sampled trees were also used to develop new local green ash biomass equations, more appropriate for use in emerald ash borer management strategies in Colorado's Northern Front Range cities. In addition, the locally-derived average specific gravity value for green ash wood was 0.57, and the locally-derived average moisture content value was 41%. These are 7.5% higher and 24% lower respectively than widely-used published values. The locally-derived values can be used to further improve the accuracy of urban forest mensuration efforts in Colorado's Northern Front Range.
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    Assessing the impact of EQIP-funded agricultural conservation practices on water quality in Colorado: the Republican, South Platte, Arkansas, and Rio Grande watersheds
    (Colorado State University. Libraries, 2021) Trotter, Brianna, author; Arabi, Mazdak, advisor; Bhaskar, Aditi, committee member; Paustian, Keith, committee member
    Water quality degradation is one of the world's most pressing environmental concerns. The implementation of Colorado Regulation 85 (5 CCR 1002-85) in 2012 has led to increased awareness of the potential water quality impacts of agricultural and other nonpoint sources of pollution. The use of agricultural conservation practices is widely accepted as a means of reducing nonpoint source pollution from agricultural runoff. The Natural Resources Conservation Service (NRCS) implemented the Environmental Quality Incentives Program (EQIP) under the 1996 Farm Bill to assist producers with applying sustainable on-farm conservation practices. However, there has been limited research to quantify the progress on water quality protection resulting from the application of EQIP-funded practices in Colorado. Water quality models have become increasingly relevant in determining watershed-level characteristics related to environmental concerns. The Soil and Water Assessment Tool (SWAT) model has been a prevailing water quality model in many studies researching the effects of agricultural nutrient runoff. SWAT simulates surface, subsurface, and shallow groundwater hydrologic processes and simulates specific farming practices and their corresponding effects, including erosion, runoff, and edge-of-field losses. In this analysis, SWAT simulation quantified the effects of specific EQIP-funded agricultural conservation practices on field runoff in the Republican, South Platte, Arkansas, and iii Rio Grande watersheds of Colorado. Practices included in this analysis were varying levels of tillage, irrigation systems, and establishment of a conservation buffer. Edge-of-field discharges of Total Nitrogen (TN) and Total Phosphorus (TP) were modeled before and after EQIP conservation practices were implemented. The modeling included EQIP conservation practices applied between 2008 and 2018 and incorporated existing Colorado State University (CSU) edge-of-field water quality data, providing a means of calibrating the model to realistic and attainable results. Results showed the most significant county-level average annual percent reductions in TN came from counties with high adoption of EQIP-funded irrigation practices, such as sprinkler or drip irrigation. On average, these counties yielded a 7.1% reduction in TN per county, which equates to 6.8 tons of TN reduced across all four watersheds. The combined reductions in TN from all EQIP-funded practices averaged 8.2% per county, which totaled approximately 19.5 tons reduced across all four watersheds over the full ten-year period of analysis. The greatest reductions in TP were observed in counties with high adoption rates of irrigation system upgrades, which yielded an average 33.5% reduction in TP per county. The implementation of all EQIP-funded practices produced a 27.7% average reduction in TP per county across all counties considered. This was equivalent to a TP reduction of 263.3 tons across all four watersheds throughout the full ten-year period of analysis. The findings indicate the modeled EQIP conservation practices are significantly reducing nutrient losses from irrigated agricultural lands.
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    Characterizing forest biomass and the impacts of bark beetles and forest management in the southern Rocky Mountains, USA
    (Colorado State University. Libraries, 2020) Vorster, Anthony Grant, author; Evangelista, Paul, advisor; Paustian, Keith, committee member; Ex, Seth, committee member; Jarnevich, Catherine, committee member
    To view the abstract, please see the full text of the document.
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    Constraints and capabilities of no-till dryland agroecosystems for bioenergy production
    (Colorado State University. Libraries, 2011) Lloyd, Grace Susanna, author; Hansen, Neil C., advisor; Brummer, Joe, committee member; Paustian, Keith, committee member; Leach, Jan, committee member
    Crop residues are receiving attention as potential feedstocks for lignocellulosic biofuels. Sustainable residue harvest may be limited by soil erosion and the need to maintain soil organic carbon (SOC). Little attention has been given to the potential for residue harvest in the semi-arid Great Plains, largely due to assumptions of low production levels and the strong erosive forces of wind. Due to expanding interest in growing dedicated biofuel crops on marginal lands, these studies examined the capabilities and constraints of harvesting agricultural residues from dryland production systems in the semi-arid Great Plains. The first study examined long-term production levels of grain and stover for wheat (Triticum aestivum), corn (Zea mays), and grain sorghum (Sorghum bicolor) at three no-till dryland cropping sites in wheat-corn-fallow and wheat-sorghum-fallow rotations, and evaluated the impact of stover removal on wind and water erosion, soil organic carbon dynamics, and future productivity. The Revised Universal Soil Loss Equation and the Wind Erosion Equation were used to simulate water and wind erosion under various levels of residue removal. The DAYCENT model was used to estimate changes in soil organic carbon, grain yields, and soil fertility, if 50% of corn and wheat stover were harvested each crop year. Model validation was performed by comparing long-term production rates and measured changes in soil organic carbon to model simulated output. Total aboveground biomass production for corn and sorghum, averaged over site and soil type, was 5550 ± 2810 kg ha-1 yr-1, with average stover production of 2750 ± 1570 kg ha-1 yr-1 and 2800 ± 1570 kg ha-1 yr-1 for grain. The total aboveground annual biomass production of wheat across all sites averaged 5840 ± 2440 kg ha-1. Wheat annual stover yields were 3940 ±1880 kg ha-1 and grain yields averaged 1950 ± 820 kg ha-1. A 50% stover removal rate only slightly increased water erosion from 0.53 Mg ha-1 yr-1 (no removal) to a maximum of 1.4 Mg ha-1 yr-1. Wind erosion was a bigger risk, with rates surpassing the tolerable erosion levels after removing 10 - 30% of corn stover, depending on site and soil landscape position. However, at all three sites, up to 80% of wheat straw could be harvested without surpassing tolerable erosion rates. Soil organic carbon (SOC) declined 6-9% after 96 years of simulating 50% removal of corn and wheat stover. Under 0% removal, SOC levels appeared relatively stable, with maximum declines of 2.0%. As SOC levels are very low in these dryland systems, these declines represent a very small net loss of SOC when compared to losses observed in more humid regions. Under current wheat-corn-fallow management, virtually all stover must remain in order to control wind erosion and maintain soil organic carbon. However, if dedicated non-grain bioenergy crops were grown on an annual basis, there could be 2500-2700 kg ha-1 of harvestable biomass yearly while still retaining enough residues to maintain SOC. Total biomass production of a dedicated non-grain energy crop could be higher than the biomass production of the grain crops examined, namely because energy is not diverted to grain, and non-grain crops are not as sensitive to the timing of water deficits. Replacement of the fallow period with a non-grain biomass crop could lower the amount of residue needed to control erosion. Elimination of the fallow period would likely reduce the amount of residue that must remain to maintain SOC, increasing the amount of biomass available for removal. The second study uses the DAYCENT model to simulate variable responses in fertility, yield, and soil organic carbon within two field landscapes in eastern Colorado. Grain yield, soil fertility, and soil carbon were all impacted by stover harvest, but the magnitude and direction of responses were dependent on soil type. Yield declines as great as 1615 and 1382 kg ha-1 were simulated for corn and wheat, respectively. Declines in annual mineralization rates as high as 13 g N m-2 yr-1 were observed with stover harvest, but simulated changes in mineralization rates were highly variable between soil types, with net mineralization rates increasing with stover removal in some years. The impact of stover harvest on soil organic carbon varied with soil type and landscape position. Results are used to highlight the variable impacts stover harvest could have within one field or management unit, and demonstrate the need for landscape scale predictive models to assess the impact of stover removal on soil fertility and SOM dynamics and transfer processes models. Simulations characterized by an average soil type are not sufficient to account for the complexity of soils or the interactions and feedbacks of sediment, nutrient, and water transport that occur within agricultural fields. In addition to predictive model support systems, management of soil-specific responses to stover harvest will likely require adoption of precision agriculture technologies and practices such as variable rate harvesting and fertilization.
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    Evaluating the sustainability of agricultural systems using life cycle assessment and techno-economic analysis
    (Colorado State University. Libraries, 2021) Summers, Hailey, author; Quinn, Jason C., advisor; Marchese, Anthony J, committee member; Paustian, Keith, committee member; Willson, Bryan, committee member
    In a time of expansive population growth, our global resources have never been so strained; our contributions to a changing climate so significant. The International Panel on Climate Change recently addressed the need for focused effort toward reducing global resource depletion and greenhouse gas emissions (GHGs). As such, special attention has been given to some of the largest GHG emitting sectors in the world: energy, industry, and agriculture. This work focuses on using sustainability analysis to further understand agricultural processes and products, both novel and emerging. To quantify the environmental component of sustainability, life cycle assessment (LCA) is used because it is a well-established method for evaluating processes and products with respect to emissions. Similarly, techno-economic analysis (TEA) is used to understand the economic viability of various processes and products. In harmony, these assessments are used to evaluate the sustainable performance of various agricultural processes and products by identifying pathways to reduce environmental impact while concurrently increasing economic viability. Results enable targeted research to be highlighted that can advance early-stage development toward a sustainable adoption. The dissertation proposal is divided into three topics all with a common theme: Using LCA and TEA to assess the sustainability of, and advance, agricultural systems. A drought tolerant crop currently grown in India, guar, was investigated to understand relative environmental impact and economic viability in the American Southwest compared to existing crops. Guar is cultivated as a source of guar gum, used primarily in hydraulic fracking fluid for shale oil and gas recovery, with demand currently met through importation. Therefore, a feasibility analysis was performed for a domestic guar supply in Arizona and New Mexico using LCA and TEA. The integrated assessment provided insight on environmental and economic performance of guar for comparison to existing crops. Results indicate that environmentally, guar has lower GHGs than many crops currently cultivated in the American Southwest. Economically, guar gum can be produced for less than the five-year average U.S. import price, with minimizing or eliminating irrigation identified as a critical area for further research. A best case scenario and sensitivity analysis are also investigated using LCA and TEA to evaluate early-stage development of adopting guar in the American Southwest. LCA is also a valuable assessment tool for emerging agricultural systems. A detailed LCA was performed for a first-of-its-kind study investigating the GHGs of commercial indoor cannabis cultivation. Since legalization, the cannabis industry has seen substantial growth with many products being cultivated inside industrialized warehouses. An engineering process model was built to track material and energy requirements of a typical indoor cannabis facility which was then translated to GHGs using LCA methodology. Results of a U.S.-wide analysis indicate that indoor cannabis production leads to substantial GHGs regardless of where it is cultivated, with regions such as the Mountain West and Midwestern United States being much more GHG intensive than East or West Coasts. Individual processes that lead to the majority of GHGs are heating, ventilation, and air conditioning (HVAC), high intensity grow lights and the addition of carbon dioxide for increased plant growth rates. Results of this work have informed the industry, consumers, and policymakers of the environmental impact from this practice while providing insight on ways to reduce GHG emissions. Despite LCA and TEA being proven methodologies for assessing novel, emerging and established processes and products, limitations do exist. Particularly, in the context of agriculture, LCA does not traditionally account for water use outside of the emissions associated with procurement and use. In the American Southwest specifically, it is critical to understand water use and associated environmental impact to make informed decisions regarding ecosystem and societal sustainability. Recently, the development of an advanced LCA method, water scarcity footprint (WSF), has enhanced that ability to understand spatial and temporal considerations of freshwater consumption. However, this method is actively emerging and therefore limitations exist, particularly for arid regions where water demand is typically higher than the amount of water available. A novel method was proposed that can improve resolution and decision-making capabilities for freshwater environmental impact when evaluating arid regions. Results include method comparisons that highlight the improved resolution between the developed method and the traditional WSF method. Furthermore, a case study shows variation of the two methods when applied to alfalfa production in the American Southwest that reveals the severity of drought in the region. The proposed method enables improved resolution when considering spatial and temporal freshwater use in arid regions which enhances decision-making capabilities for product development. Throughout this work, traditional and advanced sustainability metrics, LCA, TEA and WSF, were used to understanding the environmental impact and economic viability of various agricultural-related products. Results from these assessments, from novel and existing technology investigation, provide quantifiable results for holistic comparisons and internal process improvement. These results can serve as decision-making tools during the research and development and commercialization stages, all leading toward providing a more sustainable future.
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    Global impacts of U.S. bioenergy production and policy: a general equilibrium perspective
    (Colorado State University. Libraries, 2012) Evans, Samuel Garner, author; Hoag, Dana, advisor; Bond, Craig, committee member; Davies, Stephen, committee member; Paustian, Keith, committee member
    The conversion of biomass to energy represents a promising pathway forward in efforts to reduce fossil fuel use in the transportation and electricity sectors. In addition to potential benefits, such as greenhouse gas reductions and increased energy security, bioenergy production also presents a unique set of challenges. These challenges include tradeoffs between food and fuel production, distortions in energy markets, and terrestrial emissions associated with changing land-use patterns. Each of these challenges arises from market-mediated responses to bioenergy production, and are therefore largely economic in nature. This dissertation directly addresses these opportunities and challenges by evaluating the economic impacts of U.S. bioenergy production and policy, focusing on both existing and future biomass-to-energy pathways. The analysis approaches the issue from a global, economy-wide perspective, reflecting two important facts. First, that large-scale bioenergy production connects multiple sectors of the economy due to the use of agricultural land resources for biomass production, and competition with fossil fuels in energy markets. Second, markets for both agricultural and energy commodities are highly integrated globally, causing domestic policies to have international effects. The reader can think of this work as being comprised of three parts. Part I provides context through an extensive review of the literature on the market-mediated effects of conventional biofuel production (Chapter 2) and develops a general equilibrium modeling framework for assessing the extent to which these phenomenon present a challenge for future bioenergy pathways (Chapter 3). Part II (Chapter 4) explores the economic impacts of the lignocellulosic biofuel production targets set in the U.S. Renewable Fuel Standard on global agricultural and energy commodity markets. Part III (Chapter 5) extends the analysis to consider potential inefficiencies associated with policy-induced competition for biomass between the electricity and transportation fuel sectors.
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    High groundwater in irrigated regions: model development for assessing causes, identifying solutions, and exploring system dynamics
    (Colorado State University. Libraries, 2021) Deng, Chenda, author; Bailey, Ryan T., advisor; Grigg, Neil, committee member; Niemann, Jeffrey, committee member; Paustian, Keith, committee member
    Waterlogging occurs in irrigated areas around the world due to over-irrigation and lack of adequate natural or artificial drainage. This phenomenon can lead to adverse social, physical, economic, and environmental issues, such as: damage to crops and overall land productivity; soil salinization; and damage to homes and building foundations. Solutions to waterlogging include implementation of high-efficient irrigation practices, installation of artificial drainage systems, and increased groundwater pumping to lower the water table. However, in regions governed by strict water law, wherein groundwater pumping is constrained by impact on nearby surface water bodies, these practices can be challenging to implement. In addition, current engineering and modeling approaches used to quantify soil-groundwater and groundwater-surface water interactions are crude, perhaps leading to erroneous results. An accurate representation of groundwater state variables, groundwater sources and sinks, and plant-soil-water interaction is needed at the regional scale to assist with groundwater management issues. This dissertation enhances understanding of major hydrological processes and trade-offs in waterlogged agricultural areas, through the use of numerical modeling strategies. This is accomplished by developing numerical modeling tools to: (1) analyze and quantify the cause of high groundwater levels in highly managed, irrigated stream-aquifer systems; (2) assess the impact of artificial recharge ponds on groundwater levels, groundwater-surface water interactions, and stream depletions in irrigated stream-aquifer systems; (3) and gain a better understanding of plant-soil-water dynamics in irrigated areas with high water tables. These objectives use a combination of agroecosystem (DayCent) and groundwater flow (MODFLOW) models, sensitivity analysis, and management scenario analysis. Each of these sub-objectives is applied to the Gilcrest/LaSalle agricultural region within the South Platte River Basin in northeast Colorado, a region subject to high groundwater levels and associated waterlogging and infrastructure damage in the last 7 years. This region is also subject to strict water law, which constrains groundwater pumping due to the effect on the water rights of the nearby South Platte River. Results indicate that recharge from surface water irrigation, canal seepage, and groundwater pumping have the strongest influence on water table elevation, whereas precipitation recharge and recharge from groundwater irrigation have small influences from 1950 to 2012. Mitigation strategy implementation scenarios show that limiting canal seepage and transitioning > 50% of cultivated fields from surface water irrigation to groundwater irrigation can decrease the water table elevation by 1.5 m to 3 m over a 5-year period. Decreasing seepage from recharge ponds has a similar effect, decreasing water table elevation in local areas by up to 2.3 m. However, these decreases in water table elevation, while solving the problem of high groundwater levels for residential areas and cultivated fields, results in a decrease in groundwater discharge to the South Platte River. As the intent of the recharge ponds is to increase groundwater discharge and thereby offset stream depletions caused by groundwater pumping, mitigating high water table issues in the region can be achieved only by (1) modifying fluxes of sources and sinks of groundwater besides recharge pond seepage, or (2) modifying or relaxing the adjudication of water law, which dictates the need for offsetting pumping-induced stream depletion, in this region. The modeling tools developed in this dissertation, specifically the loose and tight coupling between DayCent and MODFLOW, can be used in the study region to quantify pumping-induced stream depletion, recharge pond induced stream accretion, and the interplay between them in space and time. In addition, these models can be used in other irrigated stream-aquifer systems to assess baseline conditions and explore possible effects of water management strategies.
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    Life cycle and technoeconomic analysis of microalgae-based biofuels
    (Colorado State University. Libraries, 2014) Batan, Liaw Yih Der, author; Bradley, Thomas H., advisor; Willson, Bryan D., advisor; Marchese, Anthony J., committee member; Graff, Gregory D., committee member; Paustian, Keith, committee member
    Microalgae are an appealing feedstock for production of biofuels due to their high productivity compared to terrestrial plant-based feedstocks, and their relative tolerance of low quality land and water. Despite these potential benefits, there are technological, environmental and economic challenges that must be overcome to enable commercialization of any microalgae-to-biofuels process. Due to the relative immaturity of the field, assessments of the environmental performance, scalability and economic performance of microalgae-based biofuels are highly uncertain, data poor, and incomparable across technologies. This dissertation seeks to study these aspects of microalgae-based biofuels so as to provide models of increased utility for technical design, investment planning, and achieving policy-level objectives. This work is divided in three primary research efforts. First, this research develops an integrated life cycle assessment of the microalgae to biofuels process using a detailed engineering model derived from a pilot-scale photobioreactor system. The life cycle assessment quantifies and compares energy consumption, greenhouse gas emissions, and scalability of the biofuel life cycle. Second, this work defines the water footprint for a photobioreactor-based biofuel production system with geographical and temporal resolution. The water footprint (WF) of microalgae biofuel is comprehensively assessed using a combined process and economic input-output lifecycle analysis method, using blue, green and lifecycle WF metrics, four different fuel conversion pathways, and 10 continental US locations with high productivity yields. Finally, a technoeconomic analysis of the baseline enclosed photobioreactor microalgae to biofuels system is performed with stochastic economic risk assessment. This section provides a range of probabilities of economic success based on the sensitivity of the microalgae-to-biofuel process to the variable economic variables and scenarios. Based on the results of these integrated assessments of microalgae biofuels, this study communicates an improved understanding of the economic and environmental performance of microalgae biofuels and their characteristics compared to petroleum and biofeedstock-based biofuels.
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    Life cycle assessment and life cycle cost of photovoltaic panels on Lake Street Parking Garage
    (Colorado State University. Libraries, 2014) Fan, Jiawei, author; Strong, Kelly, advisor; Glick, Scott, committee member; Paustian, Keith, committee member
    In the U.S., the capacity of photovoltaic panels has already reached a level close to 14GW in 2014. The goal of the solar power industry is to meet 10% of U.S. peak electricity generation capacity by 2030 (Dincer, 2011). Photovoltaic panel systems have become a new trend to produce electric power. Solar radiation is an abundant, inexhaustible, clean and cheap energy source. By using solar energy, solar panels are considered a clean and green method to produce electric power. However, photovoltaic panels have impacts on the environment in the production process and end-of-life process. This thesis uses a methodology that combines life cycle assessment (LCA) and life cycle cost (LCC) to analyze the life cycle impact and the cost of a PV system on a public garage located in Fort Collins, Colorado. The LCA method used in this thesis is a hybrid LCA, which is a combination of process based LCA and economic Input/Output LCA (EIO-LCA). The result of the analysis of LCA indicates that a solar panel power system does have some advantages in reducing greenhouse gas emissions and gaseous toxic releases. However, solar panel systems have higher toxic releases to water and land than a traditional power plant. The result of LCC points out that the solar panel system on the roof of Lake Street Parking Garage cannot recover its cost during its 25-year life span.
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    Moist synoptic transport of CO2 along midlatitude storm tracks, transport uncertainty, and implications for flux estimation
    (Colorado State University. Libraries, 2011) Parazoo, Nicholas C., author; Denning, A. Scott, advisor; Randall, David, committee member; Maloney, Eric, committee member; Kawa, Randy, committee member; Paustian, Keith, committee member
    Mass transport along moist isentropic surfaces on baroclinic waves represents an important component of the atmospheric heat engine that operates between the equator and poles. This is also an important vehicle for tracer transport, and is correlated with ecosystem metabolism because large-scale baroclinicity and photosynthesis are both driven seasonally by variations in solar radiation. In this research, I pursue a dynamical framework for explaining atmospheric transport of CO2 by synoptic weather systems at middle and high latitudes. A global model of atmospheric tracer transport, driven by meteorological analysis in combination with a detailed description of surface fluxes, is used to create time varying CO2 distributions in the atmosphere. Simulated mass fluxes of CO2 are then decomposed into a zonal monthly mean component and deviations from the monthly mean in space and time. Mass fluxes of CO2 are described on moist isentropic surfaces to represent frontal transport along storm tracks. Forward simulations suggest that synoptic weather systems transport large amounts of CO2 north and south in northern mid-latitudes, up to 1 PgC/month during winter when baroclinic wave activity peaks. During boreal winter when northern plants respire, warm moist air, high in CO2, is swept upward and poleward along the east side of baroclinic waves and injected into the polar vortex, while cold dry air, low in CO2, that had been transported into the polar vortex earlier in the year is advected equatorward. These synoptic eddies act to strongly reduce seasonality of CO2 in the biologically active mid-latitudes by 50% of that implied by local net ecosystem exchange while correspondingly amplifying seasonality in the Arctic. Transport along stormtracks is correlated with rising, moist, cloudy air, which systematically hides this CO2 transport from satellite observing systems. Meridional fluxes of CO2 are of comparable magnitude as surface exchange of CO2 in mid-latitudes, and thus require careful consideration in (inverse) modeling of the carbon cycle. Because synoptic transport of CO2 by frontal systems and moist processes is generally unobserved and poorly represented in global models, it may be a source of error for inverse flux estimates. Uncertainty in CO2 transport by synoptic eddies is investigated using a global model driven by four reanalysis products from the Goddard EOS Data Assimilation System for 2005. Eddy transport is found to be highly variable between model analysis, with significant seasonal differences of up to 0.2 PgC, which represents up to 50% of fossil fuel emissions. The variations are caused primarily by differences in grid spacing and vertical mixing by moist convection and PBL turbulence. To test for aliasing of transport bias into inverse flux estimates, synthetic satellite data is generated using a model at 50 km global resolution and inverted using a global model run with coarse grid transport. An ensemble filtering method called the Maximum Likelihood Ensemble Filter (MLEF) is used to optimize fluxes. Flux estimates are found to be highly sensitive to transport biases at pixel and continental scale, with errors of up to 0.5 PgC/year in Europe and North America.
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    Pathways of soil organic matter formation in agroecosystems as influenced by litter chemistry, root depth and aggregation
    (Colorado State University. Libraries, 2024) Fulton-Smith, Sarah E., author; Cotrufo, M. Francesca, advisor; Paustian, Keith, committee member; Ojima, Dennis, committee member; Fonte, Steven, committee member
    Soils contain more carbon (C) than any other terrestrial reservoir, and the increase of these C stocks has been targeted as a potential climate solution globally. Agroecosystems play a critical role in our ability to provide these climate solutions through increasing soil organic matter (SOM). There is significant potential for SOM accrual in agroecosystems due to the degradation of SOM typically observed in these systems. One promising approach to increasing soil C sequestration is through the selection of deep-rooted crops, such as Sorghum bicolor. However, significant questions remain about root inputs' ability to contribute to SOM in order to balance the greenhouse gas (GHG) lifecycle of a bioenergy feedstock. My dissertation aims to answer some of these questions as well as to propose a framework to integrate the study of SOM formation from crop inputs with soil aggregate structure. Bioenergy has the potential to emit fewer GHGs than other fuel sources, such as fossil fuels, yet there are some emissions during the transportation production of bioenergy feedstocks and fuels that could be offset by soil C sequestration. However, in annual bioenergy systems, aboveground biomass is typically removed from the system, meaning roots are the primary source of OM available to return to the soil. However, roots and shoots may differ significantly in their ability to contribute to SOM due to differences in litter chemistry. In Chapter 2, I conducted a field incubation to understand how sorghum root versus leaf litter, as influenced by their contrasting chemistry, affect the formation and stabilization of SOM. Using unique soil-biomass microcosms to incubate root or leaf litter in topsoil (0-30 cm) for 19 months in the field, I traced the fate of litter decomposition products by combining stable 13C and 15N isotope labeling with extensive separation of physical soil fractions, free or within different aggregate structures. I found that roots, which were lower quality than leaves, decomposed more slowly but contributed more efficiently to total SOM formation than leaves. However, leaves contributed more to the stable SOM pool (i.e. associated to minerals) while roots contributed more to less stable fractions (i.e. light particulate organic matter). Additionally, sorghum is known to produce roots to a depth of 2 meters. There is limited understanding of how roots deeper in the soil (e.g., below 30 cm) lead to SOM formation and stabilization. In Chapter 3, I used the same microcosm approach as in Chapter 2, with roots that were incubated up to a 90 cm depth to better understand how depth influences the ability of roots to contribute to the formation of SOM and what role aggregates play in this process. Results of this study showed that differences in root decomposition dynamics with depth resulted in greater accrual of root litter C in more stable mineral associated SOM pools in the surface depth while there was slower decomposition and greater accrual in the less stable particulate organic matter fractions in the deep soil. Interestingly, most of the stable fraction was recovered within soil aggregates, particularly microaggregates. The results of these experiments emphasized the important role of microaggregates in modulating SOM dynamics. In Chapter 4, I used the information gleaned from Chapters 2 and 3 as well as advances in the SOM research community to speculate on the role of aggregation, specifically microaggregates, in moderating SOM formation by presenting a conceptual framework that integrates aggregates within our current understanding of particulate and mineral associate SOM dynamics. Overall, my dissertation addresses fundamental questions about our ability to increase SOM levels and resulting soil C accrual through the production of a deep-rooted crop through a field incubation. At the same time, I have connected these relevant results to the broader SOM research community by presenting a novel conceptual model that advances our current SOM framework. My hope is that this will be a valuable contribution to the field and spark discussion and future research.
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    Potential environmental impacts from cropping-pattern and land-use changes under Thailands's ethanol production mandate
    (Colorado State University. Libraries, 2015) Suksawat, Jakrapun, author; Graff, Gregory D., advisor; Hoag, Dana L. K., committee member; Loomis, John B., committee member; Paustian, Keith, committee member
    The primary energy source meeting demand in Thailand is oil, especially in the transportation sector, which has resulted in energy import dependency and environmental impacts (Energy Policy and Planning Office, 2012). To reduce energy import and carbon emission the Thai government has announced a plan, known as “Low Carbon Society” policy that promoted bioenergy use (Ministry of Energy, 2012). The main bioenergy strategy of the Thai government is promotion of ethanol production. Ethanol production targets have been set at 3.0, 6.2, and 9.0 million liters per day, in 2008-2011, 2012-2016, and 2017-2022, respectively (Ministry of Energy, 2012). The main feedstocks for ethanol production in Thailand are cassava and molasses, a by-product from refining cane sugar. The cultivation areas of these energy crops are thus expected to increase and intensify due to expansion on ethanol production. In 2010, it was estimated that 1.61 million tonnes of cassava and 2.19 million tonnes of molasses could serve as feedstock for ethanol production of 2.25 million liters per day. Based on licensed ethanol plants and the ethanol production target for 2022, demand for cassava and molasses from the Thai ethanol industry would increase up to at least 14.34 and 3.96 million tonnes per year. While the current molasses production could serve this feedstock demand, the enormous increase in demand for cassava would significantly increase land-use for cassava cultivation. The ethanol production has been promoted for the purpose of energy security, GHG emission reduction, and economic development. However, it is unclear that the ethanol target of the Thai government is possible in both economic and political terms regardless of the cropping land-use change and thus the environmental impacts. Moreover, the planning, monitoring, and setting suitable cultivation area for ethanol feedstock could help to reduce its negative impact on land use change, deforestation, and biodiversity loss (Scarlat and Dallemand, 2011). This proposed study thus focuses on three interrelated topics: the economic and political feasibility of enacting these mandates; the potential cropping land-use change under realistic scenarios; and the potential environmental impacts of these changes. The objectives for each of these are as follow: 1. To evaluate the current economic and political feasibility to produce nine million liters per day of ethanol. The economic feasibility regards to estimate adequacy of ethanol feedstock crops and cultivate areas as compared to other major competing crops benefit. The political feasibility issues regards the competition of interests among influential parties that play important roles in the Thai energy and agricultural industries, such as the government itself, oil companies, and farmer associations. 2. To assess on the outcome of cropping land-use change when ethanol target is introduced. The significant increase in ethanol and feedstock demand is expected to dramatically alter crop cultivation areas. Moreover, energy crops and competitive crop prices would also impact on farmers’ decision. Thus, individual farmers’ economic decision when adopting ethanol feedstock crops to be cultivated instead of other competitive crops will be investigated. Various scenarios cropping land-use change when ethanol mandate is implemented and subsequent will be studied in-depth by using the Multi-criteria Analysis and Geographic Information Systems (GIS). 3. To estimate the environmental impacts of Thai ethanol mandate under these various scenarios. Ethanol mandate implementation does not only directly affect GHG reduction, but also effects GHG balance due to cropping land-use change. Other environmental impact such biodiversity can also be measured. Based on a range of realistic alternative scenarios of cropping land-use change, the range of impacts on several measures of environmental quality will be estimated. The CENTURY model will be used to account soil carbon sequestration as GHG balance. Meanwhile, the nitrous oxide, methane, and biodiversity loss from cropping land-use change are discussed.
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    The ecology of natural climate solutions: quantifying soil carbon and biodiversity benefits
    (Colorado State University. Libraries, 2021) McClelland, Shelby C., author; Schipanski, Meagan E., advisor; Cotrufo, M. Francesca, committee member; Dillon, Jasmine, committee member; Paustian, Keith, committee member
    Achieving net zero greenhouse gas emission by 2050 will require simultaneous emissions reductions and carbon dioxide removal from the atmosphere. Natural climate solutions offer the most mature opportunities to remove atmospheric carbon and sequester it in woody biomass and soils but currently these options remain at low levels of adoption in the United States. To increase the uptake of these practices by growers, there needs to be greater confidence in the expected soil carbon benefits and improved understanding of potential environmental tradeoffs from these strategies across management and environmental contexts. This dissertation quantified the influence of management decisions and environmental variables on soil carbon responses under two proposed agricultural natural climate solutions: inclusion of cover crops and additions of organic amendments. The ecological and biodiversity co-benefits under these practices were also examined. Using a meta-analysis approach, the first chapter analyzed soil carbon responses to cover crop management decisions and environmental variables. Across 181 observations of 40 publications from temperate climates, inclusion of cover crops in cropping systems increased soil organic carbon stocks from 0-30 cm by twelve percent relative to a similarly managed system without cover crops. Management and environmental variables were responsible for variation in soil C responses across studies. The second chapter evaluated the application of organic amendments to improved and semi-native pastures at a semi-arid experimental site in northern Colorado. Over eight years and two applications of a high-quality organic amendment, soil organic carbon stocks as quantified by equivalent soil mass increased 0.7 Mg C ha-1 yr-1 from 0-20 cm under the organic amendment in the improved pasture relative to the control. After accounting for the additions of carbon from the two amendment applications, soil organic carbon stocks in the improved pasture increased by 0.46 Mg C ha-1 yr-1 from 0-20 cm. In contrast, there was no net change of soil carbon stocks in the semi-native pasture. The third chapter examined changes in plant and soil community composition and function after nitrogen application at the same experimental site. A single organic nitrogen addition to the improved pasture increased forage production, plant diversity, and soil microbial community composition and function. The stronger initial plant responses and the gradual change in microbial community composition and function suggests a plant-mediated response to organic nitrogen in this system, which likely impacted soil carbon cycling. Water-limited, semi-native pastures appear to be more resistant to change under one-time organic and inorganic nitrogen additions than irrigated, improved pastures. The final chapter of this dissertation compared two recommended approaches by the Food and Agriculture Organization of the United Nations for quantifying livestock production system impacts on biodiversity. The results illustrated how indicator selection and functional unit may result in discrepancies between the two methods. Together, these findings contribute to a growing body of scientific evidence in support of natural climate solutions for their climate and environmental co-benefits.
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    The effects of soil structure on soil organic matter: a mechanistic approach
    (Colorado State University. Libraries, 2022) Even, Rebecca, author; Cotrufo, M. Francesca, advisor; Conant, Richard, committee member; Paustian, Keith, committee member
    Two key factors theorized to affect soil organic carbon (SOC) dynamics are type of plant carbon (C) inputs and soil structure (i.e., soil aggregation), both are influenced by management practices and are considerably intertwined. Research surrounding these factors has increased in the last several decades as the threat of climate change has forced policy makers to find natural based solutions to rising CO2 levels in Earth's atmosphere. Given that soil acts as the largest terrestrial C pool but has lost substantial amounts of C due to land use change and unsustainable agriculture, focus has shifted towards identifying better ways to manage arable lands that improve SOC storage. Among the conventional management practices tillage is likely the most studied, because of its damage to soil structure, leading to soil C losses. However, while research centered on tillage effects on soil aggregation and SOC cycling is vast, few studies explore how plant C input type (i.e., soluble versus structural) and disturbance (i.e., tilling) together affects SOC in soils with different degrees of aggregation. We examined the effects of soil texture, disturbance, and plant input type on soil aggregation, C mineralization, and formation and persistence of plant input-derived SOC to better understand the mechanisms by which soil aggregates help form and protect SOC, specifically as particulate and mineral associated organic carbon (POC and MAOC). POC and MAOC are expected to be formed by distinct pathways, respectively from structural and soluble inputs. Because of their different mechanisms of protection, POC and MAOC are also expected to respond differently to plant inputs and management practices, like tilling, that disturb soil aggregates. We aimed to parse the formation and persistence of POC and MAOC by adding 13C labeled plant residue separated into soluble and structural plant constituents to determine how these physically distinct plant compounds contribute to either pool when soil is intact or disturbed. In an in-lab incubation using 13C enrichment, we traced SOC over the course of one year in a factorial design with four factors: soil type*disturbance*plant input*harvest. Our results showed, as expected, that hot-water extractable (HWE) plant inputs contributed substantially to MAOC while structural plant components (SPC) inputs preferentially formed POC. Interestingly, we found that disturbance resulted in less HWE mineralized to CO2 and more MAOC formation in the highly aggregated (HA) soil suggesting that increased mineral surface area caused more efficient dissolved OM sorption. Moreover, HWE-derived MAOC persisted in both the undisturbed (U) and disturbed (D) HA soils but not in low aggregation (LA) soils, indicating that persistence of MAOC is dependent on soil type and aggregation (i.e., soil physical structure). Although we did not observe significant differences in aggregate-occluded POC (oPOC) formation between HAD and LAD soils, we did see higher oPOC persistence in HAD soil compared to LAD soil. Greater accumulation oPOC in HAD from day 22 to the end of the incubation suggests, again, that soil type influences the persistence of POC through occlusion in aggregates. To corroborate this, we also found that LAD soil had the highest CO2 mineralization of SPC plant inputs as SPC was left more unprotected in the soil with a low capacity to aggregate. Disturbance did not affect microbial biomass in either HA or LA soils. We saw more plant-derived microbial biomass C from HWE inputs compared to SPC inputs in the bulk soil, indicating that HWE inputs are assimilated into microbial biomass, thus incorporated into SOC with higher efficiency. Lastly, there was a significant drop in % plant-derived microbial biomass C in the bulk soil overtime, as expected. However, because the % HWE-derived MAOC persisted in HA soils regardless of disturbance, we illustrated the importance of microbial necromass in addition to direct DOC sorption for SOC stabilization as MAOC. Overall, my study provides mechanistic understanding for the role of soil structure and aggregation on POC and MAOC formation and persistence which can help improve the representation of these processes in models, to provide better predictions of SOC changes with changes in management practices affecting disturbance.
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    Understanding the dynamics and management of organic nutrient sources in smallholder farming systems: an interdisciplinary approach
    (Colorado State University. Libraries, 2021) Magonziwa, Blessing, author; Fonte, Steven, advisor; Davis, Jessica, committee member; Carolan, Michael, committee member; Paustian, Keith, committee member
    Smallholder farmers often face challenges in managing soil fertility due to limited inputs and high spatial variability on their farms. In many places, soil fertility, and overall soil health, is on the decline, and management of organic nutrient sources (ONS) can play a vital role in sustaining the productivity of soils. However, in mixed smallholder crop-livestock systems there is often competition for crop residues between retaining residues within fields versus feeding them to livestock. Understanding how ONS produced on-farm are managed, and the flows and drivers of this essential resource is critical for the restoration and sustainable management of soil fertility and health in smallholder agroecosystems. The objectives of this study were to: i) validate a soil health tool kit developed to facilitate smallholder research and management involving the use of ONS and other soil management strategies; ii) evaluate how different maize-based ONS (shoot, roots, manure) influence soil organic carbon (SOC) dynamics; iii) understand socio-cultural, economic, and environmental drivers of ONS allocation and use; and iv) understand management and environmental drivers SOC and nutrient (N, P and K) balances across various management scenarios. To address these objectives, a soil health tool kit to provide in-field quantitative data that are comparable to formal laboratory methods was assembled. I then validated methods used in this tool kit against standard analyses conducted at national laboratories on soils collected from 36 smallholder farms in Kenya and 115 farms in Peru. My results showed that permanganate oxidizable C and pH measured with the tool kit from Kenyan and Peruvian soils were highly correlated to the same variables measured by a standard laboratory. The tool kit and standard laboratory measures of available P were less well correlated, but also showed a significant positive relationship. Both tool kit and standard lab analyses displayed similar abilities to predict maize grain yield in Kenya. My findings suggest that the tool kit methods proposed in this study have broad applicability to smallholder farms for explaining variability in crop yields, assessing soil properties of different plots and quantifying management-induced changes in soil health. In the next study, I used a mesocosm experiment and a 13C natural abundance approach, where organic residues (maize shoots, ex-situ maize roots, in-situ maize roots and cattle manure) were incubated for 11 months to trace maize-derived C into different SOC pools. My findings indicated that there was greater stabilization of shoot-derived C (2 X more than manure and 1.6 X more than ex-situ root C) in the mineral-associated organic matter fraction. At the same time, mineral additions of N, P and S (aimed at adjusting the stochiometry of the added residue inputs) led to a 60% decrease in C stabilization in the mineral-associated fraction, compared to a control with no nutrient additions. My study highlights the potential importance of residue retention as a strategy to maintain SOC and therefore soil health and did not support the idea that strategic N, P, and S additions can facilitate C stabilization in soil over the long-term. I then used focus group discussions and conducted a survey of 184 farming households to understand socio-economic, socio-cultural, and environmental drivers of ONS allocation and use at farm scale in three contrasting agroecological zones of western Kenya. I found that the more resource endowed a farmer is, the more ONS are allocated to the main production plot within a farm. However, beyond resource endowment I observed that agroecological location, and tenure, perceived soil fertility, gender and social connections also had important influences on ONS allocation. Lastly, I examined case studies from three representative farm types within three agroecological zones in western Kenya and used a modelling approach to estimate nutrient and C flows in and out of fields. Based on the estimated flows, I then examined different scenarios representing alternative possibilities for ONS management in the region. I noted differences in inputs and allocation between the three zones, but these did not affect the overall balances, which were largely influenced by fertilizer inputs, as well as nutrient export in harvest and soil erosion. Overall nutrient balances were variable, but largely negative across the zones, farm types and field types. When exploring the different management scenarios, reducing erosion led to significantly less negative N balances in all locations. A full residue retention scenario indicated the greatest impact on K balances, while for SOC scenarios with full residue retention and lablab (a high biomass legume) incorporation resulted in at least 50 % more SOC compared to current practices. Scenarios indicate that retaining residues as well as implementing erosion control measures have the potential to effectively reduce nutrient losses as well as improve SOC stocks and that these practices should be encouraged. As research and development organizations continue to engage with smallholder farmers to reduce the burden of global food insecurity, the insights gained by this research will allow for better anticipation of drivers and obstacles to improved nutrient management in these farming landscapes and communities.
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    Using Bayesian model selection and calibration to improve the DayCent ecosystem model
    (Colorado State University. Libraries, 2020) Gurung, Ram B., author; Ogle, Stephen M., advisor; Paustian, Keith, committee member; Parton, William J., committee member; Breidt, F. Jay, committee member
    Process-based biogeochemical models have been developed and used for decades to predict the outcomes of real-world ecological processes. These models are based on a theoretical understanding of relevant ecological processes and approximated using highly complex mathematical equations and hundreds of unknown parameters—requiring calibration using physical observations of the system. These models are then used to test scientific understanding, estimate pools and fluxes, make predictions for future scenarios, and to evaluate management and policy outcomes. To provide a better understanding of the ecological processes, these models need to be simple, make accurate predictions, and account for all sources of uncertainty. The focus of this dissertation is to develop a Bayesian model analysis framework to meet the goal of developing simple and accurate models that fully address uncertainty. This framework includes variance-based global sensitivity analysis (GSA) to identify influential model parameters, a Bayesian calibration method using sampling importance resampling (SIR) to estimate the posterior distribution of unknown model parameters and hyperparameters, and a Monte Carlo analysis to estimate the posterior predictive distribution of model outputs. The framework accounts for all sources of uncertainty, including the remaining uncertainty over the fitted parameters. Additionally, Bayesian model selection is also implemented in the framework to determine the most appropriate level of complexity during model development. The framework is applied to improve the DayCent ecosystem model in agricultural applications. The DayCent model was improved with several model developments, including NH3 volatilization, the release of nitrogen (N) from controlled-release N fertilizers (CRNFs) and the inhibition of the biological process of nitrification and delay the transformation of NH+4 to NO-3 with nitrification inhibitor (NIs). The model development incorporates key 4R management practices that mitigate NH3 and N2O emissions in fertilized upland agricultural soils. In addition, I recalibrated the soil organic matter submodel to improve estimation of soil organic carbon (C) sequestration potentials to a 30 cm depth for several management practices, including organic matter amendment, adoption of no-till management, and addition of synthetic N fertilizers. The results showed that the DayCent model predictions of C sequestration and reduction in N2O flux as well as NH3 volatilization from several management practices were consistent with the field observations. The model result suggested that addition of organic amendments and adoption of no-till are viable management option for C sequestration, however, the addition of synthetic N fertilizer did not produce a significant level of C sequestration. For NH3 volatilization, the model also adequately captures the reduction potential of urease inhibitor along with the incorporation of urea by mechanical means or with immediate irrigation/rainfall. The model also shows promising results in mitigating N2O emissions with both CRNFs and NIs in comparison to field observations. The model prediction focuses on estimating greenhouse gas (GHG) mitigation potential and estimation of uncertainty arising during model prediction—enhancing DayCent as a tool for scientific understanding, regional to global assessments, policy implementation, and carbon emission trading. Overall, the model improvements enhanced the ability of the DayCent model in providing a stronger basis to support policy and management decisions associated with GHG mitigation in agricultural soils.
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    When the wells run dry: soil organic carbon dynamics during the transition from irrigated to dryland cropping systems
    (Colorado State University. Libraries, 2021) Núñez, Agustín, author; Schipanski, Meagan, advisor; Cotrufo, M. Francesca, committee member; Davis, Jessica, committee member; Paustian, Keith, committee member
    In many parts of the world, irrigation must decrease due to declining water availability and increased demand from other water users. The Ogallala Aquifer, one of the biggest aquifers in the world, is one example where declining groundwater levels threaten agricultural productivity and social communities across large parts of the semiarid High Plains. In this semiarid region, irrigation is not only fundamental for crop productivity, but it also has positive effects on soil organic carbon (SOC). However, little is known about the changes in SOC dynamics during the transition from irrigated to dryland cropping systems, which has important potential implications for the long-term productivity of these agricultural systems as well as the potential for the soils of the region to be a net sink or source of CO2. The general objective of my dissertation was to study how irrigation retirement affects SOC dynamics in semiarid agricultural systems of the Ogallala Aquifer Region. I used field experiments to quantify the early changes in crop productivity and C inputs, soil microbial communities, C outputs and SOC formation and turnover during the transition from irrigated to dryland cropping systems. Irrigation retirement had a stronger influence on C inputs than on C outputs because plants responded faster and to a greater magnitude than soil microorganisms to water limitations. Given intrinsic differences in growing season and water requirements, crops vary in their sensitivity to water stress, and wheat agroecosystems were less affected by irrigation retirement than maize agroecosystems. After three growing seasons, there was lower microbial activity and SOC formation in dryland (retired) than irrigated maize, but we did not find changes in the decomposition rate of old SOC. In winter wheat, low differences in soil moisture and crop productivity resulted in almost no changes in microbial activity and SOC dynamics after irrigation retirement. These short-term study results suggest that large losses of crop productivity and C inputs without changes in C outputs will decrease the formation of new SOC, thus affecting SOC storage on the longer term. I confirmed this outcome with on-farm observations of the longer-term effect of irrigation retirement on SOC stocks under different management options. After 7-10 years, sites that used to be irrigated and transitioned back to dryland systems had lower SOC than long-term irrigated sites and had the same SOC stocks as long-term dryland fields, confirming the relatively short legacy effect of irrigation. An exception to this was the transition from irrigated agriculture to perennial, ungrazed grasslands enrolled in the Conservation Reserve Program (CRP). Fields that transitioned into CRP were able to maintain intermediate SOC levels that did not differ from the currently irrigated controls. Taken together, the results of my dissertation indicate that there will be rapid and significant losses of SOC during the transition from irrigated to dryland cropping systems in the Ogallala Aquifer Region. These losses will occur mainly in response to changes in C inputs. Therefore, comparison of biomass and residue production could be used to rapidly identify crop and vegetation management strategies with higher potential to minimize the negative impact of irrigation retirement on SOC.