Theses and Dissertations

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Open Access
Cover crops for ecological management of U.S. agricultural systems: quantifying ecosystem services across multiple scales
(Colorado State University. Libraries, 2023) Eash, Lisa, author; Fonte, Steven J., advisor; Schipanski, Meagan E., committee member; Trivedi, Pankaj, committee member; Mooney, Daniel, committee member
Managing agricultural systems to provide multiple ecosystem services (ES) beyond food provisioning has gained considerable attention in recent years. The integration of cover crops (CC) into U.S. cropping systems presents an opportunity to support multifunctional agricultural systems, which alleviate negative environmental impacts of agriculture, mitigate greenhouse gas (GHG) emissions and support sustained crop production. However, CC impacts on these ES are variable and depend on management and site characteristics, contributing to uncertainty surrounding to what extent CC can improve ES. Reducing this uncertainty is critical to both identify appropriate environmental and management conditions for CC adoption and improve the estimated potential for CC to improve multifunctionality of U.S. cropping systems. This dissertation aims to quantify CC impacts on ES at multiple scales, exploring benefits to the soil microbiome, at the farm level, and nationally. Throughout this assessment I explore how these effects are influenced by climate and soil characteristics and how management can be leveraged to optimize the provision of ES. Chapter two estimates the potential for widespread adoption of CC to increase soil organic carbon (C) stocks and mitigate GHG emissions in the U.S. Analysis using current U.S. crop management data and a biogeochemical model revealed that the mitigation potential over a 20 year period is lower than previous estimates due to regional variability, decreasing rates of C accrual over time, and limited CC integration. Changes in N2O emissions did not offset C sequestration but introduced large uncertainty surrounding total national mitigation potential. Soil C gains due to CC offer important co-benefits to U.S. cropping systems, but the contribution of CC to achieving U.S. emissions targets will likely be lower than previously anticipated. Our spatially-explicit analysis also highlights regions where adoption of CC can have greater relative contributions to GHG mitigation. I then quantify a larger suite of ES in dryland wheat systems of the semi-arid western U.S., a particularly challenging context for CC due to lower potential productivity and associated economic trade-offs. I used two existing field trials to monitor CC impacts on soil health, cash crop productivity, and economics over a period of six years. No-till, CC planting window, and the sale of CC biomass as forage were also explored as strategies to optimize ES provision and economic viability. Chapters three and four demonstrate that the integration of CC amidst water limitations can benefit erosion control and soil structure, but also present significant productivity and economic trade-offs. The integration of fall-planted CC, no-till management, and the use of CC for forage provided the greatest potential for maximizing ES benefits in an economically viable manner. In Chapter five, I conducted a greenhouse study to examine the impact of CC type and functional diversity on microbial community composition and associated ES. Plant functional types (Poaceae, Brassicaceae, and Fabaceae) were associated with distinct increases in ES proxies, which appear to be mediated by shifts in microbial community composition. Specifically, Fabaceae (legume) CC enhanced the presence of copiotrophic microbes, which were associated with improvements in soil structure and high enzyme activity, a proxy for nutrient cycling. Poaceae and Brassicaceae led to improvements in microbial diversity. Ecosystem service benefits and microbial community shifts were conserved in diverse CC mixtures, contributing to increased multifunctionality. Across studies and scales, CC were observed to support a number of ES that address environmental concerns resulting from modern intensive agricultural practices. However, slight benefits and substantial productivity trade-offs in water-limited systems may limit the extent to which CC can mitigate GHG emissions and restore soil C reserves nationally. Management choices, such as CC composition and diversity, no-till management, and the sale of a portion of CC biomass as forage, can be leveraged to optimize the provision of ES in an economically viable manner. Overall, CC effectively contribute to multifunctional agroecosystems whose ES extend beyond food provisioning.
Open Access
Soil health indicators for water-limited regions: sensitivity to compost and cropping intensification
(Colorado State University. Libraries, 2024) Noble Strohm, Tess, author; Schipanski, Meagan, advisor; Fonte, Steven, advisor; Ross, Matthew, committee member
In the water-limited agroecosystems of the Great Plains, USA, management strategies such as compost application and cropping system intensification have been promoted to increase soil health and help adapt to climatic variability. However, accurately assessing soil health to support production systems in such regions hinges upon a selection of indicators sensitive to management and linked to essential soil functions, especially those related to soil water dynamics. Using a suite of soil physical and biological parameters, this study assessed the effects of management on soil health metrics and evaluated the extent to which these metrics were related to soil water dynamics utilizing long-term studies in Akron, CO, and Clovis, NM. Soil physical indicators included aggregate stability (mean weight diameter; MWD), bulk density and saturated hydraulic conductivity, while biological indicators included measures of soil macrofauna and microbial communities. Compost application was the primary driver of increased aggregate stability and abundance of soil biota at both sites, though effects of cropping system intensification were observed for some indicators. Measures of soil microbial abundance were correlated with MWD, but saturated hydraulic conductivity was generally not correlated with other measured variables. Our findings indicate that MWD and microbial abundance are linked and sensitive to management, and further research to connect measures of soil biological and physical health to soil water dynamics in semi-arid systems is necessary to develop regionally relevant frameworks for soil health assessments.
Open Access
Soil organic matter as a nitrogen source for Brassica napus
(Colorado State University. Libraries, 2020) Carter, Candace, author; Schipanski, Meagan, advisor; Fletcher, Richard, committee member; Vivanco, Jorge, committee member; Wallenstein, Matthew, committee member
Decreasing nitrogen (N) fertilizer losses from agricultural systems is a major focus in sustainable agriculture research. Most research to date has focused on reducing and managing N fertilizer additions in time and space. However, approximately half of the N taken up by most field crops is not from that season's fertilizer but is derived from the mineralization of soil organic matter (SOM). Despite its importance, intentionally managing crop utilization of background SOM as a source of N has received little attention. Our study explored N uptake patterns of rapeseed or canola (Brassica napus) in a greenhouse pot study and in a field setting. In the greenhouse pot study, we explored the effects of rapeseed genotypic diversity on N uptake from organic and inorganic N sources. We used dual 15N labeled ammonium-nitrate fertilizer to examine N uptake patterns of rapeseed in different N environments. Using a full factorial experiment, 10 varieties were grown under four treatments that included combinations of high and low N fertilizer and SOM. While we found limited varietal differences in N uptake dynamics, SOM was an important N source across all varieties even as N fertilizer availability increased. Our High SOM/High Fertilizer treatment obtained 64% of N from SOM, while the High SOM/Low Fertilizer obtained 89% of total N from SOM. Nitrogen source uptake was dependent on the treatment level N availability. We found evidence of enhanced SOM mineralization in higher N treatments, where high N fertilizer additions increased overall plant N uptake from SOM by 42% relative to low N fertilizer treatments. Although overall plant N uptake from SOM increased in high fertilizer treatments, microbial enzyme activity related to nutrient mineralization processes was suppressed in the high N fertilizer treatments relative to low fertilizer treatments in similar SOM environments by 16-58%. This result suggests high N fertilizer additions change microbial nutrient cycling dynamics. Based on the general results from our greenhouse study that N from SOM had an additive effect to fertilizer additions on rapeseed biomass production, we estimated the potential yield contributions of SOM increases with the adoption of conservation tillage practices in Canada. We used yield data provided by a literature search and the Canola Council of Canada to examine how the adoption of conservation tillage practices over the last 25 years has contributed to crop yield improvements in the Canadian prairies. We found that on average canola yields increase by 54.9 kg/ha per year, with 13% of annual yields attributed to agronomic practices. We estimated that the adoption of conservation tillage has increased soil N by 320 kg N/ha per year. Although N mineralization is highly variable and dependent on many factors, we estimated that 2% of total soil N is available annually for plant uptake. This translated to an additional 6.4 kg N/ha per year available for plant nutrition. We estimated that 91 to 164 kg/ha of the annual canola yield increases could be contributed to an increase in soil N availability. It is important to acknowledge the complex nature of N mineralization and plant N uptake patterns. This complexity likely leads to an underestimation of the contribution of SOM as an N source in cropping systems. Because of the dynamic and complex nature of agricultural systems, an integrated approach to N management where both N fertilizer and SOM are considered in crop breeding and system management is an important step in improving agricultural sustainability.
Open Access
The effects of irrigation retirement on soil carbon dynamics of a continuous maize agroecosystem
(Colorado State University. Libraries, 2023) Mendoza-Martinez, Violeta, author; Schipanski, Meagan, advisor; Wrighton, Kelly, committee member; Prenni, Jessica, committee member
Over half of the world's fresh water is used in crop production and, in some key agricultural regions, use far exceeds local water availability and recharge rates. With the increasing strain on freshwater resources caused by climate change and a growing population, agriculture is under pressure to reduce its water consumption and large areas of currently irrigated farmland across the Western U.S. will likely transition into dryland agriculture over the coming decades. The effects this will have on global soil carbon (C) dynamics, however, remain unclear. In 2016, a study was established in Northern Colorado to understand how stopping irrigation affects soil C turnover in a no-till, continuous maize agroecosystem. Earlier results showed limited responses of the soil microbial community to irrigation retirement, but differences in soil heterotrophic respiration (Rh) rates were detected after two years of accumulated differences in plant residue inputs, thus suggesting a possible co-limitation of water and available C to the microbial community. We continued this experiment through 2022 to further explore the relationship between soil moisture and C inputs in shaping the soil microbial community under the new watering regimes and the consequential effects on soil respiration (Rs) as an indicator of soil organic C (SOC) turnover rates. Two seasons of data collection in 2021 and 2022 showed decreases in available soil water, bacteria, fungi, protozoa and actinomycetes fatty acid methyl ester (FAME) biomarkers, activities of four extracellular enzymes and soil autotrophic respiration in response to both reductions in irrigation and plant inputs, with strong interactive effects between the two factors. However, plots under dryland conditions had higher concentrations of dissolved organic carbon (DOC) and muted differences in soil Rh when compared to their irrigated counterparts; differences in Rh between fallow treatments with (YF) and without residue inputs (LTF), on the other hand, were more pronounced. Soil Rs in fallow plots was consistently, positively correlated with field soil temperature, while correlations with moisture were weak or even negative, thus suggesting soil moisture was not a strong direct driver of Rh. We investigated the direct and indirect influences of variables collected monthly across two seasons on soil Rh to test our hypothesized model using structural equation modeling. In contrast to the cumulative treatment level impacts of plant inputs and irrigation, monthly soil moisture measurements had a stronger, direct effect on Rh than substrate availability as estimated by water-extractable DOC. The final model only explained 24% of the variability in soil Rh. Changes in global C dynamics can be expected with transition of land areas from irrigated to dryland agriculture. However, focusing on soil health, resource conservation practices and the resiliency of the soil microbiome can be the key to minimize the potential negative impacts of this transition.

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Browsing Theses and Dissertations by Subject "agroecology"

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