Browsing by Author "Schipanski, Meagan, advisor"
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Item Open Access Crop residue: a hero's journey from biomass to soil carbon in eastern Colorado dryland crop rotation systems(Colorado State University. Libraries, 2019) Schnarr, Cassandra, author; Schipanski, Meagan, advisor; Ham, Jay, committee member; Conant, Richard, committee member; Tatarko, John, committee memberCrop residues play a vital role in reducing the potential for wind erosion of agricultural soils in arid and semi-arid regions. The residues act via three modes: reducing wind speed, acting as a physical impediment to wind reaching the soil surface, and as an organic matter input to spur aggregation and aggregate stability. The interactions of crop residues, crop rotation systems, and wind erosion factors were studied at three long-term agricultural research sites along an evapotranspiration gradient near Sterling, Stratton, and Walsh, Colorado. The sites have a 30-year history of dryland, no-till management, and are divided into different cropping system intensities that vary in the frequency of summer fallow periods in the rotation. Crop rotations studied here include wheat (Triticum aestivum)-fallow, wheat-corn (Zea mays) – fallow, and continuously cropped plots with small grains and forage crops including foxtail millet (Setaria varidis) and forage sorghum (Sorghum bicolor). Forage crop and wheat residues were tracked over two growing seasons (2015 and 2016) to estimate the length of time before soil surface cover fell below a 30% threshold and to create models for residue persistence. Decomposition Days (DD), a calculation that factors in temperature and rainfall to estimate cumulative conditions that favor decomposition, was used to normalize time scales following harvest across sites and years. Wheat residue covered 82% of the soil surface following harvest and summer forage crops covered 56%. Wheat persisted longer, taking 62.5 DD to fall to the 30% cover threshold, forage crop residue remained above the threshold for 16.6 DD. The decline of forage crop residue cover followed an exponential decay model. Wheat residue surface cover had a longer, slower decline and fit a quadratic decay model. Wheat stem heights were taller following harvest and heights declined at a similar or faster rate than forage crops. To assess rotation legacy impacts on soil erodibility, soils were sampled in May 2015 and tested for dry aggregate size distribution, dry aggregate stability, and carbon distribution by size classes and between cropping intensities. No differences were found in the amount of erodible aggregate size fraction (<0.84mm) by cropping system intensity. The site with the highest amount of clay in the soil displayed a significant difference in aggregate stability by crop rotation, with wheat-fallow rotations having stability of 2.96 ln J/Kg and continuously cropped systems having 2.80 ln J/Kg. Carbon distribution did not differ by crop rotation but did differ by size class at the site with the highest potential evapotranspiration and lowest clay content where the largest aggregates contained the highest proportion of carbon. Every phase (i.e., rotation year) of each of the crop rotation systems were represented each year. There was a significant difference in mean erodible fraction and aggregate stability by cropping phase at the time of sampling at the site with the highest clay content. Taken together, the crop residue and soil aggregate portions of the study indicate that the reliable and consistent prevention of wind erosion by crop system intensity may be more dependent upon annual crop residue surface cover than longer-term management impacts on soil aggregation properties. The differences in aggregate stability by crop type could be due to the impacts of active root systems at the time of sampling. More investigation is warranted into the influence of active root systems on macro dry aggregates and whether dry aggregate stability properties differ by season. Further study into the application of residue biomass decay models to residue soil cover, particularly in crops with multiple layers of residue is also indicated.Item 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 memberIn 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.Item 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 memberDecreasing 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.Item 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 memberOver 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.Item Open Access The intensification revolution in dryland cropping systems: implications from field to landscape scale(Colorado State University. Libraries, 2017) Rosenzweig, Steven, author; Schipanski, Meagan, advisor; Stromberger, Mary, committee member; Carolan, Michael, committee member; Davis, Jessica, committee memberA global transformation in semi-arid cropping systems is occurring as dryland (non-irrigated) farmers shift from crop rotations reliant on year-long periods of bare fallow to more intensively cropped systems. Bare fallow has reduced year-to-year variability in crop yields, but it has also constrained crop productivity and, therefore, reduced carbon (C) inputs to soils. Exposure to tillage and erosion, combined with C limitation, has gradually degraded dryland soils and reduced their capacity to capture water and supply plant nutrients, requiring dryland farmers to rely on external inputs to support plant growth. However, the emergence of no-till has enabled dryland farmers to save enough water to replace bare fallows with crops, a practice called cropping system intensification. Cropping intensification has potential implications for the environment and economy of dryland agriculture as it impacts every aspect of the agroecosystem – from soil health, to weed and nutrient management, to crop yields. This dissertation seeks to unravel the economic and environmental implications of cropping system intensification at both the field and landscape scale in the US High Plains, and to understand the social dynamics underpinning this revolution. I quantified the impacts of cropping system intensification on a range of soil health parameters on 96 dryland, no-till fields in the High Plains. Three levels of cropping system intensity – wheat-fallow, mid-intensity, and continuous – were represented along a potential evapotranspiration gradient that increases from northwestern Nebraska to southeastern Colorado. I conducted in-depth interviews with farmers to examine the motivations, perceptions, and social interactions that influence decisions about whether and how much to intensify, and to collect detailed field histories including input use and crop yields. To scale up the implications of these field-level analyses, and to assess the current extent of the cropping revolution in the High Plains, I conducted a spatial analysis using high-resolution satellite crop data to examine changes in cropping patterns over time at the landscape scale. I found that cropping system intensification was positively associated with soil organic carbon, aggregation, and fungal biomass, and these effects were robust amidst variability in environmental and management factors. I also found that intensified systems were associated with greater potentially mineralizable and total nitrogen (N), and arbuscular mycorrhizal fungal colonization of wheat roots, suggesting that cropping intensity enhances internal cycling of N and phosphorus (P). Continuous dryland farmers also achieved greater total crop production using fewer external inputs than wheat-fallow farmers, leading to enhanced profitability. To explain the social dynamics underpinning the cropping system revolution, I build on Carolan's application of Bourdieusian social fields to agriculture, and find several overlapping fields within Carolan's more general fields of sustainable and conventional agriculture, which are reflected in different degrees of intensification. I identify strategies for change, some of which would serve to reshape social fields, and others which leverage existing social positions and relationships, to enable farmers to overcome the barriers constraining cropping system intensification. Results from the spatial analysis suggest that, from 2008 to 2016, the High Plains witnessed a profound shift in cropping systems, as the historically dominant wheat-fallow system was replaced by intensified rotations as the dominant systems across the landscape. I estimated that these patterns over the 9-year study period increased annual grain production and annual net farm operating income, slightly reduced herbicide use, and increased C sequestration, contributing to greenhouse gas reductions. I projected each of these implications to a scenario of 100% continuous cropping adoption to estimate the potential environmental and economic impacts of cropping system intensification in the High Plains. Overall, my findings suggest that dryland cropping systems are gradually intensifying in the High Plains, and these trends are likely reversing historical negative environmental and economic trends to enhance the profitability and environmental sustainability of dryland agroecosystems.Item Open Access Water-limited competition and the yield-density response in dryland maize (Zea mays): an ecological and economic analysis(Colorado State University. Libraries, 2023) Dewey, Caroline, author; Schipanski, Meagan, advisor; Comas, Louise, advisor; Ocheltree, Troy, committee memberThe plant dynamics of biomass production under competing resources are commonly understood through the empirical generalization of Constant Final Yield (CFY). This law has considerable utility for crop management decisions that often center on altering resources and planting density to maximize plant productivity. Dryland producers are uniquely vulnerable to variability in climactic conditions and precipitation patterns. However, most studies on yield-density relationships have focused on well-watered conditions for maize (Zea mays). In this thesis, I investigated the hypothesis that the yield-density relationship in dryland maize will approximate CFY, with the point of plateau determined by water availability (Chapter 1). This concept was tested by planting maize at a range of stand densities (20, 30, 40, 50 thousand plants/ha) under four water regimes in a semi-arid region in Colorado, USA. Plant productivity increased under greater water availability as the planting density increased. A quadratic plateau model best fit the yield-density relationship. Treatments with less water availability did not exhibit yield-limiting thresholds at the densities included. Plant functioning in terms of chlorophyll fluorescence, grain N uptake and proportional allocation to grain (i.e., harvest index) remained relatively unaffected by resource availability to the plants. Results of the study indicate dryland maize systems can reach a maximum yield while forgoing significant physiological stress. The pattern of CFY was approximated, with water availability corresponding to a higher asymptotic point at a greater population density. As a next step, a partial budget analysis was conducted to assess net returns associated with varying seeding rate and soil moisture in dryland maize cropping system (Chapter 2). Data was selected from the experimental study outlined in Chapter 1. Benefits were calculated in terms of maize grain yield. Cost estimations for each treatment included the cost of seed, representative field operations and management. Results showed that net returns responded positively to high evapotranspiration (ET) conditions. Under low ET conditions a decrease in seeding rate was more profitable. Improved soil moisture improved net returns among both seeding rates. The increase in revenue from grain yield under high ET conditions was greater than any additional costs, even though materials and services costs generally increased. Dryland producers should approach increased seeding with caution if seeking to maximize grain yield at low soil moisture.Item Open Access 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 memberIn 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.