Theses and Dissertations from UMD

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New submissions to the thesis/dissertation collections are added automatically as they are received from the Graduate School. Currently, the Graduate School deposits all theses and dissertations from a given semester after the official graduation date. This means that there may be up to a 4 month delay in the appearance of a give thesis/dissertation in DRUM

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Managing Cover Crops for Better N Efficiency and Soil Health
(2024) Stefun, Melissa; Weil, Ray; Environmental Science and Technology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Winter cover cropping is a major tool that agriculture can use to protect soil and water quality and mitigate climate change. Unlike farmland in the world at large, most Maryland cropland has seen little tillage disturbance and some level of cover cropping for decades. With that background, field experiments on two soils with contrasting textures at the Beltsville Facility of Central Maryland Research and Education Center tested the effects of cover crop management enhancements on nitrogen (N) leaching, soil health indicators, and cover crop N uptake over three years. Two cover crops (sole rye and a mixture of forage radish, crimson clover, and rye) were compared to a control where cover cropping was ceased. The cash crops were corn and soybean grown in rotation. With best nutrient management practices applied, suction lysimeter sampling at 90 cm depth from October through April showed low levels of N leaching in general, but NO3-N concentrations were significantly lower under cover crops. Overall mean concentrations of NO3-N were 2.20 mg N/L in the control but 0.43 mg N/L under cover crops. Additionally, soil water samples were digested to determine dissolved organic N (DON) which was found to make up between 44-60% of the total dissolved N in the leaching water. In additional experiments, a small fertilizer N application was made to cover crops to stimulate rapid deep rooting with the goal of accessing soluble N deep in the profile to increase N capture by more than the amount of N applied. The response to fall N fertilization failed to accomplish this goal and was not related to the surface soil NO3-N concentration as expected. In spring, cover crops were terminated on three dates from mid-April to mid-May and rye biomass doubled with each extra two weeks it was allowed to grow whether it was in the mix or alone. The effect of cover crops on soil health indicators was evident with increased soil permanganate oxidizable carbon, total soil carbon, lower bulk density, and greater aggregation. These experiments demonstrated that cover crops with enhanced management can have marked effects on an agricultural system already using sustainable practices.
NITROGEN, MICROBES, PARTICLES AND OXYGEN DEFICIENT ZONES
(2024) Huanca Valenzuela, Paulina Alejandra; Fuchsman, Clara A.; Cram, Jacob A.; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Single-celled microbes mediate most biogeochemical cycling in the ocean. Ammonium is generally the preferred reduced nitrogen form microbes use for assimilation and growth. However, ammonium is often removed to undetectable levels from offshore waters. Microorganisms can metabolize alternative organic reduced nitrogen forms in the absence of ammonium, if they possess genes encoding for the enzymes cyanase (cynS), and urease (ureC), which catalyze the decomposition of cyanate and urea respectively. It is unknown which microbes contain these genes in the environment.In my first chapter, I quantified the microbes that can use cyanate and/or urea in oxic and anoxic (ODZ) environments by using a phylogenetic read placement technique. First, I explored depth profiles of metagenomes from two Pacific Ocean regions: an oxic region represented by the nutrient limited Hawaii Ocean Time series, and two ODZ environments represented by the Eastern Tropical South and the North Pacific. A larger proportion of N2 producing anammox bacteria in ODZs have the ability to utilize cyanate than urea, while a larger proportion of nitrite oxidizing Nitrospina have the ability to utilize urea than cyanate. Ammonia-oxidizing Thaumarchaeota had the ability to use urea in deep oxic waters. Contrastingly, the majority of heterotrophic SAR11 bacteria had the ability to use urea in surface waters, but none did in deep waters. This structuring of who can utilize which reduced N form could reflect competition between microbes and N availability. For my second chapter, I examined microbial ability to use urea and cyanate across time and space using metagenomes from two oceanic Geotraces transects in the North Atlantic; GA02 a North-South spring transect, and GA03, a Fall West to East transect. The two transects differed in nutrient concentrations, affecting the composition of phytoplankton communities. Though eukaryotic phytoplankton were abundant on the spring GA02 transect, they did not have the ability to use urea or cyanate, probably because ammonia was present. However, the ability to use urea was still common in SAR11. Cyanobacteria Synechococcus was abundant on this transect and had the ability to use cyanate. In the nutrient limited fall GA03 transect, the results were similar to oxic waters in chapter 1 except that towards the east, cyanobacteria Prochlorococcus gained the ability to utilize cyanate. Both seasonal and spatial changes were observed in the distribution of ureC and cynS genes in microbial groups in the North Atlantic. My third chapter focuses on organisms living on suspended particles. Marine particles constitute a niche that provides ample nutrient and carbon sources. Large particles have been postulated to support anaerobic metabolism that cannot occur in the surrounding water. We examined how microbial diversity changes among a range of 7 different particle sizes in a depth profile at the East Pacific Rise, an area of the ocean with a distinct oxygen minimum. By combining a quantitative 16S rRNA amplicon sequencing dataset with size fractionated organic matter concentrations, we estimated numbers of each microbial taxa per gram of carbon. Results show differences in microbial composition at different particle sizes and depths.
Multispectral satellite remote sensing approaches for estimating cover crop performance in Maryland and Delaware
(2022) THIEME, ALISON; Justice, Chris; Geography; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Winter cover crops encompass a range of species planted in late summer and fall for a variety of reasons relating to soil health, nutrient retention, soil compaction, biotic diversity, and erosion prevention. As agricultural intensification continues, the practice of winter cover cropping remains a crucial practice to reduce leaching from agricultural fields. Maryland and Delaware both incentivize cover cropping to meet water quality objectives in the Chesapeake Bay Watershed. These large-scale programs necessitate methods to evaluate cover crop performance over the landscape. Cover crop quantity and quality was measured at 2,700 locations between 2006-2021 with a focus on fields planted to four cereal species: wheat, rye, barley, and triticale. Samples were GPS located and timed with satellite remote sensing observations from SPOT 4, SPOT 5, Landsat 5, Landsat 7, Landsat 8, or Sentinel-2. When paired imagery at 10-30 m spatial resolution , there is a strong relationship between the normalized difference vegetation index (NDVI) and percent ground cover (R2=0.72) as well as NDVI and biomass (as high as R2=0.77). There is also a strong relationship between Δ Red Edge (a combination of 740 nm and 783 nm bands) and nitrogen content (R2=0.75). These equations were applied to Harmonized Landsat Sentinel-2 products and used to estimate cover crop aboveground biomass in ~300,000 ha of Maryland Department of Agricultures and ~60,000 ha of Delaware Association of Conservation Districts enrolled fields from 2019-2021 and grouped by agronomic method. Wintertime and springtime cover crop biomass varied based on planting date, planting method, species, termination date, and termination method. Early planted fields had higher wintertime biomass while fields that delayed termination had higher springtime biomass. Triticale had consistently higher biomass while wheat had the lowest biomass. Fields planted using a drill followed by light tillage or no-till drill had higher biomass, likely due to the better seed-to-soil contact. Fields that were taken to harvest or terminated for on farm use (roller crimped, green chopped) also had higher springtime biomass than other termination methods. Incentives can be used to encourage specific agronomic methods and these findings can be used to inform adaptive management in the Mid-Atlantic Region.
EXAMINATION OF SOIL GREENHOUSE GAS FLUXES AND DENITRIFICATION TO ASSESS POLLUTION SWAPPING IN AGRICULTURAL DRAINAGE WATER MANAGEMENT
(2022) Hagedorn, Jacob; Davidson, Eric A; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Increases in agricultural nitrogen (N) inputs driven by synthetic N fertilizer application over the past century have led to higher crop yields but have also intensified riverine nitrate (NO3-) loading, contributing to environmental degradation. Drainage water management (DWM) is a best management practice (BMP) implemented on agricultural ditches to reduce downstream NO3- loading by slowing ditch discharge with drainage control structures that raise the in-field water table, creating anaerobic conditions. More anaerobic conditions stimulate denitrification and possibly methanogenesis. Denitrification consumes NO3-, thereby reducing the downstream N loading, but also increases production of N gases nitric oxide (NO), nitrous oxide (N2O), and dinitrogen (N2). This research examined the potential consequence of greenhouse gas (GHG) emissions, specifically methane (CH4) and N2O, as a result of DWM-induced low oxygen conditions in a replicated experimental design. Using multiple methods such as soil gas flux measurements, N isotope analyses, gases dissolved in groundwater, and N budgets, this project examined the potential pollution trade-offs between dissolved NO3- and soil GHG fluxes. In chapter 2, I quantified soil N2O and CH4 fluxes using static chambers over three years in a corn/soybean rotation system. I also measured soil environmental variables to assess controls on gas production. Results indicated that the DWM treatment raised the groundwater level near the ditch edge but did not increase the surface soil moisture, which likely led to the observation that DWM did not significantly increase soil N2O and CH4 emissions. Variation in N2O fluxes were heavily influenced by N fertilizer application events. A N budget indicated that this farm site had a lower than average N use efficiency in the U.S. and higher than average soil N2O emissions. In chapter 3, I qualitatively and quantitatively examined the role of denitrification in this DWM system by using natural abundance NO3- isotopes measured across a leaching continuum (topsoil to deep soil to groundwater to ditch water). Results demonstrated that isotopic values of δ15N and δ18O increased in residual NO3- along the leaching continuum, providing evidence of denitrification. However, the net effects of nitrification and denitrification resulted in NO3- less enriched in 15N than expected by denitrification alone. These isotopic values were then applied to a mass balance of total N and δ15N to quantitively calculate the magnitude of total gaseous N export and to constrain that estimate using a net N isotopic discrimination factor. The calculated gaseous N export and denitrification rates fell within but toward the high end of previously reported literature ranges. The N budget indicated lower hydrologic N export in the DWM treatment, suggesting increased denitrification, but uncertainty of the corresponding estimates of increased gaseous N export was greater than the difference between treatments, rendering inconclusive the hypothesis that DWM treatment causes more total gaseous N production and denitrification. Inclusion of isotopes in the N mass balance established a lower bound of total gaseous N export, which was still large relative to other budget terms. In chapter 4, I synthesized results from the previous two chapters to explore the components of total gaseous N export. I also estimated annual export via dissolved N2O and N2 in groundwater entering the drainage ditches. Soil N2 emissions were estimated by subtracting annual estimates of soil N2O and groundwater dissolved N2 and N2O from the total gaseous N export. Results showed that soil N2 emissions dominated the gaseous N export. The N2O/(N2 + N2O) ratios of soil emissions were within but on the lower side of the literature range. This study demonstrated that, at least for this farm, the decrease in hydrologic N loading due to implementation of DWM outweighed the small and statistically non-significant observed increase in GHG production. This result lends support for policies to further incentivize adoption of DWM in ditched agricultural settings. This study also provides a novel, multi-methodological approach for quantitatively assessing and constraining denitrification rates and N2 emissions. It also is the first study to incorporate measurement of multiple fractions of total gaseous N export on the farm scale as part of annualized agricultural N budget.
REMOVAL OF STORMWATER DISSOLVED ORGANIC NITROGEN MODEL COMPOUNDS THROUGH ADSORPTION AND BIOTRANSFORMATION
(2019) Mohtadi, Mehrdad; Davis, Allen P.; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Bioretention systems are stormwater control measures designed to reduce nitrogen and phosphorus transferred by stormwater to water resources. They are, however, not effectively designed to remove dissolved organic nitrogen (DON). This study concentrated on improvement of bioretention design to remove stormwater DON. Batch adsorption of eight organic nitrogenous compounds onto several adsorbents showed that coal activated carbon (AC) could be a reliable adsorbent for removal of organic nitrogenous compounds such as pyrrole and N-acetyl-D-glucosamine (NAG). The adsorption capacity of pyrrole and NAG on coal AC were 0.4 mg N/g (at equilibrium concentration, Ce = 0.02 mg N/L) and 0.71 mg N/g (at Ce = 1 mg N/L), respectively. These eight nitrogenous compounds were also tested for continuous column adsorption on a media mixture of coal AC + quartz sand, and only pyrrole showed an appreciable adsorption performance; the breakthrough and exhaustion depths for pyrrole were 88 and 499 m, respectively, at the fixed superficial velocity of 61 cm/h and influent DON concentration of 1 mg N/L. Pyrrole adsorption was also minimally affected by superficial velocity (DON removal efficiency stayed > 91% for all tested superficial velocities, 7 to 489 cm/h). Because the adsorption process was successful for removal of only one (pyrrole) out of eight examined compounds, biological treatment was also investigated for removal of organic nitrogenous compounds. Biotransformation alongside adsorption demonstrated benefits such as ammonification of bio-recalcitrant organic nitrogen compounds, e.g., pyrrole, and bioregeneration of the adsorbent (coal AC). According to the results, ammonifiction might be considered as a possible reliable mechanism for stormwater DON removal at low temperatures > 4°C. Under intermittent wetting/draining conditions, the effluent DON was less than 0.1 mg N/L after the applied depth of 48 m, indicating that DON was successfully removed through simultaneous adsorption/ammonification, although generated ammonium in the effluent must be properly addressed. Overall, based on the results from the current study, some DON types were strongly adsorbed by adsorbents, e.g., adsorption of pyrrole on coal AC, some were more bioavailable, e.g., ammonification of leucine, and some were barely adsorbable and bioavailable, e.g., Aldrich humic acid on coal AC. Accordingly, both adsorption and biotransformation should be considered to enhance stormwater DON removal as much as possible.
Nitrogen cycling by grass-brassica mixtures in the Mid-Atlantic
(2019) Gaimaro, Joshua Ruben; Tully, Kate; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Mixtures of cover crop species may be more effective than monocultures at internal nutrient cycling due to their ability to occupy different niches. Our study investigates nitrogen (N) cycling of radish (Raphanus sativus L.) and rye (Secale cereal L.) in monocultures and mixtures compared to a no cover crop control. The study was established on fine-textured soils near Laurel, MD where we estimated N leaching losses, quantified mineral soil N (to 60 cm), and cover crop biomass N for two years. Forage radish suppressed estimated N leaching in the fall, while cereal rye suppressed estimated N leaching in the spring. In this study, growing radish in a mixture with rye decreased the risk of N leaching losses and enhanced N cycling due to the difference in timing of N uptake and release. Our research indicates that grass-brassica mixtures are a flexible management tool for mitigating N leaching in the Mid-Atlantic.
SHIFTING INPUTS AND TRANSFORMATIONS OF NITROGEN IN FORESTED AND MIXED LAND USE BASINS: IMPLICATIONS FOR HYDROLOGIC NITROGEN LOSS
(2018) Sabo, Robert Daniel; Eshleman, Keith N.; Marine-Estuarine-Environmental Sciences; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Increased N inputs along with changes in population, land use, and climate have globally altered the N cycle. This alteration has been associated with increased food, energy, and fiber availability, but has also contributed to the degradation of human health conditions and diminishment of expected ecosystem services in many regions throughout the world. In this context, my research explored the impact of shifting anthropogenic N inputs and other environmental drivers on terrestrial N surpluses and linked changes in terrestrial surpluses to observed changes in N loss to aquatic systems. Working in both forested and mixed land use catchments in the eastern USA, I hypothesized that processes that reduced terrestrial N surpluses in catchments by 1) reducing N inputs, 2) increasing plant uptake, and/or 3) increasing gaseous efflux would result in decreased hydrologic N export. Identification of potential processes was accomplished by first generating long-term atmospheric, remote sensing, terrestrial, and hydrologic datasets for individual catchments. The first two components of my dissertation highlighted potential interactions between atmospheric N deposition, acidic deposition, climate, and disturbance in influencing terrestrial N availability, as indicated by N isotopes in tree rings, in forested catchments. Leveraging trend analysis and statistical models, I identified continued long-term declines in terrestrial N availability in forests, but this decline was likely being modified by disturbance and long-term reductions in acidic deposition. The final component of my dissertation involved developing a lumped conceptual model to explain water quality trends in three mixed land use catchments within the Chesapeake Bay watershed. This study assessed the relative influence of point source N loading, agricultural practices, and atmospheric N deposition on long-term trends in riverine N loss. Insights from the simple N loading model strongly suggested that declines in atmospheric N deposition and point source loading were key drivers of historical water quality improvement. Whether relying on quasi-mass balances or dendroisotopic records, findings from this research emphasize the usefulness of constructing proxy datasets of terrestrial N surpluses in identifying likely processes driving changes in hydrologic N loss in forested and mixed land use catchments.
DEEP SOIL NITROGEN CAPTURE AND RECYCLING BY EARLY-PLANTED, DEEP-ROOTED COVER CROPS
(2018) Hirsh, Sarah Marie; Weil, Ray R; Environmental Science and Technology; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
The overall purpose of this study was to improve the efficiency of nitrogen (N) cycling in Mid-Atlantic cropping systems through the use of cover crops. Our focus was on describing soil inorganic N pools (0-210 cm deep) and investigating the potential for cover crops to scavenge and recycle deep soil N. Few agronomic studies consider soil properties and processes deeper than the upper 20 to 30 cm, as the majority of roots, amendments, and practices such as fertilizer application or tillage occur on the soil surface or in the topsoil. We 1) assessed amounts of deep soil N on 29 farms in the Mid-Atlantic region, 2) used 15N tracer to investigate the capacity of various cover crops with early- or late-planting dates to capture and recycle deep soil N, and 3) investigated early-planted cover crop systems on 19 farm trials to assess their performance on farms with various soils with diverse management practices. We found that on average 253 kg N ha-1 of inorganic N remained in the soil following summer crops, 55% from 90-210 cm deep. Soil following soybean had the same amount or more of inorganic N than soil following corn throughout the soil profile. Using 15N isotopic tracer, we determined that radish, rye, and radish/rye mixes with and without crimson clover all could capture N from deep soil (60+ cm), but in order for cover crops to capture agronomically meaningful amounts of nitrate-nitrogen (NO3-N) from deep soil, they had to be planted by early-September. Cover crop trials on 19 farms indicated that, while variable site-by-site, early-planted cover crops tended to accumulate substantial N in the fall and reduce residual soil NO3-N levels substantially in the fall and spring. Cover crops also impacted subsequent corn growth and yield, with winter cereal tending to cause lower yields or increased corn N fertilizer needs compared to a no cover crop control, and forage radish sometimes leading to higher yields compared to the control. Overall, cover crops are effective at scavenging deep soil N in the fall, before winter leaching occurs, and under certain conditions, can release N for subsequent crops.
Evaluation of Nutrients and Suspended Solids Removal by Stormwater Control Measures Using High Flow Media
(2017) Landsman, Matthew Robert; Davis, Allen P; Civil Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
High Flow Media (HFM) is able to treat large runoff volumes using small- footprint systems. Seven full-scale HFM Stormwater Control Measures (SCMs) in a residential area were monitored over 11 months to assess the removal of Total Suspended Solids (TSS), Nitrogen, and Phosphorus in First Flush (FF) stormwater runoff. Excellent removal of TSS and particulate-bound nutrients was noted, but, in most SCMs, removal of dissolved species was limited. Sorption of dissolved P occurred, although most likely on captured and suspended sediment and not on the HFM itself. Mineralization and nitrification of dissolved N species during dry periods led to nitrate export. HFM grain size and organic content did not significantly impact TSS or P removal, but higher organic content was associated with higher N removal. FF was present in TSS (strongest), TN, and TP (weakest). Optimal HFM SCM design incorporates sedimentation before filtration.
Influence of Nitrogen and Sink Competition on Shoot Growth of Poplar
(2016) Egekwu, Chioma; Coleman, Gary D; Plant Science and Landscape Architecture (PSLA); Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)
Terrestrial and oceanic biomass carbon sinks help reduce anthropogenic CO2 emissions and mitigate the long-term effect of increasing atmospheric CO2. Woody plants have large carbon pools because of their long residence time, however N availability can negatively impact tree responses to elevated CO2. Seasonal cycling of internal N in trees is a component that contributes to fitness especially in N limited environments. It involves resorption from senescing leaves of deciduous trees and storage as vegetative storage proteins (VSP) in perennial organs. Populus is a model organism for tree biology that efficiently recycles N. Bark storage proteins (BSP) are the most abundant VSP that serves as seasonal N reserves. Here I show how poplar growth is influenced by N availability and how growth is influenced by shoot competition for stored N reserves. I also provide data that indicates that auxin mediates BSP catabolism during renewed shoot growth. Understanding the components of N accumulation, remobilization and utilization can provide insights leading to increasing N use efficiency (NUE) of perennial plants.

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