Browsing by Author "Strahm, Brian"

Now showing 1 - 6 of 6
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    Catchment Structure Regulates Hydrodynamic Drivers of Chemical Weathering in Shallow Forest Soils
    Pennino, Amanda (Virginia Tech, 2023-06-12)
    Determining where, when, and how subsurface flow affects soil processes and the resulting arrangement of soil development along flow paths is challenging. While hydrologic regime and soil solution acidity are known to influence weathering rates and soil transformation processes, an integrated understanding of these factors together is still lacking. This dissertation explores the effects of subsurface flow on the mobility and distribution of dissolved organic carbon (DOC) and base cations to explain spatial patterns in chemical weathering in a forested headwater catchment. In the first chapter, relationships between hydrologic behavior, fluxes of weathered elements, and the extent of soil elemental loss across landscape positions are established. The second chapter investigates what specific groundwater behavior best explains spatial patterns in solution DOC concentrations during storm events. Lastly, in the third chapter, near surface saturation dynamics are examined to determine when and where DOC mobilization might be enhanced by subsurface flow. Results show that weathering extent was greatest in the upper reaches of the catchment, where O horizon saturation frequency and DOC concentrations are highest. Annual base cation fluxes, which were also greatest in these positions, could indicate where weathering is likely still enhanced. Additionally, while O horizon saturation occurred across the catchment, spatial differences in DOC concentrations suggest there are other sources of acidity to groundwater solutions other than just leaching from O horizons. Shallow organic soils, near bedrock outcrops at the top of the catchment is likely this additional C source, in which drainage water is transported downslope to nearby mineral soils when water tables are high and hydrologic connectivity between soils is increased. Spring and fall storm events were identified as times when groundwater most frequently reached O horizons during the snow-free year, providing insight into the timing of these processes throughout the year. This dissertation highlights how catchment structure mediates DOC flushing events, which in turn, influences the spatial architecture of soil development and chemical weathering processes across the landscape.
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    Estimating site susceptibility to Scotch broom dominance in young Douglas-fir plantations for control prioritization in western Washington, USA
    Boyle, Grady John (Virginia Tech, 2023-10-11)
    Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco var. menziesii), a keystone species in western Washington, faces threats on plantations across this region from the invasive species Scotch broom (Cytisus scoparius (L.) Link), whose invasions on recently established stands can lead to mortality of Douglas-fir through overtopping. The susceptibility of sites to Scotch broom achieving dominance over Douglas-fir has been demonstrated as highly site dependent, however the site conditions that cause this have not yet been identified. Scotch broom has a demonstrated average maximum height of 3m, thus, after Douglas-fir exceeds this height, its risk of being overtopped is significantly reduced. This thesis strives to identify sites that were at the greatest risk Douglas-fir being overtopped by Scotch broom by first, identifying what factors improved growth of Douglas-fir during the period when they are at the greatest risk, and second, identifying factors that led to Douglas-fir outcompeting Scotch broom on sites they cohabitated. In Chapter 1, we utilized LiDAR scans, Soil Survey Geographic Database characteristics, and management histories to identify conditions that improved growth for Douglas-fir in ages 3-8. Individual tree detection was used to measure Douglas-fir heights, and a correction algorithm for LiDAR measured young Douglas-fir heights was established from field validation data. We identified that young Douglas-fir had improved growth on sites with lower elevation, flatter slopes, and finer textured soils. The factors identified were then transformed into four potential site index models based on mean stand elevation class, Mean stand elevation class and clay class, textural class and slope class, and textural class and Mean stand elevation class. In Chapter 2, we used paired field plots to examine Douglas-fir and Scotch broom competition on 19 sites across western Washington. Each site had 2 plots with only Douglas-fir and 2 plots with Douglas-fir and Scotch broom. Elevation, soil texture, and soil nutrient composition for carbon, nitrogen and available phosphorous were examined for influence on height and growth rate of both species. We identified that Scotch broom presence was negatively related to Douglas-fir height growth and that sites with either higher percentages of silt, lower concentrations of phosphorous, or higher percentages of Carbon were more likely to have growth patterns close to or exceeding Scotch broom.
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    Exploring the Impact of Climate and Productivity on Soil Carbon Across the Native Range of Loblolly Pine
    Price, Aaron Cooper (Virginia Tech, 2024-08-30)
    Policies intended to pursue climate mitigation through soil carbon sequestration require accurate estimates of soil carbon stocks and projections of how carbon pools will change with future climate. In this study, the process model MIMICS-CN was utilized to explore the potential changes in bulk soil organic carbon (SOC) and protected mineral-associated organic matter (MAOM) with changes in mean annual temperature (MAT) and annual net primary productivity (NPP) for the native range of loblolly pine (Pinus taeda L). A Monte Carlo method was used to calibrate a large ensemble of model parameters using observational data from loblolly pine stands. Spatial projections of carbon pools across the native range of loblolly pine were produced for nine temperature and productivity scenarios representing a range of likely futures. The standard deviation of the mean total SOC pool to a depth of 30 cm was 3.6 kg C m-2 while the standard deviation of potential changes in pool magnitude was 0.96 kg C m-2. Associated with regional climate change projections, increases in MAT caused losses in bulk SOC but did not affect MAOM. Simulated increases in NPP resulted in increased bulk SOC, MAOM and the proportion of SOC that is MAOM (MAOM:SOC) highlighting the importance of productivity in sequestering atmospheric CO2 in general and in protected C pools, specifically. SOC pools did not change consistently across the landscape. Rather, they varied by subregion due to differences in soil texture and changes in forecasted NPP. Soils with clay contents above 18% had the greatest potential to accumulate MAOM and bulk SOC under a realistic future climate scenarios. In regions with low clay content, significant increases in total SOC are driven by the accumulation of C in unprotected SOC sub-pools. With the maximum likely increase in NPP (31.4%), a regionwide increase in total SOC of 29% was projected whereas subregions in which NPP decreases showed no potential to increase soil carbon stocks. We conclude that the management of loblolly pine to increase productivity can increase the amount of SOC and the residence time of bulk SOC over decadal time scales, however regional differences in the proportion of C in MAOM and unprotected pools must be considered when planning silvicultural management.
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    Nitrogen Fate and Transformations in the Production of Containerized Specialty Crops
    Brown, Forrest Jackson (Virginia Tech, 2024-05-07)
    Nitrogen (N) fertilizer is a required mineral nutrient in containerized crop production that is necessary for crop growth and development. Due to production aspects, the N added to crops far exceeds the amount that the plant uses and such inefficiency results in adverse environmental impacts related to N gaseous and aqueous emissions from containers on the production site. Growers are responsible for optimizing nutrient usage in crop production. Three studies were conducted to investigate and better understand the fate of applied N fertilizers, the transformations associated with individual N sources, and the influence of substrate texture on losses of aqueous and gaseous N species. The first study conducted a mass balance looking at the four major avenues of N fate in an open-air container production setting (plant uptake, immobilized or bound N in a pine bark substrate, leached aqueous N, and gaseous emissions of N), the mass balance was speciated to measure applied and intermediary forms of N fertilizer species to provide insight into the overall fate of applied N. Show Off® Forsythia ×intermedia' Mindor' were grown using two control-release fertilizer (CRF) treatments [AN (ammonium-nitrate based) or UAN (urea ammonium-nitrate)] products. This study determined that 97% of the released N from the CRF treatments was lost via aqueous or gaseous pathways. The aqueous losses were inferred to be predominately composed of NO3-N, while the gaseous emissions were inferred to be predominately lost as inert nitrogen gas (N2). During a second experiment, individual N sources treatments [urea (CH4N2O), ammonium (NH4+), and nitrate (NO3-)] were applied to established containers of At LastⓇ Rosa x 'HORCOGJIL' grown in a pine bark substrate in either open wall high tunnel or a glass greenhouse to determine subsequent reaction sequence and fate based on applied N source. By applying an individual form of N it was determined that based on the N source applied, a sequential set of reactions occurs based on the N source. This study determined that the reactive N gaseous species occurred from the hydrolysis of CH4N2O-N to NH4+ and the nitrification of NH4+ to NO3- and then the denitrification of NO3- to N2. Hibiscus moscheutos' Vintage wine' was grown in either a coarse or fine texture substrate utilizing either a water-soluble fertilizer or a CRF to compare the influence of pine bark texture on N leachate losses and RN gaseous emissions. There were few differences between the two substrate texture treatments related to aqueous or gaseous N losses. In both experiments, the Hibiscus grown in the fine texture substrate resulted in higher above and below-ground biomass at experimental termination. Working with growers to develop best management practices will help to improve the use of N fertilizers and impact growers economically, while simultaneously reducing losses leading to less environmental impact on the areas surrounding production sites.
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    Physical and Biological Drivers of Wetlandscape Biogeochemistry
    Corline, Nicholas John (Virginia Tech, 2024-05-22)
    Wetlands play a vital role in regional and global biogeochemistry by controlling the movement and cycling of nutrients and carbon. While individual wetlands may provide these ecosystem services, high density wetland landscapes, referred to as wetlandscapes, can have far reaching aggregate effects on elemental cycling and solute transport. Here we use forested Delmarva bays or wetlands as a study ecosystem to explore physical and biological controls on wetland chemistry within forested wetlandscapes. The Delmarva wetlandscape consists of thousands of geographically isolated wetlands on the Delmarva Peninsula, United States, which despite their proximity to each other have highly variable sizes, shapes, hydrology, vegetative cover, and biological communities. This physical and biological variation makes the Delmarva wetlandscape an ideal ecosystem to understand spatio-temporal heterogeneity and drivers of biogeochemistry. In this dissertation, I demonstrate that water chemistry within the Delmarva wetlandscape is heterogeneous both within and between surface water and groundwater systems (Chapter 2). Surface water chemistry was primarily influenced by temporal factors (season and month), followed by local hydrology. In contrast, groundwater chemistry was strongly influenced by water level below ground surface and interaction with organic soil layers. These results are important in understanding both internal wetlandscape water chemistry dynamics and export of solutes such as dissolved organic matter (DOM) to adjacent river ecosystems. Further, these results suggest that local biological and hydrological factors strongly affect surface water chemistry in wetlands. To explore these factors, I used an observational approach to determine the role of larval amphibians on wetland biogeochemistry (Chapter 3) and employed high-resolution chemistry sensors to study the effect of hydrological changes on surface water dissolved organic matter concentrations (Chapter 4). Animal waste can contribute substantially to nutrient cycling and ecosystem productivity, yet little is known of the biogeochemical impact of animal excretion in wetland habitats. A common and abundant amphibian in Delmarva wetlands are wood frog (Lithobates sylvaticus) tadpoles. I found that wood frog tadpole aggregations elevated nutrient recycling, microbial metabolism, and carbon cycling in Delmarva wetlands. These results provide evidence for the functional and biogeochemical role of tadpole aggregations in wetland habitats, with important implications for ecosystem processes, biodiversity conservation, and ecosystem management. To further explore the role of hydrology on DOM concentrations, I utilized high-resolution fluorescent dissolved organic matter sensors (fDOM) and applied river solute transport frameworks and metrics to wetland catchments. I found that there was heterogeneity in wetland response to changing hydrology and that seasonality and potentially bathymetry influences fDOM concentrations. Together, these studies inform our understanding of wetlandscape heterogeneity and DOM export, as well as biological and hydrological drivers of biogeochemistry.
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    Relationships among Root Traits, Nitrogen Availability, and Mineral-Associated Organic Carbon
    Duston, Stephanie Ann (Virginia Tech, 2025-02-26)
    Mineral-associated organic carbon (MAOC) is a vital component of soil health and ecosystem productivity, playing a key role in carbon (C) sequestration and nutrient cycling. This dissertation investigates how plant root traits, nitrogen (N) fertilization, and cover cropping influence soil organic carbon (SOC) and MAOC. In the first chapter, a greenhouse experiment with 30 herbaceous plant species found that non-N-fixing plants exhibited stronger positive correlations with MAOC compared to N-fixing species, which were more closely linked to SOC. Root biomass contributed to decreases in MAOC, while specific root length (SRL), and coarse root traits were found to significantly contribute to increases in MAOC, highlighting the importance of plant root architecture in stabilizing C. In the second chapter which focused on relationships among N-fertilization, root traits, and MAOC, moderate N fertilization (56 and 112 kg N/ha) was found to enhance total SOC (∆SOC) and MAOC (∆MAOC) accumulation in the system, while higher N inputs (168 kg/ha) reduced C gains. Notably, the use of stable isotopes allowed for the quantification of fresh C additions, with results that indicate plant-added MAOC (PA-MAOC) was influenced more by plant species and root traits, such as coarse root length and aboveground biomass, rather than N fertilization. In the third chapter, a long-term field study demonstrated the effectiveness of cover cropping in increasing both SOC and MAOC by 43–59% and 27–36%, respectively, compared to conventional and no-till systems without cover crops. Despite triennial additions of N fertilizer over nine years, no significant increases in SOC or MAOC was observed. Additionally, root biomass exhibited positive trends with MAOC. These findings suggest that cover cropping, combined with no till practices, plays a pivotal role in enhancing MAOC by minimizing soil disturbance and promoting root-driven C inputs. This research highlights the importance of integrating plant species selection, root morphological traits, N management, and conservation practices to optimize long-term C storage (i.e., MAOC) and support sustainable soil management. Future studies should continue to include MAOC and particulate organic carbon fractions as these functional C sub pools may respond differently than bulk SOC pool. Including further studies on the interactions between root morphology, environmental factors, and C/N dynamics is necessary to develop more resilient agroecosystems capable of mitigating C losses and improving long-term soil health.