Browsing by Author "Stewart, Ryan D."

Now showing 1 - 20 of 67
  • Thumbnail Image
    Abundance, Distribution, and Geometry of Naturally Occurring Macropores in Stream Banks
    McEwen, Amiana Marie (Virginia Tech, 2018-06-13)
    Preferential flow paths are areas of substantially higher permeability than surrounding media. Macropores and soil pipes are a type of preferential flow path where conduit-like voids in the subsurface are typically greater than three millimeters in diameter. They are known to occur in agricultural and forest soils, often as a result of biological and physical processes. Macropores also exist in stream banks and have the potential to enhance the exchange of water and solutes between the channel and riparian groundwater, yet the geographic distribution of bank macropores is unknown. Here we determined the abundance, distribution, and geometry of naturally occurring surface-connected macropores in the banks of 20 streams across five physiographic provinces in the Eastern United States. We identified a total of 1,748 macropores, which were present in all 20 streams, with 3.8 cm average width, 3.3 cm average height, 11.5 cm average depth, and 27.9 cm average height above water surface elevation. Macropore abundance, distribution and geometry were statistically different between physiographic provinces, stream orders, and soil textures, with the latter being the most important. Macropores tended to be larger and more abundant in soils with a high cohesiveness and a low hydraulic conductivity compared to soils with a low cohesiveness and high hydraulic conductivity. As a result, streams with greater longitudinal heterogeneity of soil texture also had greater heterogeneity of macropore density. However, macropore size and height above baseflow water surface elevation also increased with stream order and therefore stream size. This work represents the first attempt to characterize macropores across a variety of riverine systems and presents evidence that macropores may play an important role in hyporheic exchange within stream banks. These results may have water quality implications, where macropores may enhance hyporheic exchange yet reduce the filtering capacity of riparian buffer zones.
  • Thumbnail Image
    Advances in Ecohydrology for Water Resources Optimization in Arid and Semi-Arid Areas
    Castellini, Mirko; Di Prima, Simone; Stewart, Ryan D.; Biddoccu, Marcella; Rahmati, Mehdi; Alagna, Vincenzo (MDPI, 2022-06-07)
    Conserving water resources is a current challenge that will become increasingly urgent in future due to climate change. The arid and semi-arid areas of the globe are expected to be particularly affected by changes in water availability. Consequently, advances in ecohydrology sciences, i.e., the interplay between ecological and hydrological processes, are necessary to enhance the understanding of the critical zone, optimize water resources’ usage in arid and semi-arid areas, and mitigate climate change. This Special Issue (SI) collected 10 original contributions on sustainable land management and the optimization of water resources in fragile environments that are at elevated risk due to climate change. In this context, the topics mainly concern transpiration, evapotranspiration, groundwater recharge, deep percolation, and related issues. The collection of manuscripts presented in this SI represents knowledge of ecohydrology. It is expected that ecohydrology will have increasing applications in the future. Therefore, it is realistic to assume that efforts to increase environmental sustainability and socio-economic development, with water as a central theme, will have a greater chance of success.
  • Thumbnail Image
    Analysis of Reconstructed Mine Soils on Reclaimed Coal and Mineral Sands Mined Lands in Virginia
    Angel, Hannah Zoe (Virginia Tech, 2022-09-01)
    Post-mining land use potentials are greatly influenced by mine soil properties. I analyzed and classified coal mine soils in SW VA and mineral sands mine soils in SE VA to aid development of appropriate post-mining land use interpretations. For coal mine soils, long-term pedogenesis was assessed in sandstone (SS) and/or siltstone (SiS) spoils with and without surface amendments. For mineral sands mined lands, I evaluated their spatial variability with respect to underlying relic mining influences and rowcrop productivity. Coal mine subsoils densified as root-limiting layers formed that were not observed initially and all soils had weak pedogenic development (^Bw; cambic horizons), particularly SS derived soils. Initial rock spoil type strongly influenced coal mine soils over time (i.e., SS had coarser textures and lower subsoil pH vs. SiS). However, most soil chemical properties (e.g., pH, EC, CEC) became similar in ^A horizons over time. Fe-oxides increased, and extractable-P decreased with weathering, raising concerns for long-term P-availability. Organic amendments applied to rock spoil surfaces sustained higher total-N (sawdust and biosolids) and extractable P (biosolids). Soil carbon sequestration rates did not differ among rock types or amendments (0.16 to 0.28 Mg ha yr-1). Further, mineral sands mine soils were limited by densic contacts and short-range variability of important properties (texture) appeared related to underlying relic mining features (pits and berms). Yields in reclaimed soils were more variable than in adjacent undisturbed farmland, but mean yields were only slightly reduced for soybean in 2020. Coloration and Fe-oxide spectral indices had weak to moderate negative correlations with yield. Berm positions supported better soybean growth (three out of five sites), while pit positions were redder/darker (one out of five sites). Current Soil Taxonomy conveys anthropogenic origins at the family (e.g., spolic) and subgroup (e.g., Anthroportic) levels. Existing taxa and proposed Artesols order criteria both acknowledge effective soil depth x compaction limitations (Anthrodensic subgroup), but Artesols more effectively recognizes pedogenesis (Inceptic subgroup) and acknowledges mine soils at the highest level. The biggest management limitation for both mine soil types is root-limiting compaction; thus, future efforts should focus on defining depth ranges and taxonomic interpretations for densic contacts.
  • Thumbnail Image
    Analysis of Soil Tests: Nutrients and Soil Properties to Help Farmers Improve Management Practices
    Motley, Natalie M. (Virginia Tech, 2020-05-11)
    Soil nutrients and properties are the foundation of plant growth and our health. The basis of our soil starts with its texture. Soil texture impacts infiltration rate and cation exchange capacity; which in turn affects water and nutrient availability in plants. Analysis of soil nutrients and properties is imperative for effectively managing adequate soil levels and helping prevent excessive amounts of fertilizer applications. While research studies were conducted to analyze specific nutrients, these studies do not provide farmers the importance of certain nutrients and soil testing on specific farmland parcels. This research study identified soil texture for three parcels of farmland in Dewitt County, Illinois, described soil sampling methodology used, and trends of periodic testing results. Analysis of the soil testing results was compared to recommended optimal soil test values. Key nutrients that were analyzed were phosphorus, potassium, and calcium, as well as pH and organic matter content. Management practices were also assessed. Most test results identified nutrients within the optimum values. Where there were slight variations, effective management of fertilizer applications were applied. Variable rate technology was used to spread fertilizers specifically to field locations in need. Management practices, including crop rotation, were used and based on soil test results, farmers were able to maintain a critical balance of adequate soil nutrients. Further, based on pH soil test results, farmers applied a liming program to regain optimal pH levels for annual crop production. While the organic matter content on one of the three parcels had declined below the Illinois average level, the other two parcels remained within the average range. Maintaining soil nutrient levels through periodic and consistent soil testing is paramount to help farmers more efficiently use limited resources for protecting the soil quality for future generations.
  • Thumbnail Image
    Analyzing a 10-Year Cave Drip Record in James Cave, Virginia: Implications for Storage and Recharge in Shallow Appalachian Karst Systems
    Groce-Wright, Nigel C. (Virginia Tech, 2021-07-16)
    Karst aquifers, characterized by soluble rocks such as limestone and dolostone, provide drinking water to 20-25% of the world's population and are thus critical global water sources. However, recent work suggests that rapid alteration of karst aquifers due to the impact of climate change on precipitation patterns may affect recharge to these aquifers. Much of the research on recharge in karst aquifers has relied on using patterns of spring discharge to quantify recharge. Spring outlets allow for continuous monitoring of discharge from karst aquifers, making them easily accessible monitoring sites. However, because springs can integrate multiple flow paths, it is difficult to rely on spring discharge patterns to get information on where and how karst aquifers are receiving recharge. Monitoring closer to the source of recharge through the measurement of cave drips allows for a more accurate analysis of recharge timing and mechanisms. In this study, I conducted recession analyses on cave drip hydrographs from a 10-year record (2008-2018) of three drip monitoring stations within James Cave (Pulaski Co., VA) to: 1) examine differences in hydrologic characteristics of the epikarst (the zone of soil and weathered bedrock above a karst aquifer); 2) quantify the storage volume of the epikarst and 3) investigate seasonal, and annual trends in recharge. Results of recession analysis show heterogeneity in epikarst hydrologic characteristics, reflected by calculations of the recession coefficient, , and storage volume. Calculations of the recession coefficient show subtle differences between the three drip sites, suggestive of spatial heterogeneity in permeability and storage in the overlying epikarst. The storage volume calculations show that during the recharge season (winter- spring), up to 95% of recharge through the unsaturated zone to the cave occurs through rapid pathways (i.e., fractures), and 5% through diffuse pathways (i.e., pores). However, during the recession period (spring-summer), when evapotranspiration is active, recharge through cave drips decreases and occurs predominantly through diffuse flow. Combined, these results underscore the importance of both spatial and temporal characterization of drip rates and other recharge inputs into karst aquifer systems.
  • Thumbnail Image
    Antimicrobial resistance in soil: long-term effects on microbial communities, interactions with soil properties, and transport of antimicrobial elements
    Shawver, Sarah Elizabeth (Virginia Tech, 2022-06-08)
    Since penicillin was discovered in 1928, antibiotic usage in human and veterinary medicine and prevalence of antibiotic resistant bacteria (ARB), has been increasing. While antibiotics and antibiotic resistance genes (ARGs) naturally occur in soils, increasing abundances of ARGs correlate with increased antibiotic usage in agricultural settings. When livestock are treated with antibiotics, the antibiotic compounds, ARB, and ARGs can enter soil via manure excreted onto pastures or applied to other fields as fertilizer, thereby spreading antimicrobial resistance (AMR) in the environment. In addition to human health implications, increased AMR has negative impacts on ecosystem services such as carbon and nitrogen cycling. While many studies have researched antibiotic persistence in agricultural systems and their impacts on soil microbial communities, there are still significant knowledge gaps around the long-term effects of antibiotic exposure in soils, how those impacts differ among soils, and how elements of AMR may differentially transport through soil. To address these knowledge gaps, our objectives were to 1) examine the impact of multi-year repeated additions of manure from cattle administered antibiotics on soil microbial communities, 2) determine the interactive effects of soil moisture and type on soil microbial communities exposed to antibiotics and manure, and 3) differentiate between vertical transport of AMR in the form of viable ARB or ARGs in extracellular plasmids. Our results demonstrate that soil bacterial community structures were consistently altered by 3-year additions of manure from cattle administered antibiotics compared to soil amended with antibiotic-free manure. Furthermore, ARG abundances were higher in soils with manure additions compared to soil without manure, although this was true regardless of whether the cattle were administered antibiotics, suggesting that manure and antibiotic impacts on soil microbial communities can persist over multi-year of repeated manure applications. Additionally, in microcosms, effects of manure from cattle administered antibiotics on ARG abundances, microbial community structures, respiration, and nitrogen pools in soil were seen across multiple soil types and moisture contents, suggesting environmental conditions can alter how manure and antibiotics impact microbial community structure and nutrient cycling. Finally, ARB flowed readily through saturated soil, but were also detectable in the top 5 cm of soil columns. However, ARGs on extracellular plasmids did not flow through soil columns and were not detected in soil, indicating that extracellular DNA does not persist or transport through the soil to any meaningful degree. Overall, these results indicate a nuanced approach is required to mitigate the environmental spread of AMR. Soil management strategies for addressing the AMR crisis should consider the broader context of manure management, as high ARG abundances can come from application of manure from antibiotic-free cattle, and soil microbial communities in individual environments may have varied responses to manure antibiotic exposure. Furthermore, the transport of AMR through soil is complex and dynamic, as elements of AMR may transport differently through soil and require separate consideration in modeling and management. Future AMR management practices that consider diverse factors that affect persistence and spread of AMR in the environment can help protect livestock productivity and maintain the efficacy of antibiotics to protect human and animal health.
  • Thumbnail Image
    Assessing landscape and seasonal controls on CO2 fluxes in a karst sinkhole
    Thompson, Taryn Karie (Virginia Tech, 2022-01-06)
    Karst landscapes can serve as carbon sinks when carbon dioxide (CO2) reacts with water to form carbonic acid, which then weathers carbonate rocks. However, CO2 can also move through the subsurface via gas diffusion, a process that is not well-understood in karst systems. This study focused on quantifying CO2 diffusion within a karst sinkhole. The objectives of this study were to: 1) identify the depth of the zero-flux plane (i.e., depths of local maximum CO2 concentrations), analyze the distributions of concentration gradients, and investigate the validity of a uniform concentration gradient throughout the profile; and 2) assess the influences of vertical position and seasonality on CO2 fluxes within this sinkhole. The study site contained three locations within the sinkhole, including shoulder, backslope, and toeslope locations. Each location had three soil CO2 and three soil water content/temperature sensors placed at 20, 40, and 60 cm depths. Zero-flux planes were seldom detectable during the warm season (April-September) but were frequently found near the surface (20 or 40 cm) during the cool season (October-March). The common assumption of a uniform concentration gradient was often invalid based on relative concentrations between sensor pairs. As for the second objective, CO2 fluxes generally followed a trend of upward fluxes in warmer months that was partially offset by downward fluxes during the cooler months. These study results provide new insight into CO2 dynamics in a karst system, and suggest that subsurface processes such as chemical weathering and cave ventilation affect the direction and magnitude of CO2 fluxes.
  • Thumbnail Image
    Assessing Potential Solutions to Mitigate Pollution from Neonicotinoid Seed Coatings
    Morrison, Benjamin Anthony (Virginia Tech, 2020-12-08)
    Thiamethoxam and clothianidin are two neonicotinoids used in seed coatings for crops such as corn and soybeans. Both neonicotinoids have high solubility in water, so they are prone to transport via leaching and runoff. This thesis is comprised of two studies that evaluated potential solutions to mitigate neonicotinoid transport from fields. The first study examined the relationship between soil organic carbon content and neonicotinoid transport in a field planted in soybeans. Soils with increased organic carbon leached less thiamethoxam and clothianidin during early growing season leaching peaks; however, at the end of the season, higher organic carbon content only decreased leached mass of clothianidin. The second study was to determine neonicotinoid uptake of different ground covers used as cover crops or edge-of-field buffer strips, as well as the partitioning of thiamethoxam and clothianidin throughout the plants. Ground covers, such as crimson clover, had the highest recovery of applied thiamethoxam, meaning that it may be a good candidate to retain this pesticide in fields. Thiamethoxam and clothianidin concentrations were higher in leaf tissues than in stems or roots, indicating that above-ground biomass removal may be an effective way to reduce neonicotinoid loading in the environment. From these studies, I concluded 1) practices that raise the amount of organic carbon in the soil may help decrease early-season neonicotinoid transport, resulting in lower concentrations in surrounding waterways, and 2) careful selection of plant species, such as crimson clover, may help reduce neonicotinoid transport in the environment, while potentially reducing exposure to beneficial insects.
  • Thumbnail Image
    BEST-WR: An adapted algorithm for the hydraulic characterization of hydrophilic and water-repellent soils
    Di Prima, Simone; Stewart, Ryan D.; Abou Najm, Majdi R.; Ribeiro Roder, Ludmila; Giadrossich, Filippo; Campus, Sergio; Angulo-Jaramillo, Rafael; Yilmaz, Deniz; Roggero, Pier Paolo; Pirastru, Mario; Lassabatere, Laurent (Elsevier, 2021-12-01)
    Water-repellent soils usually experience water flow impedance during the early stage of a wetting process followed by progressive increase of infiltration rate. Current infiltration models are not formulated to describe this peculiar process. Similarly, simplified methods of soil hydraulic characterization (e.g., BEST) are not equipped to handle water-repellent soils. Here, we present an adaptation of the BEST method, named BEST-WR, for the hydraulic characterization of soils at any stage of water-repellency. We modified the Haverkamp explicit transient infiltration model, included in BEST for modeling infiltration data, by embedding a scaling factor describing the rate of attenuation of infiltration rate due to water repellency. The new model was validated using analytically generated data, involving soils with different texture and a dataset that included data from 60 single-ring infiltration tests. The scaling factor was used as a new index to assess soil water repellency in a Mediterranean wooded grassland, where the scattered evergreen oak trees induced more noticeable water repellency under the canopies as compared to the open spaces. The new index produced results in line with those obtained using the water drop penetration time test, which is one of the most widely test applied for quantifying soil water repellency persistence. Finally, we used BEST-WR to determine the hydraulic characteristic curves under both hydrophilic and hydrophobic conditions.
  • Thumbnail Image
    Brief Overview: Field Measurements of Soil Cracks
    Stewart, Ryan D.; Abou Najm, Majdi R. (2018-05)
  • Thumbnail Image
    Building Healthy Soil with Best Management Practices
    Poukrel, Sapana; Maguire, Rory O.; Thomason, Wade E.; Stewart, Ryan D. (2022-10-12)
  • Thumbnail Image
    Characterizing the physical and hydraulic properties of pine bark soilless substrates
    Wolcott, Caroline Courtney (Virginia Tech, 2023-11-06)
    Soilless substrates, such as peat, pine bark, and coir, are widely used as growing media in containerized crops for their favorable characteristics, including low bulk density, balanced air exchange and water retention, disease resistance, and low pH and salinity. However, improper irrigation of these media can have negative outcomes such as root asphyxia, pathogen development, and reduced plant growth. Understanding pore size distributions, water dynamics, and gas diffusivity of these substrates is essential to promote plant growth. The effects of different particle sizes of soilless media on processes such as infiltration, hydraulic conductivity, and gas diffusivity are also not well understood. The characterization of these effects is important for the overall improvement of container crop production. This thesis presents three studies that aimed to characterize the physical and hydraulic properties of pine bark substrates, both unamended and amended with peat or coir. The first study looked at three substrate types: unamended, unscreened pine bark, peat-amended pine bark, and coir amended pine bark. Three methods were employed to quantify pore distributions: non-equilibrium infiltration measurements, equilibrium water retention characterization, and scanning electron microscopy. We characterized pore distributions during wetting and drainage for the three substrates. Coir-amended bark had the largest water-conducting porosity, highest hydraulic conductivity, and most water retention. Unamended pine bark had the highest microporosity, and the addition of peat and coir lowered macroporosity, with peat having the greater effect. The total porosity inferred from the infiltration method was significantly smaller than that inferred from drainage experiments due to assumptions related to pore shape. The second study focused on defining hydraulic conductivity and water retention for pine bark substrates of five different particle sizes, <1 mm, 1-2 mm, 2-4 mm, 4-6 mm, and an unscreened fraction. We utilized the same methods from the first study. The resulting data showed that the smallest particle sizes (i.e., <1 mm and 1-2 mm) had the highest hydraulic conductivity and greatest water retention. The three larger sizes had lower hydraulic conductivity and poor water retention, including the unscreened fraction, which more closely followed the results of the 2-4 mm size. The final study examined gas diffusivity of the five pine bark particle sizes at different moisture levels: 60% moisture content (initial conditions), saturated at the bottom of the sample, near-saturated at the sample bottom, and drained from saturation to container capacity. We used a one-chamber gas diffusion setup to find gas diffusion coefficients (Ds). The results displayed an inverse relationship between Ds values and substrate water content. In addition, the larger particle sizes were less sensitive to changes in water content due to their well-draining large pores. Proper balance of aeration and water retention is necessary for the success of soilless growing media. Overall, the smaller particle size fractions had the best water retention and hydraulic conductivity rates while the larger fractions had the largest Ds coefficients. This work contributes valuable knowledge on the physical and hydraulic properties of different size fractions of pine bark substrates, which can assist nursery growers in optimizing water usage for sustainable container crop production.
  • Thumbnail Image
    Conservation management decreases surface runoff and soil erosion
    Du, Xuan; Jian, Jinshi; Du, Can; Stewart, Ryan D. (Keai Publishing Ltd, 2022-06)
    Conservation management practices - including agroforestry, cover cropping, no-till, reduced tillage, and residue return - have been applied for decades to control surface runoff and soil erosion, yet results have not been integrated and evaluated across cropping systems. In this study we collected data comparing agricultural production with and without conservation management strategies. We used a bootstrap resampling analysis to explore interactions between practice type, soil texture, surface runoff, and soil erosion. We then used a correlation analysis to relate changes in surface runoff and soil erosion to 13 other soil health and agronomic indicators, including soil organic carbon, soil aggregation, infiltration, porosity, subsurface leaching, and cash crop yield. Across all conservation management practices, surface runoff and erosion had respective mean decreases of 67% and 80% compared with controls. Use of cover cropping provided the largest decreases in erosion and surface runoff, thus emphasizing the importance of maintaining continuous vegetative cover on soils. Coarse- and medium-textured soils had greater decreases in both erosion and runoff than fine-textured soils. Changes in surface runoff and soil erosion under conservation management were highly correlated with soil organic carbon, aggregation, porosity, infiltration, leaching, and yield, showing that conservation practices help drive important interactions between these different facets of soil health. This study offers the first large-scale comparison of how different conservation agriculture practices reduce surface runoff and soil erosion, and at the same time provides new insight into how these interactions influence the improvement or loss of soil health. (C) 2021 International Research and Training Center on Erosion and Sedimentation, China Water and Power Press, and China Institute of Water Resources and Hydropower Research. Publishing services by Elsevier B.V. on behalf of KeAi Communications Co. Ltd.
  • Thumbnail Image
    Corn and Wheat Residue Management Effects on Greenhouse Emissions in the Mid-Atlantic USA
    Battaglia, Martin L.; Thomason, Wade E.; Fike, John H.; Evanylo, Gregory K.; Stewart, Ryan D.; Gross, Cole D.; Seleiman, Mahmoud F.; Babur, Emre; Sadeghpour, Amir; Harrison, Matthew Tom (MDPI, 2022-06-05)
    Greenhouse gas (GHG) emissions from crop residue management have been studied extensively, yet the effects of harvesting more than one crop residue in a rotation have not been reported. Here, we measured the short-term changes in methane (CH4), nitrous oxide (N2O), and carbon dioxide (CO2) emissions in response to residue removal from continuous corn (Zea mays L.) (CC) and corn–wheat (Triticum aestivum L.)–soybean (Glycine max L. Merr.) (CWS) rotations in the Mid-Atlantic USA. A first experiment retained five corn stover rates (0, 3.33, 6.66, 10, and 20 Mg ha−1) in a continuous corn (CC) in Blacksburg, VA, in 2016 and 2017. Two other experiments, initiated during the wheat and corn phases of the CWS rotation in New Kent, VA, utilized a factorial combination of retained corn (0, 3.33, 6.66, and 10.0 Mg ha−1) and wheat residue (0, 1, 2, and 3 Mg ha−1). Soybean residue was not varied. Different crop retention rates did not affect CO2 fluxes in any of the field studies. In Blacksburg, retaining 5 Mg ha−1 stover or more increased CH4 and N2O emissions by ~25%. Maximum CH4 and N2O fluxes (4.16 and 5.94 mg m−2 day−1) occurred with 200% (20 Mg ha−1) retention. Two cycles of stover management in Blacksburg, and one cycle of corn or wheat residue management in New Kent did not affect GHG fluxes. This study is the first to investigate the effects of crop residue on GHG emissions in a multi-crop system in humid temperate zones. Longer-term studies are warranted to understand crop residue management effects on GHG emissions in these systems.
  • Thumbnail Image
    Counterions, smectite, and palygorskite increase microstructural stability of saline-sodic soils
    Javaheri, F.; Esfandiarpour-Boroujeni, Isa; Farpoor, Mohammad Hady; Holthusen, D.; Stewart, Ryan D. (Elsevier, 2022-02-01)
    Saline-sodic soils are susceptible to wind and water erosion when the dispersive effect of sodium overcomes inter-particle bonds. Rheological parameters of viscoelasticity can help to quantify inter-particle attractive forces and account for the effect of salinity in these soils. The main objective of the present study was to investigate the viscoelasticity behavior of saline-sodic soils of the Sirjan playa in south-central Iran. Three representative pedons were excavated and described by horizon. Soil physicochemical properties and rheological properties were determined, namely the micromechanical parameters flow point (γf), loss factor tan δ, and integral z, with samples analyzed at three matric potentials (0, −6, and −15 kPa). Results showed that soil microstructural stiffness was mainly influenced by soil texture, clay minerals, gypsum, calcium carbonate equivalent (CCE), and matric potential. The dispersive effect of sodium, as indicated by low integral z and γf values, decreased with increasing gypsum content in − 6 and − 15 kPa matric potentials (0.6 < r < 0.8) and CCE percentage in the quasi-saturated (0 kPa) condition (r > 0.8). However, greater microstructural stability (i.e., higher integral z and γf) was observed for fine-textured soils with relatively high amounts of smectite and palygorskite and low pH. Furthermore, integral z and γf increased with lower matric potentials due to the stabilizing effect of menisci forces. Therefore, the viscoelastic behavior of the saline-sodic soils was negatively associated with water content and high sodium concentration, while the presence of smectite, palygorskite, gypsum, and CCE improved the soil physical conditions and thus the rigidity of the porous system. These results demonstrate that rheological measurements can identify saline-sodic soils that have strongly degraded microstructural stability and would most benefit from active management and amelioration.
  • Thumbnail Image
    Coupling time-lapse ground penetrating radar surveys and infiltration experiments to characterize two types of non-uniform flow
    Di Prima, Simone; Giannini, Vittoria; Ribeiro Roder, Ludmila; Giadrossich, Filippo; Lassabatere, Laurent; Stewart, Ryan D.; Abou Najm, Majdi R.; Longo, Vittorio; Campus, Sergio; Winiarski, Thierry; Angulo-Jaramillo, Rafael; Del Campo, Antonio; Capello, Giorgio; Biddoccu, Marcella; Roggero, Pier Paolo; Pirastru, Mario (Elsevier, 2021-09-17)
    Understanding linkages between heterogeneous soil structures and non-uniform flow is fundamental for interpreting infiltration processes and improving hydrological simulations. Here, we utilized ground-penetrating radar (GPR) as a non-invasive technique to investigate those linkages and to complement current traditional methods that are labor-intensive, invasive, and non-repeatable. We combined time-lapse GPR surveys with different types of infiltration experiments to create three-dimensional (3D) diagrams of the wetting dynamics. We carried out the GPR surveys and validated them with in situ observations, independent measurements and field excavations at two experimental sites. Those sites were selected to represent different mechanisms that generate non-uniform flow: (1) preferential water infiltration initiated by tree trunk and root systems; and (2) lateral subsurface flow due to soil layering. Results revealed links between different types of soil heterogeneity and non-uniform flow. The first experimental site provided evidence of root-induced preferential flow paths along coarse roots, emphasizing the important role of coarse roots in facilitating preferential water movement through the subsurface. The second experimental site showed that water infiltrated through the restrictive layer mainly following the plant root system. The presented approach offers a non-invasive, repeatable and accurate way to detect non-uniform flow.
  • Thumbnail Image
    Cover Cropping: A Strategy to Healthy Soil and Nitrogen Management in Corn
    Pokhrel, Sapana (Virginia Tech, 2023-05-31)
    Economic and environmental concerns surrounding nitrogen (N) have motivated efforts to improve estimates of plant available N in soil in order to improve crop N management decisions. Cover crops have been recognized as an effective tool for protecting soil and enhancing soil function including N cycling. This recognition has increased the adoption of cover cropping in the United States. Despite this growing popularity, there is lack of consistent response of cover crop on soil health and only a few experiments have examined how cover crop impacts nitrogen (N) management in corn. Therefore, the objectives of this study were to: a) quantify the impacts of cover crops on various soil health indicators like permanganate oxidizable carbon (POXC), CO2 burst, autoclaved-citrate extractable (ACE)-soil protein, b) investigate the relationship between soil health indicators, soil nitrate (NO3-N) and ammonium (NH4-N), corn N requirement and corn yield, and c) study the decomposition and N release of different cover crops. In a comparison of cover crop treatments, there was a trend of increasing permanganate oxidizable carbon (POXC) and CO2 burst with cover crop compared to no-cover crop control in the short term (1 year). Additionally, CO2 burst values were significantly increased for a long-term site with cover crops compared to the control in both 2021 and 2022. A long-term cover crop study at 25 sites showed a weak relationship of CO2 burst, POXC, soil protein and NO3-N with agronomic optimum N rate (AONR), with r values ranging from 0.00 to 0.48, suggesting these indicators may not be reliable predictors of N available in soil and corn yield. However, there was significant relationship between NO3-N at N sidedress time and relative yield (r = 0.65) at these long-term sites. In short-term cover crop study (Chapter 1), Presidedress nitrate test (PSNT) nitrate concentration was >15 mg kg-1 at 5 sites and in long term cover crop study (Chapter 2), 15 sites had nitrate concentration > 15 mg kg-1 indicating potential of N sidedressing reduction when compared to current pre-sidedress N test (PSNT) N recommendation in Virginia, which is currently only recommended for sites receiving manure or biosolids. A cover crop decomposition study at Kentland showed that hairy vetch had a faster decomposition rate (k = 0.0377 g g-1 d-1) than rye and vetch mix (k = 0.0292 g g-1 d-1) or cereal rye (0.0227 g g-1 d-1) with 0 N fertilizer and released more N than cereal rye and rye and vetch mix. The difference in C: N ratio (hairy vetch (9-11:1), cereal rye (31-46:1), rye and vetch mix (19-20:1)) may have affected decomposition rate and N release of cover crops. Hairy vetch released significant amounts of N within a month of incubation, with 103 kg N ha-1 in 2021 and 57 kg N ha-1 in 2022. Overall, this study showed that cover crops did not have a consistent or significant effect on soil health indicators in short term. However, cover crops improved CO2 burst at long term cover crop site compared to no-cover, control. Future studies should focus on understanding best methods of predicting N available to subsequent crop and conduct cover crop decomposition studies across the state with different cover crop species and their mixture.
  • Thumbnail Image
    Culturable antibiotic-resistant fecal coliform bacteria in soil and surface runoff after liquid dairy manure surface application and subsurface injection
    Hilaire, Sheldon Shervon; Chen, Chaoqi; Radolinski, Jesse; Leventhal, Talia; Preisendanz, Heather; Kleinman, Peter J. A.; Maguire, Rory O.; Stewart, Ryan D.; Saporito, Lou S.; Xia, Kang (Wiley, 2022-02-05)
    Land application of manure, while beneficial to soil health and plant growth, can lead to an overabundance of nutrients and introduction of emerging contaminants into agricultural fields. Compared with surface application of manure, subsurface injection has been shown to reduce nutrients and antibiotics in surface runoff. However, less is known about the influence of subsurface injection on the transport and persistence of antibiotic-resistant microorganisms. We simulated rainfall to field plots at two sites (one in Virginia and one in Pennsylvania) 1 or 7 d after liquid dairy manure surface and subsurface application (56 Mg ha–1) and monitored the abundance of culturable antibiotic-resistant fecal coliform bacteria (ARFCB) in surface runoff and soils for 45 d. We performed these tests at both sites in spring 2018 and repeated the test at the Virginia site in fall 2019. Manure subsurface injection, compared with surface application, resulted in less ARFCB in surface runoff, and this reduction was greater at Day 1 after application compared with Day 7. The reductions of ARFCB in surface runoff because of manure subsurface injection were 2.5–593 times at the Virginia site in spring 2018 and fall 2019 and 4–5 times at the Pennsylvania site in spring 2018. The ARFCB were only detectable in the 0-to-5-cm soil depth within 14 d of manure surface application but remained detectable in the injection slits of manure subsurface-injected plots even at Day 45. This study demonstrated that subsurface injection can significantly reduce surface runoff of ARFCB from manure-applied fields.
  • Thumbnail Image
    Current Insights into Nonuniform Flow across Scales, Processes, and Applications
    Abou Najm, Majdi R.; Lassabatere, Laurent; Stewart, Ryan D. (2019-12-05)
    This introduction to the special section Nonuniform Flow across Vadose Zone Scales is a brief summary of this special section's diverse contributions covering nonuniform flow across a wide range of scales, processes, and applications. We summarize the 17 articles constituting this special section and hope that those contributions are positive steps toward a new, generalizable, and comprehensive paradigm to modeling flow and transport in porous media.
  • Thumbnail Image
    A database for global soil health assessment
    Jian, Jinshi; Du, Xuan; Stewart, Ryan D. (2020-01-13)
    Field studies have been performed for decades to analyze effects of different management practices on agricultural soils and crop yields, but these data have never been combined together in a way that can inform current and future cropland management. Here, we collected, extracted, and integrated a database of soil health measurements conducted in the field from sites across the globe. The database, named SoilHealthDB, currently focuses on four main conservation management methods: cover crops, no-tillage, agro-forestry systems, and organic farming. These studies represent 354 geographic sites (i.e., locations with unique latitudes and longitudes) in 42 countries around the world. The SoilHealthDB includes 42 soil health indicators and 46 background indicators that describe factors such as climate, elevation, and soil type. A primary goal of this effort is to enable the research community to perform comprehensive analyses, e.g., meta-analyses, of soil health changes related to cropland conservation management. The database also provides a common framework for sharing soil health, and the scientific research community is encouraged to contribute their own measurements.