VTechWorks staff will be away for the Thanksgiving holiday beginning at noon on Wednesday, November 27, through Friday, November 29. We will resume normal operations on Monday, December 2. Thank you for your patience.

Browsing by Author "Strahm, Brian D."

Now showing 1 - 20 of 102
  • 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
    Belowground Carbon and Nitrogen Cycling in a Loblolly Pine Forest Managed for Bioenergy Production
    Minick, Kevan J. (Virginia Tech, 2014-10-21)
    Concern over rising atmospheric CO2 due to fossil fuel combustion has intensified research into carbon-neutral energy and fuel production. Therefore, bioenergy production has expanded during the last decade, increasing demand for forest-based bioenergy feedstocks. Millions of acres of privately and industrially owned pine plantations exist across the southeastern US, representing a vast area of land that could be utilized to produce bioenergy without significant land-use change or diversion of agricultural resources from food production. Furthermore, loblolly pine (Pinus taeda L.) plantations offer the unique opportunity to utilize space between rows of planted trees to grow an herbaceous bioenergy crop, such as switchgrass (Panicum virgatum L.). This novel forest management regime has the potential to provide positive environmental and economic services, but hinges in part on impacts to soil carbon (C) and nitrogen (N) cycling, availability of belowground resources, and potential negative impacts of competition between pine and switchgrass on plant productivity. Three specific objectives were addressed in this study: 1) compare different bioenergy management regimes in regards to temporal dynamics of N cycling and availability following forest establishment (see Chapter 2); 2) determine the impact of loblolly pine and switchgrass intercropping on microbial N cycling processes (see Chapter 3); and 3) evaluate chemical and physical mechanisms of soil organic matter (SOM) stabilization and test their sensitivity to pine-switchgrass intercropping (see Chapter 4). The study site was located in the Lower Coastal Plain physiographic province in Lenoir County, North Carolina, USA (35-12'59'' N; 077-26'13'' W). Soils were mapped as Pantego (fine‐loamy, siliceous, semiactive, thermic Umbric Paleaquults) or Rains (fine‐loamy, siliceous, semiactive, thermic Typic Paleaquults) soil series, both of which are very poorly drained. However, previous site management in the late 1960’s and early 1970’s included installation of ditches to lower the water table and reduce saturation at the soil surface. Additionally, bedding of soil in rows was used to raise root systems of planted loblolly pine seedlings above the water table, increase soil aeration, and reduce competition. Space between bedded rows of pine trees was referred to as the interbed. Results from Chapter 2 showed that switchgrass significantly reduced interbed soil NH4 + and NO3 - concentrations by 39% and 60%, respectively, over the course of the timeframe (30 months) of this study. Surprisingly, in beds of the pine-switchgrass treatment significant increases in NO3 - concentration were measured from July - December 2011. From Chapter 3, gross N mineralization rates ranged from 0.18 - 4.7 µg N g -1 soil d-1 , while gross nitrification rates ranged from 0.02 - 0.47 µg N g-1 soil d-1 . At the 0-5 cm depth in switchgrass interbeds, gross N mineralization was reduced from April to November potentially reflecting microbial C limitations due to reduced soil C concentrations. At the 0-5 cm depth in beds of the pine-switchgrass treatment, gross N mineralization rates were elevated by 1.29 µg N iii g -1 soil d-1 in November and 1.02 µg N g-1 soil d-1 in February on average corresponding to a 305% and 193% increase, respectively. From Chapter 4, total C content in beds and interbeds ranged from 15 to 88 Mg C ha-1 and was reduced by 27% in beds of the pine-switchgrass treatment. Average C concentration for aggregate fractions was significantly lower in beds of the pine-switchgrass treatment at 0-5, 15- 30, and 30-45 cm depths, amounting to ~23%, ~28%, and ~34% reduction, respectively. Values of δ 13C for the >2000 µm aggregate size fraction at the 0-5 cm depth were diluted, corresponding to estimates of 13 - 25% of the >2000 µm C pool comprised of new pine-derived C. For SOM fractionated by density, elevated C concentrations were found in the occluded light fractions in both beds and interbeds of the pine-switchgrass treatment. Enriched δ13C in occluded light fractions led to estimates of 2.5 - 12.5% of this C fraction comprised of new switchgrass-derived C. In the free light fraction, new pine-derived C accounted for 15% and 9% of C at the 5-15 and 15-30 cm depth, respectively. Three overarching conclusions were generated from my research: 1) switchgrass grown between loblolly pine trees effectively utilized excess soil NH4 + and NO3 - when N availability was high following harvesting of a mature plantation proceeded by establishment of a second rotation of loblolly pine (see Chapter 2); 2) gross N mineralization rates were reduced under switchgrass during the growing season when soil C availability was low, but were elevated under switchgrass and adjacent loblolly pines when switchgrass was dormant and C availability was likely higher (see Chapter 3); and 3) SOM stabilized by physical or chemical mechanisms responded differently to pine-switchgrass intercropping, with losses in aggregate-stabilized C and gains in occluded, mineral-stabilized C. Furthermore, losses of aggregate C was associated with a significant reduction in total soil C in beds of the pine-switchgrass treatment. Results from 13C mass balance suggested incorporation of switchgrass-derived C into occluded light fractions of beds and interbeds. Finally, incorporation of new pine-derived C into the >2000 µm aggregate size fraction and free light fraction indicate pine inputs of particulate organic matter into these SOM fractions in beds of the pine-switchgrass treatment (see Chapter 4). I hypothesize that loblolly pines have increased root growth in beds in response to competition with switchgrass for N in the interbed, thereby alleviating seasonal microbial C limitations and stimulating microbial N cycling processes and increasing plant-available N. Overall, this research suggests that soil C and N cycling in pine plantations is altered by intercropping of pine and switchgrass. Through a mechanistic understanding of how C and N are cycled in forests and the impact of various forest management regimes on soil C and N cycling, effective management strategies can be implemented to utilize forests for intensive biomass production while limiting loss of soil C and N, and in some cases even enhancing soil C and N retention. Future research initiatives should seek to unravel the complex belowground interactions between roots of different plant species and soil microbial communities competing for limiting resources. Understanding how these interactions drive soil C storage, N cycling and availability, and forest productivity will ultimately improve resource utilization in these managed ecosystems as well as our basic understanding of how natural and managed ecosystems function.
  • Thumbnail Image
    Biochar amendment as a tool for improving soil health and carbon sequestration in agro-ecosystems
    Drew, Sophia Eliza (Virginia Tech, 2022-09-14)
    Conventional farming practices and land-use conversions drive carbon out of soil and into the atmosphere, where it contributes to climate change. Biochar, a soil amendment produced by pyrolyzing organic feedstocks under low-oxygen conditions, is a promising tool to restore soil carbon and draw down atmospheric carbon dioxide. Biochar has received considerable attention from scientists, growers, and environmentalists in the last 20 years, but there is still a gap between academic research and practical recommendations on biochar production and application that are relevant to small-scale growers. Here I present the results from two complementary studies that demonstrate the utility of local-scale biochar systems and provide some recommendations for those looking to work with biochar. The first study sought to determine the impact of biochar amendments on soil carbon and nutrient retention on three working farms across a variety of soil types, cropping systems, and climates in the United States. The effect of biochar amendment depended on initial soil characteristics and the properties of the biochar applied. Biochar amendments increased soil carbon in all three sites and increased soil nitrogen at two of the three. In this study pyrolysis conditions appeared to be as important as local soils and climate influences on the efficacy of biochar treatments. The second study was a life cycle assessment using SimaPro software to quantify the carbon balance and global warming potential of biochar produced from three local feedstocks (softwood, hardwood, and hay) applied to pasture soils in Southwest Virginia. Feedstock type, pyrolysis gas yield, and transportation distance significantly contributed to variation in the carbon balance of each agro-ecosystem. Biochar made from softwood lumber scraps performed best, with the highest net carbon storage and lowest global warming potential, followed by biochar made from hardwood scraps. Hay biochar performed worst, with positive carbon emissions (i.e., more carbon released than stored over its life cycle) in most scenarios tested, mainly because of its low biochar yield and the carbon emissions associated with agronomic production and transportation. Together these studies demonstrate the potential of local biochar systems to improve both soil health and carbon sequestration, and reinforce how important it is to know the characteristics of the soil and the production history and properties of the biochar being applied in order to meet soil health and carbon sequestration goals.
  • Thumbnail Image
    Biosolids as a source of soil conditioning and fertility for turfgrass
    Badzmierowski, Mike J. (Virginia Tech, 2019-11-04)
    Wastewater treatment plants are shifting towards producing exceptional quality (EQ) biosolids to increase recycling rates to land, especially urban areas. Other methods of improving the environmental impact of wastewater treatment includes additions of iron (Fe) to reduce phosphorus (P) concentrations in outgoing treated water and precipitate the P into the biosolids. Proper management of biosolids to rehabilitate anthropogenically disturbed urban soils for improved plant growth and effects on the cycling of nutrients requires further study. Our objectives were: 1) to determine whether various EQ biosolids could be managed to improve degraded soil properties and turfgrass quality while minimizing risk of P loss in a field study; and 2) to use spectral reflectance data to compare relationships of vegetation indices to soil and turfgrass parameters. We found that after an initial lag-time of one year, biosolids amendments increased turfgrass clipping biomass and aesthetic quality greater than did synthetic fertilizer. Repeated topdressing applications of biosolids reduced soil bulk density and increased soil organic carbon (OC) and nitrogen (N) stocks. Biosolids applied at the agronomic N rate did not increase water-soluble P (15 and 18 mg P kg-1 of soil) compared to biosolids applied at the agronomic P rate (9.6 mg P kg-1 of soil) and synthetic fertilizer (13 mg P kg-1 of soil) after five years. We further demonstrated at this field site that collecting continuous data improves spectral reflectance vegetation indices relationships to turfgrass quality, clipping biomass, and tissue N accumulation. Soil volumetric water content was best correlated to the water band index (r = 0.60) and the green-to-red ratio index (r = 0.54) vegetation indices. Differences in soil and turfgrass measured parameters were best detected when there was drought-stressed versus irrigated turfgrass.
  • Thumbnail Image
    Burkholderia phytofirmans strain PsJN effects on drought resistance, physiological responses and growth of switchgrass
    Wang, Bingxue (Virginia Tech, 2015-02-09)
    To decrease dependency of fossil fuels and avoid direct competition with food crops, massive research efforts are investigating next-generation cellulose biofuel crops such as switchgrass (Panicum virgatum). A low-input, sustainable switchgrass production could be achieved by reducing traditional management practices though applying plant growth promoting rhizobacteria (PGPR), of which our understanding is still rather limited. To elucidate physiological mechanisms behind PGPR's beneficial effects, we inoculated switchgrass seedlings with Burkholderia phytofirmans strain PsJN. Two experiments were conducted to determine the initial and long-term responses of switchgrass to PsJN inoculation by tracking growth and leaf physiology. In a third experiments, we tested the effects of PsJN on growth and leaf-level physiology of switchgrass under a moderate pre-drought conditioning and a successive severe drought stress. PsJN inoculation increased biomass and promoted elongation of shoots within 17 days following inoculation. The enhanced root growth in PsJN inoculated plants lagged behind the shoot response, resulting in greater allocation to aboveground growth (p=0.0041). Lower specific root length (p=0.0158) and higher specific leaf weight (p=0.0029) were also observed in PsJN inoculated seedlings, indicating advanced development. Photosynthetic rates (Ps) were higher in PsJN inoculated seedlings after 17 days (54%, p=0.0016), which were related to higher stomatal conductance, greater water use efficiency, and lower non-stomatal limitation of Ps. These rapid changes in leaf physiology are at least partially responsible for switchgrass growth enhancement from PsJN treatment. The early growth enhancement in PsJN inoculated switchgrass linearly decreased with plant age. PsJN inoculation increased Ps of upper canopy leaves by 13.6% but reduced Ps of lower canopy leaves by 8.2%. Accelerated leaf senescence and early flowering were observed in PsJN-inoculated switchgrass, which might contribute to slightly lower aboveground biomass at final harvesting. Drought preconditioning increased Ps of PsJN-inoculated switchgrass during a later severe drought; whereas, control switchgrass only benefited from drought preconditioning when leaf water potential dropped below -1 MPa. This study verified early growth enhancement and accelerated development of switchgrass due to PsJN inoculation. Rapid improvement in leaf physiology is related to enhanced productivity. PsJN inoculation also improve drought tolerance of switchgrass.
  • Thumbnail Image
    Carbon-Mercury interactions in Spodosols Assessed through Density Fractionation, Radiocarbon Analysis, and Soil Survey information
    Nave, Lucas E.; Ornelas, A. Covarrubias; Drevnick, P. E.; Gallo, Adrian C.; Hatten, Jeff A.; Heckman, Katherine A.; Matosziuk, Lauren M.; SanClements, Michael D.; Strahm, Brian D.; Veverica, T. J.; Weiglein, Tyler L.; Swanston, Christopher W. (2019-02-21)
    Soils comprise the largest terrestrial pool of C and Hg on Earth, and these elements have critical feedbacks to problems ranging from atmospheric pollution and climate change to public health. Empirical evidence suggests these elements cycle closely in a wide range of soils, but mechanistic studies of their interactions within distinct soil organic matter (SOM) pools and between different soil types are needed. Here, we report findings of a novel approach to investigate C-Hg interactions, primarily in Spodosols, in which we: (i) examined density separated topsoil and illuvial horizons of four contrasting Spodosols, and used radiocarbon to investigate interactions between Hg and C cycling in distinct SOM pools; (ii) assessed broader patterns across Spodosols and other soil orders using USDA soil survey laboratory data. Consistent with other studies, C and Hg concentrations of individual soil horizons were positively related across the four contrasting Spodosols. Carbon and Hg were also positively related in the density fractions comprising individual soil horizons, but radiocarbon analysis revealed fundamental differences in Hg retention in modern, C-rich fractions vs. low-C fractions containing less modem radiocarbon. The lack of significant site-to-site variation in C and Hg across these sites (and Spodosols more broadly), contrasted against significant differences between horizons and fractions, su ests processes controlling C-Hg interactions are consistent across the taxonomic order. Furthermore, significant differences between other soil orders indicate that processes controlling soil formation-as represented by soil taxonomy-can explain differences in C-Hg interactions and their distribution across soils.
  • Thumbnail Image
    Climate Effects on Subsoil Carbon Loss Mediated by Soil Chemistry
    Possinger, Angela R.; Weiglein, Tyler L.; Bowman, Maggie M.; Gallo, Adrian C.; Hatten, Jeff A.; Heckman, Katherine A.; Matosziuk, Lauren M.; Nave, Lucas E.; SanClements, Michael D.; Swanston, Christopher W.; Strahm, Brian D. (American Chemical Society, 2021-12-07)
    Subsoils store at least 50% of soil organic carbon (SOC) globally, but climate change may accelerate subsoil SOC (SOCsub) decomposition and amplify SOC-climate feedbacks. The climate sensitivity of SOCsub decomposition varies across systems, but we lack the mechanistic links needed to predict system-specific SOCsub vulnerability as a function of measurable properties at larger scales. Here, we show that soil chemical properties exert significant control over SOCsub decomposition under elevated temperature and moisture in subsoils collected across terrestrial National Ecological Observatory Network sites. Compared to a suite of soil and site-level variables, a divalent base cation-to-reactive metal gradient, linked to dominant mechanisms of SOCsub mineral protection, was the best predictor of the climate sensitivity of SOC decomposition. The response was "U"-shaped, showing higher sensitivity to temperature and moisture when either extractable base cations or reactive metals were highest. However, SOCsub in base cation-dominated subsoils was more sensitive to moisture than temperature, with the opposite relationship demonstrated in reactive metal-dominated subsoils. These observations highlight the importance of system-specific mechanisms of mineral stabilization in the prediction of SOCsub vulnerability to climate drivers. Our observations also form the basis for a spatially explicit, scalable, and mechanistically grounded tool for improved prediction of SOCsub response to climate change.
  • Thumbnail Image
    A Continental-Scale Investigation of Factors Controlling the Vulnerability of Soil Organic Matter in Mineral Horizons to Decomposition
    Weiglein, Tyler Lorenz (Virginia Tech, 2019-07-30)
    Soil organic matter (SOM) is the largest terrestrial pool of organic carbon (C), and potential carbon-climate feedbacks involving SOM decomposition could exacerbate anthropogenic climate change. Despite the importance of SOM in the global C cycle, our understanding of the controls on SOM stabilization and decomposition is still developing, and as such, SOM dynamics are a source of major uncertainty in current Earth system models (ESMs), which reduces the effectiveness of these models in predicting the efficacy of climate change mitigation strategies. To improve our understanding of controls on SOM decomposition at scales relevant to such modeling efforts, A and upper B horizon soil samples from 22 National Ecological Observatory Network (NEON) sites spanning the conterminous U.S. were incubated for 52 weeks under conditions representing site-specific mean summer temperature and horizon-specific field capacity (-33 kPa) water potential. Cumulative CO2 respired was periodically measured and normalized by soil organic C content to obtain cumulative specific respiration (CSR). A two-pool decomposition model was fitted to the CSR data to calculate decomposition rates of fast- (kfast) and slow-cycling pools (kslow). Post-LASSO best subsets multiple linear regression was used to construct horizon-specific models of significant predictors for CSR, kfast, and kslow. Significant predictors for all three response variables consisted mostly of proximal factors related to clay-sized fraction mineralogy and SOM composition. Non-crystalline minerals and lower SOM lability negatively affected CSR for both A and B horizons. Significant predictors for decomposition rates varied by horizon and pool. B horizon decomposition rates were positively influenced by nitrogen (N) availability, while an index of pyrogenic C had a negative effect on kfast in both horizons. These results reinforce the recognized need to explicitly represent SOM stabilization via interactions with non-crystalline minerals in ESMs, and they also suggest that increased N inputs could enhance SOM decomposition in the subsoil, highlighting another mechanism beyond shifts in temperature and precipitation regimes that could alter SOM decomposition rates.
  • Thumbnail Image
    Destination of Isotopic Nitrogen Fertilizer Under Varying Herbicide Regimes in a Mid-Rotation Loblolly Pine (Pinus taeda L.) Plantation in the Piedmont of Virginia, USA
    Van-Spanje, Megan (Virginia Tech, 2023-05-24)
    Mid-rotation fertilization and vegetation control are some of the most common silvicultural treatments in loblolly pine (Pinus taeda L.) plantations in the southeastern United States. Competing vegetation is commonly thought to sequester fertilizer nitrogen (N) and reduce the potential growth response to a mid-rotation fertilization treatment. This experiment aims to identify what proportion of applied N fertilizer is retained in the crop tree pine foliage, and the degree to which understory vegetation is competing for this resource. Our mid-rotation loblolly pine plantation received an application of 15N fertilization (urea 365 kg/ha, at 46% N by weight, i.e. 168 kg/ha of N) and a portion of plots received an understory vegetation control (basal spray application of triclopyr; 13.6% active ingredient) treatment either before fertilization or not at all. One-year post-fertilization, 15N contents within pine foliage, leaf fall/leaf litter, forest floor, and soil were measured, as was competing vegetation presence. There was significant variation in applied nitrogen acquisition among the different ecosystem components measured, with 0-15 cm soils retaining a majority at 32-37% added 15N. Differences in fertilizer N acquisition in pine foliage between plots with and without understory vegetation control was marginally significant (p = 0.06) with pine foliage in plots without understory vegetation capturing greater 15N (4.3% greater). Red maple (Acer rubrum) and oak species (Quercus spp.) were the most common competitors but neither had a uniquely pronounced effect on pine nitrogen sequestration. My data indicate that increasing competition reduces fertilizer N foliar concentrations in crop pine trees but at a modest rate and equally across species groups. An unrefined threshold determining when fertilizer N capture in crop pine trees was affected was found at 3.1 m2/ha of competing vegetation basal area. This site will continue to be monitored over time to assess fertilizer N retention in loblolly pine each year after fertilization and evaluate the fertilizer N capture within competing vegetation.
  • Thumbnail Image
    Determination of fertility rating (FR) in the 3-PG model for loblolly pine (Pinus taeda L.) plantations in the southeastern United States
    Subedi, Santosh (Virginia Tech, 2015-05-22)
    Soil fertility is an important component of forest ecosystem, yet evaluating soil fertility remains one of the least understood aspects of forest science. Phytocentric and geocenctric approaches were used to assess soil fertility in loblolly pine plantations throughout their geographic range in the United States. The model to assess soil fertility using a phytocentric approach was constructed using the relationship between site index and aboveground productivity. Geocentric models used physical and chemical properties of the A-horizon. Soil geocentric models were constructed using two modeling approaches. In the first approach, ordinary least squares methods of multiple regression were used to derive soil fertility estimated from site index using soil physical and chemical properties from the A-horizon. Ordinary least squares methods were found unsuitable due to multicollinearity among the soil variables. In the second approach, a multivariate modeling approach, partial least squares regression, was used to mitigate multicollinearity effects. The best model to quantify soil fertility using soil physical and chemical properties included N, Ca, Mg, C, and sand percentage as the significant predictors. The 3-PG process-based model was evaluated for simulating the response of loblolly pine to changes in soil fertility. Fertility rating (FR) is a parameter in 3-PG that scales soil fertility in the range of 0 to 1. FR values estimated from phytocentric and geocentric approaches were tested against observed production. The 3-PG model prediction of aboveground productivity described 89% percent of the variation in observed aboveground productivity using FR derived from site index and 84% percent of the vari- ation in observed aboveground productivity using FR derived from physical and chemical properties of the A-horizon. A response function to model dynamics of FR (∆FR) due to one time midrotatoin fertilization of N and P was developed using the Weibull function. The magnitude of ∆FR varied with intensity of N and time since application of fertilizer. The hypothesis that repeated fertilization with N and P eliminate major nutrient deficiency in the southeastern US was tested and a relationship between baseline fertility rating and fertilizer response was developed. An inverse relationship was observed between fertilizer response and baseline FR.
  • Thumbnail Image
    Development of a lateral topographic weathering gradient in temperate forested podzols
    Bower, Jennifer A.; Ross, Donald S.; Bailey, Scott W.; Pennino, Amanda M.; Jercinovic, Michael J.; McGuire, Kevin J.; Strahm, Brian D.; Schreiber, Madeline E. (Elsevier, 2023-11)
    Mineral weathering is an important soil-forming process driven by the interplay of water, organisms, solution chemistry, and mineralogy. The influence of hillslope-scale patterns of water flux on mineral weathering in soils is still not well understood, particularly in humid postglacial soils, which commonly harbor abundant weatherable primary minerals. Previous work in these settings showed the importance of lateral hydrologic patterns to hillslope-scale pedogenesis. In this study, we hypothesized that there is a corresponding relationship between hydrologically driven pedogenesis and chemical weathering in podzols in the White Mountains of New Hampshire, USA. We tested this hypothesis by quantifying the depletion of plagioclase in the fine fraction (≤2 mm) of closely spaced, similar-age podzols along a gradient in topography and depth to bedrock that controls lateral water flow. Along this gradient, laterally developed podzols formed through frequent, episodic flushing by upslope groundwater, and vertically developed podzols formed through characteristic vertical infiltration. We estimated the depletion of plagioclase-bound elements within the upper mineral horizons of podzols using mass transfer coefficients (τ) and quantified plagioclase losses directly through electron microscopy and microprobe analysis. Elemental depletion was significantly more pronounced in the upslope lateral eluvial (E horizon-dominant) podzols relative to lateral illuvial (B horizon-dominant) and vertical (containing both E and B horizons) podzols downslope, with median Na losses of ∼74 %, ∼56 %, and ∼40 %, respectively. When comparing genetic E horizons, Na and Al were significantly more depleted in laterally developed podzols relative to vertically developed podzols. Microprobe analysis revealed that ∼74 % of the plagioclase was weathered from the mineral pool of lateral eluvial podzols, compared to ∼39 % and ∼23 % for lateral illuvial podzols and vertically developed podzols, respectively. Despite this intense weathering, plagioclase remains the second most abundant mineral in soil thin sections. These findings confirm that the concept of soil development as occurring vertically does not accurately characterize soils in topographically complex regions. Our work improves the current understanding of pedogenesis by identifying distinct, short-scale gradients in mineral weathering shaped by local patterns of hydrology and topography.
  • Thumbnail Image
    Development of Ecosystem Structure and Function on Reforested Surface-Mined Lands
    Avera, Bethany Noel (Virginia Tech, 2015-01-30)
    Surface mining in the central Appalachian coalfield disturbs landscapes. Post-mining reforestation efforts now achieve successful reestablishment and growth; however, it is unclear whether reforestation efforts also restore the native forest ecosystem functions. We quantified rates of return of key ecosystem functions and structural attributes of the post-mining forested ecosystem. A chronosequence of four reforested mine sites and an unmined reference stand were studied in southwestern Virginia. Total soil nitrogen (N) and component (mineral soil, forest floor, root, and aboveground biomass) ecosystem carbon (C) pools were quantified. Throughout the growing season, soil gas fluxes [i.e., carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4)], soil inorganic-N [nitrate (NO3-) and ammonium (NH4+)], and total and active microbial biomass were measured. Soil organic C (SOC) and total ecosystem C are returning to the mined landscape. Ecosystem C was correlated with N (r= 0.80; p= 0.0003) and with total and active microbial biomass (r= 0.92; p=<.0001 and r= 0.86; p=<.0001). Available soil inorganic-N and CO2 and N2O fluxes showed no significant differences among study sites; however, the reforested mine soils showed a diminished capacity for CH4 uptake. Although some ecosystem components and functions rapidly returned to the mined landscape, others did not. Our results indicate that reforestation on surface mined lands is largely successful at restoring many ecosystem functions, yet certain functions are decoupled from the redeveloping ecosystem structure. Improved understanding of relationships between ecosystem functions and structural measures in this context can aid development of ecosystem restoration science and mine reclamation practice.
  • Thumbnail Image
    Dissolved Organic Matter Sources from Soil Horizons with Varying Hydrology and Distance from Wetland Edge
    Wardinski, Katherine Mary (Virginia Tech, 2021-09-03)
    Understanding hydrologic controls on carbon accumulation and export within geographically isolated wetlands (GIW) has implications for the success of wetland restoration efforts intended to produce carbon sinks. However, little is known about how hydrologic connectivity along the aquatic-terrestrial interface in GIW catchments influences carbon dynamics, particularly regarding dissolved organic matter (DOM) transport and transformation. The organic matter (carbon) that accumulates in wetland soils may be released into water, generating DOM. DOM is mobile and reactive, making it influential to aquatic metabolism and water quality. To understand the role of different soil horizons as potential sources of DOM, extractable soil organic matter (ESOM) was measured in soil horizons collected from upland to wetland transects at four Delmarva Bay GIWs on the Delmarva Peninsula in the eastern United States. ESOM quantity and quality were analyzed to provide insights to organic matter sources and chemical characteristics. Findings demonstrated that ESOM in shallow organic horizons had increased aromaticity, higher molecular weight, and plant-like signatures. ESOM from deeper, mineral horizons had lower aromaticity, lower molecular weights, and protein-like signatures. Organic soil horizons had the largest quantities of ESOM, and ESOM decreased with increasing soil depth. ESOM quantities also generally decreased from the upland to the wetland, suggesting that continuous soil saturation leads to a decreased quantity of ESOM. Despite wetland soils having lower ESOM, these horizons are thicker and continuously hydrologically connected to wetland surface water, leading to wetland soils representing the largest potential source of DOM to the Delmarva Bay wetland system. Knowledge of which soil horizons are most biogeochemically significant for DOM transport in Delmarva and other GIW systems will become increasingly important as climate change is expected to alter the hydrologic connectivity of wetland soils to the surface water-groundwater continuum and as wetlands are more frequently designed for carbon sequestration.
  • Thumbnail Image
    Divergent controls on carbon concentration and persistence between forests and grasslands of the conterminous US
    Heckman, Katherine A.; Nave, Lucas E.; Bowman, Maggie M.; Gallo, Adrian C.; Hatten, Jeff A.; Matosziuk, Lauren M.; Possinger, Angela R.; SanClements, Michael D.; Strahm, Brian D.; Weiglein, Tyler L.; Rasmussen, C.; Swanston, Christopher W. (2020-12)
    Variation in soil organic C (%OC) concentration has been associated with the concentration of reactive Fe- and Al-oxyhydroxide phases and exchangeable Ca, with the relative importance of these two stabilizing components shifting as soil pH moves from acid to alkaline. However, it is currently unknown if this pattern is similar or different with regard to measures of soil C persistence. We sampled soils from 3 horizons (uppermost A, uppermost B, C or lowest B horizons) across a pH gradient of 11 grass-dominated and 13 deciduous/mixed forest-dominated NEON sites to examine similarities and differences in the drivers of C concentration and persistence. Variation in C concentrations in all soils could be linked to abundances of Fe, Al and Ca, but were not significantly linked to variation in soil C persistence. Though pH was related to variation in Delta(OC)-O-14, higher persistence was associated with more alkaline pH values. In forested soils, depth explained 75% of the variation in Delta(OC)-O-14 (p < 0.0001), with no significant additional correlations with extractable metal phases. In grasslands, soil organic C persistence was not associated with exchangeable Ca concentrations, but instead was explained by depth and inorganic C concentrations (R-2 = 0.76, p < 0.0001), implying stabilization of organic C through association with carbonate precipitation. In grasslands, measures of substrate quality suggested greater persistence is also associated with a more advanced degree of decomposition. Results suggest that explanatory variables associated with C concentrations differ from those associated with persistence, and that reactive Fe- and Al-oxyhydroxide phases may not be present in high enough concentrations in most soils to offer any significant protective capacity. These results have significant implications for our understanding of how to model the soil C cycle and may suggest previously unrecognized stabilization mechanisms associated with carbonates and forms of extractable Si.
  • Thumbnail Image
    Drivers and Impacts of Smoldering Peat Fires in the Great Dismal Swamp
    Link, Nicholas Turner (Virginia Tech, 2022-05-26)
    Peatlands are a diverse type of wetland ecosystem, characterized by high levels of soil organic matter, that provide a wide array of ecosystem services including water storage and filtration, carbon sequestration, and unique habitats. Draining peatlands degrades their resilience to future disturbances, notably including high intensity, soil-consuming fires. Peat soil fires are unique in that they can smolder vertically through the soil column, with consequences ranging from large carbon emissions to altered hydrology and dramatic shifts in vegetation communities. In this work we had two complementary objectives to understand both the drivers and impacts of smoldering fires at the Great Dismal Swamp (VA and NC, USA). First, we developed and verified a new method to model peat burn depths with readily available water level and peat hydraulic property data. Our findings suggest that drainage weakens both short- and long-term controls on peat burn depths by reducing soil moisture and by decreasing peat water holding capacity. To address the impacts of smoldering fires, we quantified the abundance of the noxious Phragmites australis in a large fire scar and the extent to which altered hydrology influenced its occurrence. We did so by leveraging satellite imagery, random forest models, LiDAR data, and water table observations. Our results suggest that P. australis is aided by a hydrologic regime generated, in part, from the combined effects of drainage and deep smoldering fires. Our conclusions from these two studies contribute to the scientific understanding of smoldering peat fires and can inform management efforts.
  • Thumbnail Image
    Drought resistance and gum yield performances in a Senegalia senegal (L.) Britton progeny trial in Senegal
    Sarr, Mame Sokhna; Seiler, John R.; Sullivan, Jay; Diallo, Adja M.; Strahm, Brian D. (2021-01-18)
    With continued global change as a result of land use changes, invasive species and changing climatic patterns, it is becoming increasingly important to understand the adaptability of Senegalia senegal provenances to maximize resilience in managed and natural populations of this species. The objective of this study is to investigate Senegalia senegal genotypic differences in water use efficiency (WUE) measured by stable C-13 isotope composition in foliage according to their ploidy levels. Secondary objectives are to discuss inherent adaptive variation related to soil pH, survival, growth indexes, gum arabic yield and WUE within provenance in the climate change context. A Senegalia senegal progeny trial, in Dahra, Senegal was used in this study: 443 adult trees consisting of 60 families nested within 4 provenances were assessed in this study. Results showed significant differences in gum yield among provenances (P = 0.0002) and families (P < 0.0001). Diamenar and Ngane provenances showed overall similar annual gum yield despite a lower tree survival rate of Ngane than Diamenar. Growth traits, especially stem volume index and crown area index were larger on Ngane provenance, which also displayed significantly higher foliar WUE and lower leaf area index (LAI) than the other provenances. WUE was positively correlated with gum yield (P = 0.0302), but the coefficient of determination was only 2%. Foliar delta C-13 varied significantly (P < 0.0001) between diploids (- 27.91 parts per thousand) and polyploids (- 27.12 parts per thousand). However, within each provenance no significant difference was found. Only 15% of isotope compositions could be explained by ploidy level variation. Differences found in growth and gum yield may be attributed to genotype-specific variation. However, a significant correlation between soil pH and tree survival rate was found (P = 0.0051; r = 0.60). This study confirmed a possible improvement of the gum arabic sector through genotype based selection. Ngane and Diamenar seem to be more profitable to grow in Dahra than the other tested provenances. Future research should investigate the effect of soil pH, other soil physical and chemical properties, and management activities to improve site quality on tree survival and gum yields among provenances. Further, more research is needed to clarify inherent traits underlying drought tolerance in the field and gum yield performance.
  • Thumbnail Image
    Drying-Wetting Cycles: Effect on Deep Soil Carbon
    Qi, Ji; Markewitz, Daniel; Foroughi, Maryam; Jokela, Eric J.; Strahm, Brian D.; Vogel, Jason G. (MDPI, 2018-01-09)
    In the Southeast United States (U.S.), the climate is predicted to be warmer and have more severe drought in the summer. Decreasing rainfall in summer months should create more severe soil drying, which will eventually affect re-wetting cycles deeper in the soil profile. Changing drying-wetting cycles in this deeper portion of the profile may impact the soil C pool, the largest pool of terrestrial C globally. The aim of this research is to study the effect of drying-wetting cycles on deep soil C. A soil incubation experiment was established using four soils that are part of a simulated drought experiment in Oklahoma, Virginia, Georgia, and Florida. Soils were incubated from as many as eight layers up to a depth of 3.0 m. During incubations, soil respiration was generally greatest in surface soils and declined with depth. When compared to soils that were kept constantly moist, drying-wetting cycles did not consistently stimulate more soil respiration. Soil respiration as a proportion of total soil C, however, was higher in soils below 1 m than above. Total C (R2 = 0.82) and hydrolysable C (R2 = 0.77) were the best predictors for soil respiration. Assuming that there was no other factor (i.e., new carbon inputs) affecting soil respiration at depth other than soil moisture cycles, this study indicates that there would be no significant change to soil respiration in deep soils under more severe drying-wetting cycles.
  • Thumbnail Image
    Ecohydrology and self-organization of black ash wetlands
    Diamond, Jacob S. (Virginia Tech, 2019-04-19)
    Wetlands self-organize through reciprocal controls between vegetation and hydrology, but external disturbance may disrupt these feedbacks with consequent changes to ecosystem state. Imminent and widespread emerald ash borer (EAB) infestation throughout North America has raised concern over possible ecosystem state shifts in forested wetlands (i.e., to wetter, more herbaceous systems) and loss of forest function, calling for informed landscape-scale management strategies. In this dissertation, I use black ash wetlands as a model system to understand complex ecohydrological dynamics, and I use these dynamics to explain the self-organization of observed patterns in vegetation, hydrology, and microtopographic structure. The combined inferences from the three research chapters strongly implicate black ash trees as autogenic ecosystem engineers, who, through the process of improving their local growing conditions, cause a cascade of environmental changes that result in a unique ecosystem structure. This unique ecosystem structure is under existential threat from the invasive EAB. Through experiment, I show that loss of black ash trees to EAB induces persistent shifts in hydrology that result from reduced evapotranspiration and subsequent changes to water table regime (Chapter 2). These results suggest the potential for catastrophic shifts of black ash wetlands from forested to non-forested, marsh-like states under a do-nothing EAB management scenario. However, research presented here suggests that preemptive management of black ash wetlands can potentially mitigate loss of desirable forested conditions. Forest management to replace black ash with other wetland canopy species may be a slow and steady path towards forest maintenance, and harvesting may facilitate establishment of alternative species. In the case of preemptive harvesting of black ash, I posit that maintenance of microtopographic structure, either through leaving downed woody debris or through physical creation, is paramount to forest recovery. Microtopography in these ecosystems provides crucial relief from anaerobic stress generated by higher water tables, allowing woody species to persist on elevated microsites (e.g., 30 cm above base soil elevation). Moreover, I show that microtopography in black ash wetlands has clear structure and pattern and that its presence arises from self-organizing processes, driven by feedbacks among hydrology, biota, and soils (Chapter 3). I further show that this structured and non-random microtopography has profound influence on biogeochemical processes in black ash wetlands, controlling plant richness and biomass, and soil chemistry gradients (Chapter 4). Based on this work, I propose that structured wetland microtopography is a diagnostic feature of strongly coupled plant-water interactions, and these interactions may be important for ecosystem resilience to disturbance.
  • Thumbnail Image
    Ecotypic Variation in Johnsongrass in Its Invaded U.S. Range
    Lakoba, Vasiliy T. (Virginia Tech, 2021-05-28)
    Biological invasions have been observed throughout the world for centuries, often with major consequences to biodiversity and food security. Tying invasion to species identity and associated traits has led to numerous hypotheses on why, and where, some species are invasive. In recent decades, attention to intraspecific variation among invaders has produced questions about their adaptation to climate, land use, and environmental change. I examined the intraspecific variation of invasive Johnsongrass's (Sorghum halepense (L.) Pers.) seedling stress response, propagule cold tolerance, and large-scale niche dynamics for correlation with populations' climatic and ecotypic (i.e., agricultural vs. non-agricultural) origin. Overall, I found a greater number of home climate effects than ecotypic effects on various traits. Non-agricultural seed from cold climates and agricultural seed from warm climates germinated more and faster, while non-agricultural seedlings showed uniform chlorophyll production regardless of home soil carbon origin, unlike their agricultural counterparts. Neither seedling stress response nor propagule cold tolerance interacted with ecotype identity; however, drought stress varied with population origins' aridity and soil fertility, and seed from warm/humid and cold/dry climates was most germinable. Comparison of seed and rhizome cold tolerance also suggested that the latter is a conserved trait that may be limiting S. halepense poleward range expansion. This physiological limit, an unchanged cold temperature niche boundary between continents and ecotypes, and a narrowed niche following transition to non-agricultural lands all imply low likelihood of spread based on climatic niche shift. Instead, evidence points to range expansion driven primarily by climate change and highlights agriculture's role in facilitating invasibility. This tandem approach to climate and land use as drivers of intraspecific variation is transferable to other taxa and can help refine our conception of and response to invasion in the Anthropocene.
  • Thumbnail Image
    The Effects of Biochar and Reactive Iron Additions on Soil Carbon and Nitrogen Retention
    Conner, Jared P. (Virginia Tech, 2022-06-02)
    Soil organic matter (SOM) is a critical biogeochemical pool that can be managed as part of global efforts to conserve nutrients and enhance carbon (C) sequestration. But reliably increasing SOM has proven difficult because most of the organic matter that enters soil as plant litter and organic amendments (i.e., compost, manure) is susceptible to decomposition by soil microorganisms and eventually is lost to the environment as greenhouse gases and non-point source pollution. Many soils lack the physical and/or chemical properties that enable some human-modified soils (e.g., terra preta soils in the Amazon Basin) to stabilize and retain C and nutrients in SOM while maintaining relatively high levels of productivity compared to surrounding natural soils that formed under similar conditions. I hypothesized that two of the major stabilizers of organic matter common to terra preta soils of the Amazon basin – black carbon (biochar) and poorly crystalline, reactive iron (Fe) minerals – could be applied to a fine-textured soil from Southwest Virginia to improve the accumulation and retention of C and nitrogen (N). I used a field experiment to compare the effects of three types of locally-produced biochars applied with and without an organic N fertilizer (blood meal) on soil C and N availability. I then used an incubation experiment featuring the soils from the aforementioned field experiment to examine the effects of applying Fe2+ -treated manure effluent on the retention of C and N in unamended and hardwood biochar-amended soils. I found that biochar adsorbed inorganic N in all cases, while providing a reliable, stable increase in SOM due to its recalcitrant nature. However, the manure effluent used in the incubation experiment stimulated the decomposition of mineral-associated organic matter (MAOM), with the addition of Fe2+ to the manure mitigating this apparent positive priming effect and the presence of biochar actually reversing this effect and promoting an increase in MAOM following manure application to biochar-amended soil. Overall, biochar stimulated the retention of N by decreasing the leachable inorganic N in the soil and enhanced soil C stocks. Additionally, biochar applications had the added benefit of promoting the accumulation of manure in soil as stable, microbially-processed MAOM, while co-applying Fe2+ with manure only served to inhibit the priming of native soil C.