Soil organic matter (SOM) is a well known indicator of soil quality due to its direct influence on soil properties such as structure, soil stability, water availability, cation exchange capacity, nutrient cycling, and pH buffering and amelioration. Study sites were selected in the Valley region of Virginia with the study objectives to: i) compare the efficiency of density solutions used in recovering free-light fraction (FLF) organic matter; ii) compare different soil organic fractions as sensitive indices of short-term changes in SOM due to management practices; iii) investigate on-farm effects of tillage management on soil organic carbon (SOC) and soil organic nitrogen (SON) stocks; and iv) evaluate the role of SOM in controlling soil available nitrogen (N) for corn uptake. The efficiency of the density solutions sodium iodide (NaI) and sodium polytungstate (SPT) in recovering FLF was the same at densities of 1.6 and 1.8 g cm⁻³, with both chemicals presenting less variability at 1.8 g cm⁻³. The sensitivity of SOM fractions in response to crop and soil management depended on the variable tested with particulate organic matter (POM) being the most sensitive when only tillage was tested, and FLF being the most sensitive when crop rotation and cover crop management were added. The on-farm investigation of tillage management on stocks of SOC and total soil N (TSN) indicated significant increases at 0–15 cm depth by increasing the duration (0 to 10 years) of no-tillage (NT) management (0.59 ± 0.14 Mg C ha⁻¹ yr⁻¹ and 0.05 ± 0.02 Mg N ha⁻¹ yr⁻¹). However, duration of NT had no significant effect on SOC and TSN stocks at 0–60 cm depth. Soil available N as controlled by SOM was modeled using corn (<i>Zea mays</i> L.) plant uptake as response and several soil N fractions as explanatory variables. The final model developed for 0–30 cm depth had 6 regressors representing the different SOM pools (active, intermediate, and stable) and a 𝑅² value of 65%. In summary, this study provides information about on-farm management affects on SOM levels; measurement of such effects in the short-term; and estimation of soil available N as related to different soil organic fractions. / Ph. D.

Autotrophs and heterotrophs differ in their demand, acquisition and use of materials, but fundamentally nutrient demand is inherently linked to metabolism based on the stoichiometry of biochemical reactions. The differences between these two groups of organisms confound straightforward regression approaches to quantifying the relationship between nutrient demand and metabolism at an ecosystem level. We address how nutrient demand in headwater streams changes with shifts in organic matter supply and associated microbial activity by investigating these relationships in the predominantly heterotrophic conditions of a southern Appalachian stream. We measured litter input, organic matter standing crops, litter respiration rates and nitrate demand several times during the course of decomposition. There was a strong relationship between leaf standing crop and nitrate uptake efficiency across dates with maximal efficiency occurring when litter standing crops were highest. There was also an increase in nitrogen (N) uptake rate relative to respiration rates as breakdown progressed, which appears to be due to a shift in nutrient supply from the substrate to the water column associated with the depletion of labile, high quality organic matter in the substrate. It is our contention that streams establish a gradient of resource supply from particulate to dissolved sources that coincides with the movement of materials from terrestrial to marine systems. / Master of Science

This research investigated the long-term effects of transitioning from intensive tillage to no-till (NT) practices with cover crop (CC) incorporation on soil quality, agronomic performance, and economic returns in Virginia's Coastal Plain. Nine years after integrating NT practices and CCs, improvements in soil physical and chemical properties were observed, including a 22% to 65% increase in soil organic matter (SOM) in the top 5 cm, a 4% reduction in bulk density, and enhanced soil moisture retention in corn production. Timing of CC termination played a crucial role in optimizing biomass production and nutrient accumulation. Overall accumulation rates were 44.4 kg dry biomass ha-1 d-1, 1.22 kg N ha-1 d-1, 0.16 kg P ha-1 d-1, 1.36 kg K ha-1 d-1, and 0.08 kg S ha-1 d-1 of delayed termination between March 15 and April 30. Each additional day of cover crop growth contributed to a fertilizer value of $3.91 ha-1, highlighting the economic advantage of extending CC growth during this critical period. In 2023, CC effects on corn N fertilizer demand and yields were assessed by applying variable N rates of 0, 56, 112, and 168 kg N ha-1 at sidedressing. Greatest corn yields at each N rate were observed following hairy vetch and a vetch-dominant CC mix, which had low C:N ratios (≤12:1) and accumulated 134 to 186 kg N ha-1 in their aboveground biomass. Corn yields after these CCs were 8.5 to 9.3 Mg ha-1 at the zero N sidedressing rate, increasing to 10.8 to 11.3 Mg ha-1 at the 168 kg N ha-1 rate. However, increasing the N rate yielded minimal economic benefits for these treatments. Vetch treatments produced the highest net returns, with greater returns at lower N rates, as vetch generated an additional US$1,012 ha-1 at the zero N sidedressing rate compared to the no CC control. Conversely, cereal rye produced a negative net return across all N rates, with positive returns achievable only with state cost-share payments. The findings underscore the importance of adaptive N management strategies and policy adjustments to support environmentally and economically sustainable cover crop practices in corn production. / Doctor of Philosophy / This research examines long-term benefits of switching from intensive tillage to no-till (NT) farming with cover crops (CC) on soil health, crop performance, and economic returns in Virginia's Coastal Plain. After nine years of using NT and CCs, we saw significant improvements in soil quality: soil organic matter in the top 5 cm increased by 22% to 65%, bulk density decreased by 4%, and soil moisture retention improved in corn crops. The timing of cover crop termination was crucial for maximizing biomass and nutrient benefits. Delaying termination from March 15 to April 30 resulted in additional dry biomass and nutrients, translating into a fertilizer value of $3.91 per hectare for every day of extra growth. In 2023, we assessed how different nitrogen (N) rates affected corn yields and fertilizer needs. Best yields were achieved with hairy vetch and vetch-dominant cover crops, which had low carbon-to-nitrogen (C:N) ratios and accumulated significant N in their biomass. Although these cover crops improved yields, increasing N rates returned minimal economic gains. Vetch treatments provided the highest net returns, especially at lower N rates, generating an additional $1,012 per hectare compared to no cover crop. In contrast, cereal rye resulted in negative returns across all N rates, unless state cost-share payments were applied. These results highlight the need for flexible N management strategies and policy changes to support effective and profitable cover crop practices in corn farming.

Central Appalachian headwater streams in coalfield areas are prone to mining disturbances, and compensatory mitigation is required in cases of documented impacts. Stream construction on reclaimed mines is a common mitigation strategy. Streams constructed as compensatory mitigation are meant to restore structural and functional attributes of headwater streams and are often evaluated by measuring structural ecosystem characteristics. However, replacement of stream ecosystem functions is essential for mitigation of mining disturbances from an ecosystem perspective. This research compared selected structural and functional measures in eight constructed streams on mined areas to those of four forested reference streams across two years. Three organic matter functions were evaluated: riparian litterfall input, leaf breakdown, and periphyton accrual. Constructed streams were typically warmer than reference streams and also had elevated specific conductance, elevated oxidized nitrogen concentrations, depressed benthic macroinvertebrate richness, and lower levels of canopy cover. Functionally, litterfall input and total leaf breakdown means for constructed streams were approximately 25% and 60% of reference means, respectively. Leaf breakdown in constructed streams appeared to be inhibited as a result of reduced processing by benthic macroinvertebrates as well as inhibition of microbial and physicochemical pathways. Constructed streams with total breakdown rates most similar to reference-stream levels had the coldest stream temperatures. Areal periphyton biomass, benthic algal standing crop, and senescent autotrophic organic matter in constructed streams were roughly quadruple, double, and quintuple those of reference streams, respectively. Indicator ratios also suggested stream-type differences in periphyton structure. Mean algal accrual was greater in constructed streams than in reference streams during leaf-on seasons. My results suggest that light is likely the primary factor driving accrual rate differences during summer and fall, but that temperature may also be important during fall. Planting a diverse assemblage of native riparian trees and ensuring their successful development can inhibit benthic irradiance and thermal energy inputs while providing similar quantity and quality of OM to constructed streams, thereby fostering replacement of reference-like OM functions in some streams. / Ph. D.

Land application of biosolids has been demonstrated to improve nutrient availability (mainly N and P) and improve organic matter in soils, but the effects of biosolids on C sequestration and N cycling in the Mid-Atlantic region is not well understood. The objectives were: 1) to investigate soil C sequestration at sites with a long-term history of biosolids either in repeated application or single large application; 2) to characterize and compare soil C chemistry using advanced 13C nuclear magnetic resonance (NMR) and C (1s) near edge x-ray absorption fine structure (NEXAFS) spectroscopic techniques; and 3) to compare biosolids types and tillage practices on short-term N availability in the Coastal Plain soils. Biosolids led to C accumulation in the soil surface (< 15 cm) after long-time application in both Piedmont and Coastal Plain soils. The C saturation phenomenon occurred in Coastal Plain soils, thus additional soil C accumulation was not achieved by increasing C inputs from biosolids to the Coastal Plain. Soil organic C from profiles in the field sites was not different at depths below the plow layer (15-60 cm). The quantitative NMR analyses concluded that O-alkyl C was the dominant form in the particulate organic matter (POM), followed by aromatic C, alkyl C, COO/N-C=O, aromatic C-O, OCH3 / NCH and ketones and aldehydes. The aliphatic C and aromatic C were enriched but the O-alkyl C was decreased in the biosolids-amended soils. The changes indicated that the biosolids-derived soil C was more decomposed and, thus, more stable than the control. The NEXAFS spectra showed that O-alkyl C was the dominant form in the POM extracted from biosolids-amended soils, followed by aromatic C, alkyl C, carboxylic C and phenolic C groups. These results were similar to those from NMR analysis. The regression and correlation analyses of C functional groups in the POM between NEXAFS and NMR indicated that both techniques had good sensitivity for the characterization of C from biosolids-amended soils. To evaluate short-term biosolids N availability, a three-year field study to investigate the effects of lime-stabilized (LS) and anaerobically digested (AD) biosolids on N availability in a corn-soybean rotation under conventional tillage and no-tillage practices was set up in 2009-2011. Results showed that both LS and AD biosolids increased spring soil nitrate N, plant tissue N at silking, post-season corn stalk nitrate N, grain yield, and soil total N by the end of the growing season. The same factors used to calculate plant available N for incorporated biosolids can be used on biosolids applied to no-till systems in coarse-textured soils. All these results indicated that the application of biosolids affects the long-term quantification and qualification of soil organic C and also improve short-term N availability in the Mid-Atlantic region. / Ph. D.

Effective coagulation is critical to the production of safe, potable drinking water, but variations in the chemical composition of source water can present challenges in achieving targeted contaminant removal and predicting coagulation outcomes. A critical literature review describes factors affecting the hydrolysis reactions of metal salt coagulants and the resulting precipitates. Properties of two key contaminants, turbidity and natural organic matter (NOM), are explored in the context of removal during coagulation, and the influence of co-occurring ions is described. While it is apparent that NOM character determines the minimum achievable organic carbon residual, the effects of water quality—including pH, NOM character and concentration, and concentrations of synergistic and competitive ions—on overall coagulation efficacy and NOM removal may be underestimated. An experimental research plan was devised to investigate the influence of water quality in coagulation and provide data to support the development of a predictive coagulation model. NOM is capable of interfering with ferric iron hydrolysis and influencing the size, morphology, and identity of precipitates. Conversely, calcium is known to increase the size and aggregation of Fe3+ precipitates and increase surface potential, leading to more effective coagulation and widening the pH range of treatment. Experiments and modeling were conducted to investigate the significance of the Fe/NOM ratio and the presence of calcium in coagulation. At the high Fe/NOM ratio, sufficient or excess ferric hydroxide was available for NOM removal, and coagulation proceeded according to expectations based upon the literature. At the low Fe/NOM ratio, however, NOM inhibited Fe3+ hydrolysis, reduced zeta potential, and suppressed the formation of filterable Fe flocs, leading to interference with effective NOM removal. In these dose-limited systems, equilibrating NOM with 1 mM Ca2+ prior to dosing with ferric chloride coagulant increased the extent of Fe3+ hydrolysis, increased zeta potential, decreased the fraction of colloidal Fe, and improved NOM removal. In dose-limited systems without calcium, complexation of Fe species by NOM appears to be the mechanism by which coagulation is disrupted. In systems with calcium, data and modeling indicate that calcium complexation by NOM neutralizes some of the negative organic charge and minimizes Fe complexation, making Fe hydrolysis species available for growth and effective coagulation. Experiments were conducted to investigate the influence of aqueous silica and pH on the removal of natural organic matter (NOM) by coagulation with ferric chloride. Samples with preformed ferric hydroxide were also compared to samples coagulated in situ to assess the role of coprecipitation. The moderate (10 mg/L) and high (50 mg/L) SiO2 concentrations both demonstrated interference with NOM removal at pH 6.5-7.5. In turn, NOM at 2 mg/L as DOC interfered with silica sorption at the moderate silica level and in samples with preformed ferric hydroxide at the high silica level. The combination of NOM and high silica led to decreases in DOC sorption and unexpected increases in silica sorption in the coprecipitated samples. The fraction of colloidal Fe passing a 0.45-μm filter also increased in the coprecipitated samples with both NOM and high silica. It is hypothesized that the combination of NOM and high silica synergistically interfered with Fe precipitation and particle growth processes, with NOM having the greater effect at lower pH and shorter reaction times, and silica exerting greater influence at higher pH and longer reaction time. Direct competition for surface sites and electrostatic repulsion were also influential. An overall goal for this research was the development of a quantitative coagulation model. Previous attempts to model coagulation have been limited by the inherent complexities of simultaneously predicting ligand sorption, metal complexation, floc surface charge, and particle removal. A diffuse layer (DLM) surface complexation model was formulated to simultaneously predict sorption of NOM and other key species, including silica, calcium, and carbonate alkalinity. Predictions of surface potential were used to estimate zeta potential and resulting regimes of effective aggregation and turbidity removal. The model provided good predictive ability for data from bench-scale experiments with synthetic water and jar tests of nine U.S. source waters. Under most conditions, the model provides excellent capability for predicting NOM sorption, calcium sorption, and particle destabilization and adequate capability for predicting silica sorption. Model simulations of hypothetical scenarios and experimental results help to explain practical observations from the literature. The DLM can be optimized to site-specific conditions and expanded to include sorption of additional species, such as arsenic. / Ph. D.

Much of Haitian agriculture is characterized by subsistence farming systems on eroded and nutrient-poor soils. Implementation of Conservation Agriculture systems has proven effective at improving soil quality and crop yield in many areas of the world, including areas similar to those in Haiti. While most Haitian smallholder farmers are highly resource-limited and adoption of new technologies is limited, these farmers are known to adopt new crops and practices if benefits that outweigh risks are demonstrated. Cover crops that help provide soil cover and increase nutrient mineralization are one of the most potentially beneficial changes that could be made on most smallholder farms. However, before specific cover crop recommendations can be made, their potential benefits need to be quantified. One field experiment in the summer of 2013 assessed decomposition rates and nutrient mineralization from common cash crops and two potential cover crops either on the soil surface or buried at 15 cm. The relative difficulty and expense of conducting these types of field trials led to the development and assessment of a laboratory-based system that could be used to simulate plant residue decomposition and nutrient release under controlled conditions. Additional benefits of a laboratory-based study include the ability to test significantly more treatment combinations than would likely be possible under field conditions and to control nearly all other experimental variables, other than the desired treatment comparisons. / Master of Science

Streams and terrestrial ecosystems are linked through allochthonous organic matter inputs from streamside vegetation. This allochthonous material makes up the energy base for forested aquatic food webs. Therefore, removal of riparian vegetation associated with agricultural land use affects stream ecosystem structure and function. The objectives of this study were to measure and compare allochthonous input and leaf breakdown rates along a gradient of agricultural land use in southern Appalachian streams. Study streams were placed into the following land use categories: forest and light, moderate, and heavy agriculture. Several physical, chemical, and biological parameters also were measured including discharge, temperature, nutrient concentrations, macroinvertebrate abundance and density, periphyton biomass, and chlorophyll a concentration. In forested, light agricultural, and moderate agricultural streams, the quantity and quality of allochthonous input were not significantly different. However, the timing and composition of allochthonous materials were related to land use. Chlorophyll a and periphyton biomass did not vary among land use types. Leaf breakdown rates were significantly faster in light and moderate agricultural streams in comparison to forested and heavy agricultural streams. Slow breakdown rates in forested streams resulted from low nutrient concentration and cool stream temperature. The scarcity of shredding macroinvertebrates and sedimentation probably limited leaf breakdown in heavy agricultural streams. Though limited riparian vegetation along agricultural streams resulted in an energy supply equivalent to forested streams, agricultural land use may still have long term impacts on stream structure including nutrient concentrations, temperature, macroinvertebrate community, and sedimentation thus affecting stream ecosystem function. / Master of Science

Wetlands play a vital role in regional and global biogeochemistry by controlling the movement and cycling of nutrients and carbon. While individual wetlands may provide these ecosystem services, high density wetland landscapes, referred to as wetlandscapes, can have far reaching aggregate effects on elemental cycling and solute transport. Here we use forested Delmarva bays or wetlands as a study ecosystem to explore physical and biological controls on wetland chemistry within forested wetlandscapes. The Delmarva wetlandscape consists of thousands of geographically isolated wetlands on the Delmarva Peninsula, United States, which despite their proximity to each other have highly variable sizes, shapes, hydrology, vegetative cover, and biological communities. This physical and biological variation makes the Delmarva wetlandscape an ideal ecosystem to understand spatio-temporal heterogeneity and drivers of biogeochemistry. In this dissertation, I demonstrate that water chemistry within the Delmarva wetlandscape is heterogeneous both within and between surface water and groundwater systems (Chapter 2). Surface water chemistry was primarily influenced by temporal factors (season and month), followed by local hydrology. In contrast, groundwater chemistry was strongly influenced by water level below ground surface and interaction with organic soil layers. These results are important in understanding both internal wetlandscape water chemistry dynamics and export of solutes such as dissolved organic matter (DOM) to adjacent river ecosystems. Further, these results suggest that local biological and hydrological factors strongly affect surface water chemistry in wetlands. To explore these factors, I used an observational approach to determine the role of larval amphibians on wetland biogeochemistry (Chapter 3) and employed high-resolution chemistry sensors to study the effect of hydrological changes on surface water dissolved organic matter concentrations (Chapter 4). Animal waste can contribute substantially to nutrient cycling and ecosystem productivity, yet little is known of the biogeochemical impact of animal excretion in wetland habitats. A common and abundant amphibian in Delmarva wetlands are wood frog (Lithobates sylvaticus) tadpoles. I found that wood frog tadpole aggregations elevated nutrient recycling, microbial metabolism, and carbon cycling in Delmarva wetlands. These results provide evidence for the functional and biogeochemical role of tadpole aggregations in wetland habitats, with important implications for ecosystem processes, biodiversity conservation, and ecosystem management. To further explore the role of hydrology on DOM concentrations, I utilized high-resolution fluorescent dissolved organic matter sensors (fDOM) and applied river solute transport frameworks and metrics to wetland catchments. I found that there was heterogeneity in wetland response to changing hydrology and that seasonality and potentially bathymetry influences fDOM concentrations. Together, these studies inform our understanding of wetlandscape heterogeneity and DOM export, as well as biological and hydrological drivers of biogeochemistry. / Doctor of Philosophy / Wetlands control the movement of nutrients and carbon at local, regional, and global scales. There is a large body of knowledge demonstrating the importance of wetlands to the transport of dissolved water constituents, such as dissolved organic matter (DOM) and nutrients. However, there is little information on what controls surface water chemistry in these wetland landscapes and less is known about belowground water chemistry. In this study I examined the role of water level, wetland shape, and time (i.e., year, month of the year, and season) on surface and groundwater chemistry in wetlands. I found that water chemistry was different between surface and groundwater and that differences were primarily due to seasons or months in surface water wetlands, while water level and flooding of organic matter-rich soil layers controlled groundwater chemistry. These results indicate that there are differences in water chemistry between surface water and groundwater that are controlled by unique drivers. These results also suggested that biological processes such as animal presence may influence wetland chemistry. To understand the role of animals in wetland chemistry, I studied the effect of wood frog (Lithobates sylvaticus) tadpole waste on nutrient concentrations in wetlands and found large tadpole groups are significant recyclers of nitrogen and phosphorous, which were used by microbes as nutrients, leading to enhanced leaf litter break-down in wetlands. These findings imply that tadpoles have an important role in wetland ecosystems by creating locations of enhanced nutrient and carbon cycling and that conservation of amphibian species may also preserve ecosystem processes in wetlands. Additionally, my initial study suggested that hydrology influences DOM concentrations in wetlands. I used high-frequency chemistry sensors to detect fluorescent dissolved organic matter (fDOM) concentrations, which represents a fraction of DOM. I found that relationships and patterns in fDOM concentration were complex, and that season and wetland shape were important in wetland DOM dynamics. Overall, this dynamic behavior across seasons and between wetlands indicates that wetland response to water levels can drive differences in water chemistry between wetlands and is important in our understanding of wetland response to storm events. The information gained from these studies is important in understanding how large wetland landscapes function and control movement of nutrients and carbon. Further, my research has uncovered the role of animal species in controlling nutrient and carbon cycling in wetland environments as well as the complex response of fDOM to water level changes in individual wetlands.

Current Virginia nitrogen (N) fertilizer recommendations do not include site-specific estimates of N supply from cover crops (CCs) or soil organic matter (SOM). Recent research successfully predicted the contribution of N from SOM and CCs to corn (Zea mays L.) in Pennsylvania. The objective of this work was to validate the biophysical model developed in Pennsylvania under Virginia conditions and to evaluate the decomposition rates of different surface-applied CC residues and the relationship between their chemical composition and decomposition rate. For the first objective, 83 N response trials were conducted in different regions of Virginia across 9 years using a randomized complete block design with four replications. The model was able to explain 47% and 15% of variability in unfertilized corn yield (RMSE = 1.6 Mg ha-1) and economical optimum N rate (EONR) (RMSE = 30 kg N ha-1) respectively. Efforts to improve the model by adding economically unresponsive sites improved the model performance to explain 45% of the variability in EONR. For the second objective, a lab incubation was performed to compare carbon (C) and N mineralization from four different CCs {Cereal Rye (CR), Hairy vetch (HV), Crimson clover (Cc) and Rapeseed (R)} on a sandy loam soil. Destructive sampling was performed at 6 different sampling dates – 3, 7, 14, 28, 56 and 112 days. ANOVA test revealed that the effects of CC species, incubation days and their interaction had a significant effect on mass decomposed, plant biochemical composition and net N mineralization. Variation in mass loss was positively related to lignin content for all the CCs while it was moderately correlated to C:N ratio for CR and R and weakly to HV and Cc. Biomass loss and N release was highest in HV followed by Cc, R and CR. Net N mineralization was highest in HV followed by R, Cc and CR amended soils. / Master of Science / Current Virginia nitrogen (N) fertilizer recommendations do not include site-specific estimates of N supply from cover crops or soil organic matter, both of which can influence crop N need. Recent research successfully predicted the contribution of N from cover crops and soil to corn (Zea mays L.) in Pennsylvania. The objectives of this work were to validate the biophysical model developed in Pennsylvania under Virginia conditions and to evaluate the decomposition rates of different surface-applied cover crop residues and the relationship between their chemical composition and decomposition rate. The Pennsylvania-developed model was able to successfully estimate the economical optimum N rate for corn and predict the yield of unfertilized corn. Corn yield did not increase with increasing N rates in some fields. When these sites were omitted, the accuracy of the model improved. For the second objective, a lab incubation study was performed comparing C and N released from Cereal Rye (CR), Hairy vetch (HV), Crimson clover (Cc) and Rapeseed (R)} on a sandy loam soil. Destructive sampling was performed at 6 different sampling dates – 3, 7, 14, 28, 56 and 112 days. Variation in mass loss was positively related to lignin content for all the cover crops while it was moderately correlated to C:N ratio for CR and R and weakly to HV and Cc. Biomass loss and N release was highest in HV followed by Cc, R and CR.