Agricultural activities have significant impacts on global biogeochemical cycles, particularly carbon and nitrogen. Conventional row-crop agriculture accelerates the decomposition of soil organic matter, contributing to atmospheric carbon and declining soil fertility. Planting perennial warm season grasses is a useful management alternative to row crop agriculture because these species have been shown to be effective at increasing soil carbon storage and retaining nitrogen. The objectives of this research were to examine how converting row crops to a native perennial warm season grass (Panicum virgatum L., common name switchgrass) influences the recovery of soil organic matter fractions and nitrogen retention within an agricultural watershed in the Shenandoah Valley of Virginia. Soil samples were analyzed for total carbon and nitrogen, three particulate organic matter fractions, root biomass, mineralizable carbon and nitrogen pools, and microbial biomass. Surprisingly, I observed significant declines in bulk soil organic matter and surface particulate organic matter pools following switchgrass establishment. There were no differences in mineralizable carbon and microbial biomass pools between row crop and switchgrass soils, but labile carbon pools and nitrogen immobilization increased as switchgrass stands matured. These results are potentially due to switchgrass litter inputs stimulating microbial communities and accelerating the decomposition of recalcitrant soil organic matter, leading to declines in soil organic carbon stocks. The results from this study will be used to understand the environmental and economic benefits of implementing switchgrass plantings in agricultural watershed as a means to mitigate agriculturally-induced effects on carbon storage and nitrogen retention in soils. / Master of Science

The priming effect (PE), characterized as the enhanced microbial processing of bio-recalcitrant organic matter with the addition of labile substrates, has been hypothesized to moderate carbon (C) cycling in aquatic systems. In this study, aquatic PEs were evaluated through bacterial respiration and dissolved organic C consumption in incubations of water collected from three locations along the York River estuary. Incubations from White’s Landing on the Pamunkey River, a tidal freshwater tributary of the York, and from Croaker Landing in the middle of the estuary, displayed positive PEs when amended with labile C. In contrast, amended incubations from Gloucester Point, near the mouth of the estuary, displayed negative PEs, or reduced relative C metabolism, based on our calculations, This study provides empirical evidence for the occurrence of aquatic PEs and serves to elucidate how they may enhance or retard the processing and mineralization of organic C during transport to the ocean.

The current study examined two means of reducing uncertainty in visual search: 1) visual relatedness of a prime to the target (a data-driven, bottom-up processing) and 2) expectancy (a top-down process based on the proportion of validly primed trials). The two processes were decoupled using a short and a long inter-stimulus interval (ISI) to examine their time course in visual search. Competing hypotheses were contrasted in order to determine whether relatedness is associated with iconic memory (Neely, 1977) or a longer lasting visual-structural implicit memory (Schacter and Cooper, 1995) and what role participant expectancy plays in visual search performance. Twelve participants engaged in a discrimination task and a visual search task. The obtained results suggest that visual relatedness is a bottom-up process, probably mediated by a short-term iconic store that affects search performance early, but whose effects rapidly decay. They also suggest that expectancy is a top-down process that requires time to build up before it can affect visual search performance, but whose effects are more long lasting than visual relatedness.

Identifying and locating specific objects amidst irrelevant, distracting items can be difficult when one is unsure of where, or even what, to look for. Priming the perceptual/cognitive system for specific features or objects is one way of helping observers to locate and identify target items (e.g., Grice&Gwynne, 1985; Laarni and Hakkinen, 1994). Past research has demonstrated that priming single features does indeed affect search performance (e.g., Hailston&Davis, 2006; Huang&Pashler, 2005). But, what happens when more than one feature is primed? Does priming two features result in better performance than priming only one? What about three features? How does feature priming compare to simply priming the entire object itself? The current research addressed these questions with a series of three visual search experiments. In the first experiment performance in simple feature search was compared against triple-conjunction search performance. Three prominent models of visual search were compared to see which best predicted actual performance. In the second and third experiments the effects of multiple feature priming on search accuracy were examined in a triple-conjunction search (Experiment 2) and a whole-object search (Experiment 3). Moreover, in Experiment 3 the effectiveness of whole-object primes were compared to multiple-features primes. Results show that none of the three models can accurately predict performance in all cases, suggesting some modification of each is necessary. Furthermore, valid primes resulted in performance benefits, and these benefits increased with the number of primed features. Finally, no performance costs of invalid priming were observed in the current experiments.

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Mineral-associated organic carbon (MAOC) is a vital component of soil health and ecosystem productivity, playing a key role in carbon (C) sequestration and nutrient cycling. This dissertation investigates how plant root traits, nitrogen (N) fertilization, and cover cropping influence soil organic carbon (SOC) and MAOC. In the first chapter, a greenhouse experiment with 30 herbaceous plant species found that non-N-fixing plants exhibited stronger positive correlations with MAOC compared to N-fixing species, which were more closely linked to SOC. Root biomass contributed to decreases in MAOC, while specific root length (SRL), and coarse root traits were found to significantly contribute to increases in MAOC, highlighting the importance of plant root architecture in stabilizing C. In the second chapter which focused on relationships among N-fertilization, root traits, and MAOC, moderate N fertilization (56 and 112 kg N/ha) was found to enhance total SOC (∆SOC) and MAOC (∆MAOC) accumulation in the system, while higher N inputs (168 kg/ha) reduced C gains. Notably, the use of stable isotopes allowed for the quantification of fresh C additions, with results that indicate plant-added MAOC (PA-MAOC) was influenced more by plant species and root traits, such as coarse root length and aboveground biomass, rather than N fertilization. In the third chapter, a long-term field study demonstrated the effectiveness of cover cropping in increasing both SOC and MAOC by 43–59% and 27–36%, respectively, compared to conventional and no-till systems without cover crops. Despite triennial additions of N fertilizer over nine years, no significant increases in SOC or MAOC was observed. Additionally, root biomass exhibited positive trends with MAOC. These findings suggest that cover cropping, combined with no till practices, plays a pivotal role in enhancing MAOC by minimizing soil disturbance and promoting root-driven C inputs. This research highlights the importance of integrating plant species selection, root morphological traits, N management, and conservation practices to optimize long-term C storage (i.e., MAOC) and support sustainable soil management. Future studies should continue to include MAOC and particulate organic carbon fractions as these functional C sub pools may respond differently than bulk SOC pool. Including further studies on the interactions between root morphology, environmental factors, and C/N dynamics is necessary to develop more resilient agroecosystems capable of mitigating C losses and improving long-term soil health. / Doctor of Philosophy / This dissertation explores soil organic carbon, an important component of soil organic matter, which is vital for soil health and fertility. Soil organic matter promotes plant growth and agricultural productivity, and is critical for mitigating climate change by acting as a carbon sink, absorbing carbon dioxide from the atmosphere, which is why maintaining and building soil carbon is important. Soil carbon that is bound to minerals such as clays, is referred to as mineral-associated carbon (MAOC), which holds the largest pool of carbon on land and is often believed to persist over longer time-scales. Cover crops, their plant roots, and nitrogen fertilizer may have different relationships with MAOC when compared to total soil carbon, and these responses are not well understood. To address some of these knowledge gaps, this dissertation measured MAOC, plant root traits such as root size and structure, among different types of plants commonly used in agriculture and land reclamation practices. Results from this work show that root traits play an important role in increasing MAOC during short-term plant growth, with different effects depending on whether the plants can fix nitrogen from the air. Nitrogen fertilization was found to strongly impact MAOC, with moderate levels increasing amounts of MAOC, but low or too much nitrogen caused losses. Additionally, plant traits like root and stem biomass had a stronger influence on fresh carbon inputs from plants to the soil versus nitrogen alone. Cover crops proved to be a highly effective strategy for improving MAOC storage over 9 years, while additional nitrogen fertilizer had little long-term effect. These findings highlight the importance of balancing plant selection, fertilization, and sustainable practices to maintain healthy, carbon-rich soils.