General introduction; Land use impact on the stability of soil microbial community composition and enzyme activities to heat shock; Shifts in microbial community composition and physiological profiles across a gradient of induced soil degradation (GRIND); Development and validation of a soil quality index based on the equilibrium between soil organic matter and biochemical properties in an undisturbed forest ecosystem. The objectives of this thesis were to evaluate the responses of soil microbial communities to physical and chemical disturbances, and associate these responses with soil functional stability and changes in soil quality. The first study consisted of application of heat shocks (HS) to soils with contrasting land use history to evaluate differences in the stability of soil enzymes (laccase, cellulase and fluorescein diacetate hydrolysis) and microbial community composition as determined by phospholipid fatty acid (PLFA) analysis. The conversion of land use from forest to agriculture resulted in a new microbial community that was less functionally stable. Loss of stability was indicated by the reduced of laccase and cellulase activities in the agricultural soil, which suggested a less diverse community of microorganisms capable of producing these enzymes. The second study examined changes in microbial community composition and diversity that occurred across a gradient of soil disturbance. Disturbances were simulated by tillage events applied at different intensities to a 12-year-old fallow area. These treatments caused degradation of several soil physico-chemical properties, and alterations in microbial structure based on PLFA and terminal restriction fragment length polymorphism (T-RFLP) analyses, and in metabolic potential based on community level physiological profiles (CLPPs). Multivariate ordination of soil properties revealed the formation of a linear gradient of soil degradation that was significantly correlated with CLPPs, but not with T-RFLP and PLFA profiles. Nevertheless, changes observed in microbial community structure were significantly associated with decreases in soil organic C and field hydraulic conductivity. The third study demonstrated that undisturbed forest soils from western Oregon express an equilibrium between soil organic matter and biochemical properties. A model fitted through multiple regression analysis showed that phosphatase activity and microbial biomass were able to explain 97% of the soil organic C in these soils. This equilibrium was disrupted when a soil from an old-growth site was submitted to chemical stresses (Cu addition or pH alteration) and physical disturbances (wet-dry or freeze-thaw cycles). The magnitude of this disruption was consistently expressed by the ratio between soil C predicted by the model (Cp), and soil C that was measured (Cm). This ratio is proposed as biochemically-based index of soil quality.

Changes in the type and amount of plant inputs can occur gradually, as with succession, or rapidly, as with harvesting or wildfire. With global change it is anticipated that both gradual and immediate scenarios will occur at increasing frequency. Changes in vegetation inputs alter the quality and quantity of soil organic matter inputs, thus influencing the composition of soil microbial communities and the nutrient cycles they mediate. Understanding the relationship of soil organic matter inputs on soil microbial communities and nutrient cycles will be beneficial in predicting responses to changes in vegetation inputs. During the last 100-150 years, the vegetation of the Rio Grande Plains of the United States has been shifting from grasslands/savannas to woodlands as the result of encroachment of N2-fixing trees and their associated plant communities. The structure and diversity of soil microbial communities were examined under woody species and remnant grasslands. In addition, relationships between soil microbial communities and soil physical and chemical characteristics were explored. Soil microbial communities differed in soils under N2-fixing trees and associated vegetation compared to remnant grasslands. Differences in both fungal and bacterial communities were anticipated with vegetation shifts; however, only fungal communities correlated with vegetation, whereas bacterial communities were influenced by spatial heterogeneity. Soil microbial N cycling was investigated in long-term (>10 years) organic matter manipulations in an old-growth forest, dominated by large Pseudotsuga menziesii (Mirb.) Franco (Douglas-fir). The objectives of this research were to: 1) determine if long-term organic matter manipulations in old-growth forests altered microbial N cycling, 2) determine the contribution of litter to N cycling, and 3) determine if litter quality (low C/N red alder and high C/N Douglas-fir) affected the contribution of litter-derived N to N transformations. Long-term organic matter manipulations were found to affect microbial C and N cycling, but to a lesser degree than anticipated. After 10 years of organic matter exclusions and additions, microbial communities in all treatments remained N limited, although N limitation was less severe in organic matter exclusion treatments. Adding leached litter to control and organic matter exclusion soils initially altered N processes but differences dissipated during a 151-day incubation. Litter quality had little impact on the N cycling and litter made modest contributions to N mineralization and nitrification. The exclusion of organic matter altered the functionality of the microbial community to access litter-derived N. Both the gradual establishment of woody clusters on grassland and abrupt manipulations of old-growth vegetation inputs elicited responses in microbial communities and N cycling. Although some responses were subtle, they nonetheless support the responsiveness and importance of microbial communities to soil processes. Understanding feedbacks among plant inputs, microbial communities and nutrient cycles will aid in predicting microbial, ecosystem, and global responses to vegetation changes.

Biodiversity-the genetic variety of life-is an exuberant product of the evolutionary past, a vast human-supportive resource (aesthetic, intellectual, and material) of the present, and a rich legacy to cherish and preserve for the future. Two urgent challenges, and opportunities, for 21st-century science are to gain deeper insights into the evolutionary processes that foster biotic diversity, and to translate that understanding into workable solutions for the regional and global crises that biodiversity currently faces. A grasp of evolutionary principles and processes is important in other societal arenas as well, such as education, medicine, sociology, and other applied fields including agriculture, pharmacology, and biotechnology. The ramifications of evolutionary thought also extend into learned realms traditionally reserved for philosophy and religion. The central goal of the In the Light of Evolution (ILE) series is to promote the evolutionary sciences through state-of-the-art colloquia-in the series of Arthur M. Sackler colloquia sponsored by the National Academy of Sciences-and their published proceedings. Each installment explores evolutionary perspectives on a particular biological topic that is scientifically intriguing but also has special relevance to contemporary societal issues or challenges. This tenth and final edition of the In the Light of Evolution series focuses on recent developments in phylogeographic research and their relevance to past accomplishments and future research directions.

The Arthur M. Sackler Colloquia of the National Academy of Sciences address scientific topics of broad and current interest, cutting across the boundaries of traditional disciplines. Each year, four or five such colloquia are scheduled, typically two days in length and international in scope. Colloquia are organized by a member of the Academy, often with the assistance of an organizing committee, and feature presentations by leading scientists in the field and discussions with a hundred or more researchers with an interest in the topic. Colloquia presentations are recorded and posted on the National Academy of Sciences Sackler colloquia website and published on CD-ROM. These Colloquia are made possible by a generous gift from Mrs. Jill Sackler, in memory of her husband, Arthur M. Sackler.

Determining how factors such as disturbance and nutrient availability affect species diversity in a community has been a major goal of community ecology. The purpose of this study was to look at how species diversity and composition of soil bacterial communities are affected by nutrient addition and disturbance. I characterized soil microbial communities at the long-term ecological research site at the West Research Campus (WRC) located in Pitt County, NC. Briefly, DNA extracted from soils was analyzed using amplicon sequencing of the 16S ribosomal RNA gene. The Illumina Platform was used to sequence the bacterial DNA from each sample, and the Mothur Pipeline was used to analyze the DNA sequences. I hypothesized that changes in nutrient availability and disturbance would impact soil microbial community composition and diversity through direct and indirect effects mediated by plant-soil interactions. My research complemented previous work carried out in the WRC determining the effects of nutrient addition and disturbance on plant communities. Analysis of 2013 plant data showed that mowing increased plant species richness, and fertilization decreased plant species richness significantly. The experimental treatments as well as the proximity of the blocks to a drainage ditch all had significant effects on plant community composition. Analysis of the microbial community data showed that both fertilization and mowing significantly increased mean species richness. Relative abundance microbial community composition varied significantly due to the proximity of the blocks to the ditch. Presence/absence microbial community analyses showed significant effects of the treatments, as well as ditch proximity on microbial composition differences. Also, unknown microbial communities showed significant variation of the communities due to the treatments. The results of the presence/absence analysis and the unknown microbial community analysis show the importance of rare taxa and unknown microbial communities to the differences in composition of our soil microbial communities. Analysis of the soil chemical and physical data showed very little variation due to the treatments. This study will contribute to our understanding of how both plant and soil bacterial community diversity are affected by anthropogenic nutrient addition and disturbances. Maintaining diversity is important for ecosystem stability and functioning.