Active Transport, Public Transportation, and Obesity in Metropolitan Areas of the United States.

There is a well established relationship between exercise and weight in individuals. Recently, relationships between less urban sprawl and more leisure exercise and between certain urban characteristics usually associated with less sprawl and exercise for transportation have been found. This paper completes the less-sprawl-more exercise for transportation-lower weight sequence by finding that counties in metropolitan areas where more people complete their journey to work by walking, biking, or taking public transportation have fewer people who are overweight.

Interlayer tunneling in counterflow experiments on the excitonic condensate in quantum Hall bilayers

The effect of tunneling on the transport properties of} quantum Hall double layers in the regime of the excitonic condensate at total filling factor one is studied in counterflow experiments. If the tunnel current $I$ is smaller than a critical $I_C$, tunneling is large and is effectively shorting the two layers. For $I > I_C$ tunneling becomes negligible. Surprisingly, the transition between the two tunneling regimes has only a minor impact on the features of the filling-factor one state as observed in magneto-transport, but at currents exceeding $I_C$ the resistance along the layers increases rapidly

SOIL MICROBIAL COMMUNITY CARBON AND NITROGEN DYNAMICS WITH ALTERED PRECIPITATION REGIMES AND SUBSTRATE AVAILABILITY

Understanding the nature and extent of the feedback between soil microorganisms and ecosystem processes is of great concern as we are faced with multiple elements of global environmental change. In this dissertation, I explore how anthropogenically induced environmental changes affect soil microorganisms' resource use, and how, in turn, changes in microbial resource use alters ecosystem processes. These explorations were conducted in grassland systems, which contain 12% of global soil carbon (C) stocks and can serve as large C sources or sinks depending on environmental conditions such as nitrogen (N) availability and precipitation regimes. Nitrogen availability in grasslands can control plant primary productivity as well as rates soil organic matter decomposition and the fate of soil organic C. In grassland systems undergoing N addition through fertilization, resource inputs to soils increase in both quality and quantity. In Chapter 1, I investigate the fate of enhanced biomass inputs due to N addition by determining the direct and indirect effects of N addition on the activity of the soil microbial decomposers. Through measurement of extracellular enzyme activities and isotopic analysis of the microbial biomass relative to substrate sources, I found enhanced mineralization of newly incorporated soil organic C with N addition. This increase in soil C break down was associated indirectly with N addition through increases in plant litter quality and not directly with increased soil N availability. These results suggest that increased biomass input resulting from N addition does not necessarily result in increased soil C accrual. Climate change in the Great Plains region will likely cause increases in drought severity and precipitation event size with little change in annual precipitation totals. Precipitation events, particularly those following periods of drought, can create large flushes of resources for microbial communities, but these same pulses also can cause high levels of physiological stress and disturbance. When faced with increased soil moisture stress and re-wetting disturbance, microorganisms must accumulate and release protective osmolytes. The acquisition and release of protective osmolytes, apparently of sufficient magnitude to influence ecosystem level N and C fluxes, makes understanding the mechanisms behind these fluxes critical for predicting not only microbial community responses to global change, but ecosystem responses as well. In Chapter 2, 3 and 4, I use soils from four locations across the Great Plains precipitation gradient in a combination of laboratory and in situ soil incubations to explore the effects of soil moisture stress on flows of C and N though the microbial biomass. In Chapter 2, I focus on links between soil moisture stress and resource use efficiency by manipulating the frequency and magnitude of soil wetting and drying cycles in laboratory soil incubations. As soil moisture stress was increased with longer drought intervals and larger water pulse events, I saw a decline in C use efficiency and a 360 - 4800% increase in net N mineralization in soils from four sites along the Great Plains precipitation gradient. In Chapter 3, I employed the use of stable isotopes at the end of a similar incubation, to trace the C and N during a soil wetting-drying cycle. In this study I found that increased levels of soil moisture stress shifted microbial preference from N-rich protective osmolytes to N-free osmolytes. I also found that soils from the mesic end of the precipitation gradient were more sensitive to changes in soil moisture stress than soils from the semi-arid end of the gradient and that nitrification appeared to be less sensitive than denitrification, leading to increased soil nitrate concentrations and a decoupling in the N cycle. Finally, in Chapter 4, I reciprocally transferred soils between four study sites along the precipitation gradient and allowed them to incubate in situ for 1.5 and 2.5 y. After collection I assessed nitrification and denitrification potentials and the abundance of functional genes associated with these processes. I compared effects of both the initial community composition and the change in environment on the process rates. I found that as soil moisture stress increased across the precipitation gradient, nitrification potential decreased and nitrification functional gene abundance increased. Depending on soil origin, denitrifiers were either sensitive, resistant or functionally redundant after 1.5 y of altered precipitation regimes. In contrast, after 2.5 y denitrifiers in soils of all origins exhibited declines in process rates and functional gene abundance with increased soil moisture stress. Overall, I found that microbial communities are sensitive to environmental change, and as these communities shift in structure and function C and N cycling in these grasslands is altered. In particular, the perturbations explored in this dissertation, N addition and climate change, may induce increased rates of C release and N loss from these grassland soils

Exciton condensate at a total filling factor of 1 in Corbino 2D electron bilayers

Magneto-transport and drag measurements on a quasi-Corbino 2D electron bilayer at the systems total filling factor 1 (v_tot=1) reveal a drag voltage that is equal in magnitude to the drive voltage as soon as the two layers begin to form the expected v_tot=1 exciton condensate. The identity of both voltages remains present even at elevated temperatures of 0.25 K. The conductance in the current carrying layer vanishes only in the limit of strong coupling between the two layers and at T->0 K which suggests the presence of an excitonic circular current

Exciton Condensation and Perfect Coulomb Drag

Coulomb drag is a process whereby the repulsive interactions between electrons in spatially separated conductors enable a current flowing in one of the conductors to induce a voltage drop in the other. If the second conductor is part of a closed circuit, a net current will flow in that circuit. The drag current is typically much smaller than the drive current owing to the heavy screening of the Coulomb interaction. There are, however, rare situations in which strong electronic correlations exist between the two conductors. For example, bilayer two-dimensional electron systems can support an exciton condensate consisting of electrons in one layer tightly bound to holes in the other. One thus expects "perfect" drag; a transport current of electrons driven through one layer is accompanied by an equal one of holes in the other. (The electrical currents are therefore opposite in sign.) Here we demonstrate just this effect, taking care to ensure that the electron-hole pairs dominate the transport and that tunneling of charge between the layers is negligible.Comment: 12 pages, 4 figure

Feshbach spectroscopy and analysis of the interaction potentials of ultracold sodium

We have studied magnetic Feshbach resonances in an ultracold sample of Na prepared in the absolute hyperfine ground state. We report on the observation of three s-, eight d-, and three g-wave Feshbach resonances, including a more precise determination of two known s-wave resonances, and one s-wave resonance at a magnetic field exceeding 200mT. Using a coupled-channels calculation we have improved the sodium ground-state potentials by taking into account these new experimental data, and derived values for the scattering lengths. In addition, a description of the molecular states leading to the Feshbach resonances in terms of the asymptotic-bound-state model is presented.Comment: 11 pages, 4 figure