691 research outputs found
Spring Clipping, Fire, and Simulated Increased Atmospheric Nitrogen Deposition Effects on Tallgrass Prairie Vegetation
Defoliation aimed at introduced cool-season grasses, which uses similar resources of native grasses, could substantially reduce their competitiveness and improve the quality of the northern tallgrass prairie. The objective was to evaluate the use of early season clipping and fire in conjunction with simulated increased levels of atmospheric nitrogen deposition on foliar canopy cover of tallgrass prairie vegetation. This study was conducted from 2009 to 2012 at two locations in eastern South Dakota. Small plots arranged in a split-plot treatment design were randomized in four complete blocks on a warm-season grass interseeded and a native prairie site in east-central South Dakota. The whole plot consisted of seven treatments: annual clip, biennial clip, triennial clip, annual fire, biennial fire, triennial fire, and undefoliated control. The clip plots consisted of weekly clipping in May to simulate heavy grazing. Fire was applied in late April or early May. The subplot consisted of nitrogen applied at 0 or 15 kg N · ha−1 in early June. All treatments were initially applied in 2009. Biennial and triennial treatments were reapplied in 2011 and 2012, respectively. Canopy cover of species/major plant functional groups was estimated in late August/early September. Annual clipping was just as effective as annual fire in increasing native warm-season grass and decreasing introduced cool-season grass cover. Annual defoliation resulted in greater native warm-season grass cover, less introduced cool-season grass cover, and less native cool-season grass cover than biennial or triennial defoliation applications. Low levels of nitrogen did not affect native warm-season grass or introduced cool-season cover for any of the defoliation treatments, but it increased introduced cool-season grass cover in the undefoliated control at the native prairie site. This study supports the hypothesis that appropriately applied management results in consistent desired outcomes regardless of increased simulated atmospheric nitrogen depositions
Leaf-litter stoichiometry is affected by streamwater phosphorus concentrations and litter type
The stoichiometric ratios of organisms and their food resources can influence C and nutrient
dynamics in aquatic ecosystems. Several investigators have quantified linkages between nutrient enrichment and consumer stoichiometry for stream detritivores, but very few have systematically quantified the effect of P enrichment on leaf-litter stoichiometry. Here, we examine the potential stoichiometric changes of 2 species of leaf litter subjected to varying levels of P enrichment in laboratory
microcosms and mixed species across a natural P gradient of streams in the Ozark Highlands Region, Arkansas, USA. Leaf-litter %P content increased and C:P ratios decreased with increasing levels of P enrichment and with increasing lability of the leaf species. In the laboratory study, C:P of maple and oak
leaves in the control treatment was ,2500, whereas this ratio decreased to 500 and 1000 in the high-P treatments, respectively. Total P (TP) was inversely related to leaf-litter C:P along the natural P gradient of streams in the Ozarks. Our results add to the growing body of information on the potential bottom-up
effects of anthropogenic nutrient loading in streams and the influence of water-column nutrients and leaf quality on this response
Quantifying the UV-continuum slopes of galaxies to z ˜ 10 using deep Hubble+Spitzer/IRAC observations
Measurements of the UV-continuum slopes β provide valuable information on the physical properties of galaxies forming in the early universe, probing the dust reddening, age, metal content, and even the escape fraction. While constraints on these slopes generally become more challenging at higher redshifts as the UV-continuum shifts out of the Hubble Space Telescope bands (particularly at z > 7), such a characterization actually becomes abruptly easier for galaxies in the redshift window z = 9.5-10.5 due to the Spitzer/Infrared Array Camera 3.6 μm-band probing the rest-UV continuum and the long wavelength baseline between this Spitzer band and the Hubble Hf160w band. Higher S/N constraints on β are possible at z ˜ 10 than at z = 8. Here, we take advantage of this opportunity and five recently discovered bright z = 9.5-10.5 galaxies to present the first measurements of the mean β for a multi-object sample of galaxy candidates at z ˜ 10. We find the measured βobs's of these candidates are -2.1 ± 0.3 ± 0.2 (random and systematic), only slightly bluer than the measured β's (βobs ≈ -1.7) at 3.5 < z < 7.5 for galaxies of similar luminosities. Small increases in the stellar ages, metallicities, and dust content of the galaxy population from z ˜ 10 to z ˜ 7 could easily explain the apparent evolution in β
Detailed dust modelling in the L-Galaxies semi-analytic model of galaxy formation
We implement a detailed dust model into the L-Galaxies semi-analytical model which includes: injection of dust by type II and type Ia supernovae (SNe) and AGB stars; grain growth in molecular clouds; and destruction due to supernova-induced shocks, star formation, and reheating. Our grain growth model follows the dust content in molecular clouds and the inter-cloud medium separately, and allows growth only on pre-existing dust grains. At early times, this can make a significant difference to the dust growth rate. Above z ∼ 8, type II SNe are the primary source of dust, whereas below z ∼ 8, grain growth in molecular clouds dominates, with the total dust content being dominated by the latter below z ∼ 6. However, the detailed history of galaxy formation is important for determining the dust content of any individual galaxy. We introduce a fit to the dust-to-metal (DTM) ratio as a function of metallicity and age, which can be used to deduce the DTM ratio of galaxies at any redshift. At z ≲ 3, we find a fairly flat mean relation between metallicity and the DTM, and a positive correlation between metallicity and the dust-to-gas (DTG) ratio, in good agreement with the shape and normalisation of the observed relations. We also match the normalisation of the observed stellar mass – dust mass relation over the redshift range of 0 − 4, and to the dust mass function at z = 0. Our results are important in interpreting observations on the dust content of galaxies across cosmic time, particularly so at high redshift
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Highly Unstable Double-Diffusive Finger Convection in a Hele-Shaw Cell: Baseline Experimental Data for Evaluation of Numerical Models
An experimental investigation was conducted to study double-diffusive finger convection in a Hele-Shaw cell by layering a sucrose solution over a more-dense sodium chloride (NaCl) solution. The solutal Rayleigh numbers were on the order of 60,000, based upon the height of the cell (25 cm), and the buoyancy ratio was 1.2. A full-field light transmission technique was used to measure a dye tracer dissolved in the NaCl solution. They analyze the concentration fields to yield the temporal evolution of length scales associated with the vertical and horizontal finger structure as well as the mass flux. These measures show a rapid progression through two early stages to a mature stage and finally a rundown period where mass flux decays rapidly. The data are useful for the development and evaluation of numerical simulators designed to model diffusion and convection of multiple components in porous media. The results are useful for correct formulation at both the process scale (the scale of the experiment) and effective scale (where the lab-scale processes are averaged-up to produce averaged parameters). A fundamental understanding of the fine-scale dynamics of double-diffusive finger convection is necessary in order to successfully parameterize large-scale systems
Structural and Functional Characterization of Sulfonium Carbon-Oxygen Hydrogen Bonding in the Deoxyamino Sugar Methyltransferase TyIM1
The N-methyltransferase TylM1 from Streptomyces fradiae catalyzes the final step in the biosynthesis of the deoxyamino sugar mycaminose, a substituent of the antibiotic tylosin. The high-resolution crystal structure of TylM1 bound to the methyl donor S-adenosylmethionine (AdoMet) illustrates a network of carbon-oxygen (CH•••O) hydrogen bonds between the substrate’s sulfonium cation and residues within the active site. These interactions include hydrogen bonds between the methyl and methylene groups of the AdoMet sulfonium cation and the hydroxyl groups of Tyr14 and Ser120 in the enzyme. To examine the functions of these interactions, we generated Tyr14 to phenylalanine (Y14F) and Ser120 to alanine (S120A)mutations to selectively ablate the CH•••O hydrogen bonding to AdoMet. The TylM1 S120A mutant exhibited a modest decrease in the catalytic efficiency relative to wild type (WT) enzyme, whereas the Y14F mutation resulted in an approximately 30-fold decrease in catalytic efficiency. In contrast, site-specific substitution of Tyr14 by the noncanonical amino acid p-aminophenylalanine partially restored activity comparable to the WT enzyme. Correlatively, quantum mechanical calculations of the activation barrier energies of WT TylM1 and the Tyr14 mutants suggest that substitutions which abrogate hydrogen bonding with the AdoMet methyl group impair methyl transfer. Together, these results offer insights into roles of CH•••O hydrogen bonding in modulating the catalytic efficiency of TylM1
Detection of Native-State Nonadditivity in Double Mutant Cycles via Hydrogen Exchange
Proteins have evolved to exploit long-range structural and dynamic effects as a means of regulating function. Understanding communication between sites in proteins is therefore vital to our comprehension of such phenomena as allostery, catalysis, and ligand binding/ejection. Double mutant cycle analysis has long been used to determine the existence of communication between pairs of sites—proximal or distal—in proteins. Typically, non-additivity (or “thermodynamic coupling”) is measured from global transitions in concert with a single probe. Here, we have applied the atomic resolution of NMR in tandem with native-state hydrogen exchange (HX) to probe the structure/energy landscape for information transduction between a large number of distal sites in a protein. Considering the event of amide proton exchange as an energetically quantifiable structural perturbation, m n-dimensional cycles can be constructed from mutation of n-1 residues, where m is the number of residues for which HX data is available. Thus, efficient mapping of a large number of couplings is made possible. We have applied this technique to one additive and two non-additive double mutant cycles in a model system, eglin c. We find heterogeneity of HX-monitored couplings for each cycle, yet, averaging results in strong agreement with traditionally measured values. Furthermore, long-range couplings observed at locally exchanging residues indicate that the basis for communication can occur within the native state ensemble, a conclusion which is not apparent from traditional measurements. We propose that higher-order couplings can be obtained and show that such couplings provide a mechanistic basis for understanding lower-order couplings, via “spheres of perturbation”. The method is presented as an additional tool for identifying a large number of couplings with greater coverage of the protein of interest
A Combined Very Large Telescope and Gemini Study of the Atmosphere of the Directly Imaged Planet, Beta Pictoris b
We analyze new/archival VLT/NaCo and Gemini/NICI high-contrast imaging of the young, self-luminous planet Beta Pictoris b in seven near-to-mid IR photometric filters, using advanced image processing methods to achieve high signal-to-noise, high precision measurements. While Beta Pic b's near-IR colors mimic those of a standard, cloudy early-to-mid L dwarf, it is overluminous in the mid-infrared compared to the field L/T dwarf sequence. Few substellar/planet-mass objects-i.e., And b and 1RXJ 1609B-match Beta Pic b's JHKsL photometry and its 3.1 micron and 5 micron photometry are particularly difficult to reproduce. Atmosphere models adopting cloud prescriptions and large (approx. 60 micron)dust grains fail to reproduce the Beta Pic b spectrum. However, models incorporating thick clouds similar to those found forHR8799 bcde, but also with small (a fewmicrons) modal particle sizes, yield fits consistent with the data within the uncertainties. Assuming solar abundance models, thick clouds, and small dust particles (a = 4 micron), we derive atmosphere parameters of log(g) = 3.8 +/- 0.2 and Teff = 1575-1650 K, an inferred mass of 7+4 3 MJ, and a luminosity of log(L/L) approx. 3.80 +/- 0.02. The best-estimated planet radius, is approx. equal to 1.65 +/- 0.06 RJ, is near the upper end of allowable planet radii for hot-start models given the host star's age and likely reflects challenges constructing accurate atmospheric models. Alternatively, these radii are comfortably consistent with hot-start model predictions if Beta Pic b is younger than is approx. equal to 7 Myr, consistent with a late formation well after its host star's birth approx. 12+8 4 Myr ago
A scoping review of biopsychosocial risk factors and co-morbidities for common spinal disorders
OBJECTIVE: The purpose of this review was to identify risk factors, prognostic factors, and comorbidities associated with common spinal disorders.
METHODS: A scoping review of the literature of common spinal disorders was performed through September 2016. To identify search terms, we developed 3 terminology groups for case definitions: 1) spinal pain of unknown origin, 2) spinal syndromes, and 3) spinal pathology. We used a comprehensive strategy to search PubMed for meta-analyses and systematic reviews of case-control studies, cohort studies, and randomized controlled trials for risk and prognostic factors and cross-sectional studies describing associations and comorbidities.
RESULTS: Of 3,453 candidate papers, 145 met study criteria and were included in this review. Risk factors were reported for group 1: non-specific low back pain (smoking, overweight/obesity, negative recovery expectations), non-specific neck pain (high job demands, monotonous work); group 2: degenerative spinal disease (workers\u27 compensation claim, degenerative scoliosis), and group 3: spinal tuberculosis (age, imprisonment, previous history of tuberculosis), spinal cord injury (age, accidental injury), vertebral fracture from osteoporosis (type 1 diabetes, certain medications, smoking), and neural tube defects (folic acid deficit, anti-convulsant medications, chlorine, influenza, maternal obesity). A range of comorbidities was identified for spinal disorders.
CONCLUSION: Many associated factors for common spinal disorders identified in this study are modifiable. The most common spinal disorders are co-morbid with general health conditions, but there is a lack of clarity in the literature differentiating which conditions are merely comorbid versus ones that are risk factors. Modifiable risk factors present opportunities for policy, research, and public health prevention efforts on both the individual patient and community levels. Further research into prevention interventions for spinal disorders is needed to address this gap in the literature
Coagulation Calculations of Icy Planet Formation at 15--150 AU: A Correlation Between the Maximum Radius and the Slope of the Size Distribution for Transneptunian Objects
We investigate whether coagulation models of planet formation can explain the
observed size distributions of transneptunian objects (TNOs). Analyzing
published and new calculations, we demonstrate robust relations between the
size of the largest object and the slope of the size distribution for sizes 0.1
km and larger. These relations yield clear, testable predictions for TNOs and
other icy objects throughout the solar system. Applying our results to existing
observations, we show that a broad range of initial disk masses, planetesimal
sizes, and fragmentation parameters can explain the data. Adding dynamical
constraints on the initial semimajor axis of `hot' KBOs along with probable TNO
formation times of 10-700 Myr restricts the viable models to those with a
massive disk composed of relatively small (1-10 km) planetesimals.Comment: Text: 44 pages, Tables: 5, Figures: 17; Accepted for publication in
the Astronomical Journa
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