This paper describes the general environmental conditions on the Tanana River floodplain in the vicinity of the Bonanza Creek Experimental Forest. It describes and discusses environmental ...measurements taken in three successional stages (stage III, an open willow stand; stage V, a closed balsam poplar - alder stand; and stage VIII, a mature white spruce stand) and compares these measurements with those taken in adjacent artificial clearings. The impact of vegetation-mediated changes on soil temperatures, ground surface evaporation, and precipitation regimes is substantial, but the magnitude of these changes declines with advancing succession. Removing vegetation in the three successional stages resulted in warming of the mineral soil and increased surface evaporation, even though there were only slight changes in air temperature. Vertical soil moisture movement occurs along a tension gradient from the water table for distances of up to 50 cm in the mineral soil and can be correlated with fluctuations in the river level. Both fluvial and biotic controls interact to influence the course of vegetation and environmental changes in the successional sequence on the floodplain.
Forest floors and mineral soils from early (open willow), middle (poplar-alder), and late (white spruce) floodplain primary successional stages were examined for nitrogen fixation and ...denitrification. The acetylene-reduction and acetylene-inhibition techniques were used separately and in combination to measure nitrogenase and denitrification activities, both in laboratory and field studies. In situ N2O production was undetectable at all sites and during all sampling periods. Denitrifying activity measured in the field with acetylene amendments was low to undetectable, except after a brief flood in the open willow stand when N2O production ranged from undetectable to 34 ng N.cm-2.h-1 within the newly deposited alluvium - old mineral soil interface. Intact core assays also had low to undetectable denitrification activities; the highest activities (259 ng N.g-1.h-1) were measured in the poplar-alder forest floor in the fall. Laboratory studies showed that potential denitrification enzyme activity (DEA) was also greatest in the poplar-alder forest floor (4332 ng N.g-1.h-1), once again occurring in the fall. In early and midsuccessional stages, the interactive effects of temperature, carbon, and NO3(-1) limited denitrification, yet even with the addition of the limiting amendments, low to undetectable DEA was observed in mineral soils. The later white spruce successional stage also had low to undetectable DEA, increasing only with the addition of the full DEA media and independent of temperature changes. Nonsymbiotic nitrogenase activities were highly variable, ranging from undetectable to 30 ng N cm-2. h-1. Highest activities were seen in the open willow, newly deposited alluvium - old mineral soil interface immediately after a flood and approximately 1 month after the flood on the newly deposited silt surface. Only the white spruce forest floor had measurable nonsymbiotic nitrogenase activity at all sampling times. Alder root nodule nitrogenase activity showed no significant differences between sampling periods. The estimated annual nitrogen fixation rate of 164 kg N.ha-1 for alder root nodules is a substantial N contribution to the alder stand and to the floodplain ecosystem in general.
Cold climatic conditions govern the productivity of taiga forests, yet within a successional sequence the microclimatic and biogeochemical variations also have a major effect on soil microbial ...activities, thus affecting plant productivity through nutrient availability. Nitrogen mineralization and nitrification were measured in primary-successional floodplain forests of interior Alaska. Forest floor and mineral soils from an early (open willow), middle (poplar-alder), and late (white spruce) successional stage were used. The effects of temperature, moisture, and NH4(+) were tested in the laboratory for each of the successional stages. Potential nitrification was estimated using the chlorate-inhibition technique. Surface mineral soils and white spruce forest floor had low to undetectable rates of nitrogen mineralization and nitrification (< 1-3 micrograms N.g-1.d-1). The poplar-alder forest floor had the most pronounced seasonal patterns and the highest rates of net NH4(+) mineralization (< 1-7 micrograms N.g-1.d-1) and net nitrification (< 1-21 micrograms N.g-1.d-1). Temperature was limiting in early and midsuccessional stages, and both moisture and temperature were limiting in the later white spruce stage. Ammonium additions increased nitrification only in the poplar-alder forest floor, suggesting the NH4(+) is not limiting in the other successional stages. The chlorate inhibition assay indicated that a considerable portion of the nitrification in the poplar-alder forest floor may be due to heterotrophic activity.
Abstract
We present the occurrence rates for rocky planets in the habitable zones (HZs) of main-sequence dwarf stars based on the Kepler DR25 planet candidate catalog and Gaia-based stellar ...properties. We provide the first analysis in terms of star-dependent instellation flux, which allows us to track HZ planets. We define
η
⊕
as the HZ occurrence of planets with radii between 0.5 and 1.5
R
⊕
orbiting stars with effective temperatures between 4800 and 6300 K. We find that
η
⊕
for the conservative HZ is between
(errors reflect 68% credible intervals) and
planets per star, while the optimistic HZ occurrence is between
and
planets per star. These bounds reflect two extreme assumptions about the extrapolation of completeness beyond orbital periods where DR25 completeness data are available. The large uncertainties are due to the small number of detected small HZ planets. We find similar occurrence rates between using Poisson likelihood Bayesian analysis and using Approximate Bayesian Computation. Our results are corrected for catalog completeness and reliability. Both completeness and the planet occurrence rate are dependent on stellar effective temperature. We also present occurrence rates for various stellar populations and planet size ranges. We estimate with 95% confidence that, on average, the nearest HZ planet around G and K dwarfs is ∼6 pc away and there are ∼4 HZ rocky planets around G and K dwarfs within 10 pc of the Sun.
We present a low-resolution (R = l/ lambda -90), 5.5-38 km spectral sequence of a sample of M, L, and T dwarfs obtained with the Infrared Spectrograph (IRS) on board the Spitzer Space Telescope. The ...spectra exhibit prominent absorption bands of H sub(O) at 6.27 km, CH sub(4) at 7.65 km, and NH sub(3) at 10.5 km and are relatively featureless at l>15 km. Three spectral indices that measure the strengths of these bands are presented; H sub(O) absorption features are present throughout the MLT sequence, while the CH sub(4) and NH sub(3) bands first appear at roughly the L/T transition. Although the spectra are, in general, qualitatively well matched by synthetic spectra that include the formation of spatially homogeneous silicate and iron condensate clouds, the spectra of the mid-type L dwarfs show an unexpected flattening from roughly 9 to 11 km. We hypothesize that this may be a result of a population of small silicate grains that are not predicted in the cloud models. The spectrum of the peculiar T6 dwarf 2 MASS J0937+2931 is suppressed from 5.5 to 7.5 km relative to typical T6 dwarfs and may be a consequence of its mildly metal-poor/high surface gravity atmosphere. Finally, we compute bolometric luminosities of a subsample of the M, L, and T dwarfs by combining the IRS spectra with previously published 0.6-4.1 km spectra and find good agreement with the values of Golimowski et al., who use L' - and M' -band photometry to account for the flux emitted at l> 2.5 km.
Supernova (SN) 2018oh (ASASSN-18bt) is the first spectroscopically confirmed Type Ia supernova (SN Ia) observed in the Kepler field. The Kepler data revealed an excess emission in its early light ...curve, allowing us to place interesting constraints on its progenitor system. Here we present extensive optical, ultraviolet, and near-infrared photometry, as well as dense sampling of optical spectra, for this object. SN 2018oh is relatively normal in its photometric evolution, with a rise time of 18.3 0.3 days and Δm15(B) = 0.96 0.03 mag, but it seems to have bluer B − V colors. We construct the "UVOIR" bolometric light curve having a peak luminosity of 1.49 × 1043 erg s−1, from which we derive a nickel mass as 0.55 0.04 M by fitting radiation diffusion models powered by centrally located 56Ni. Note that the moment when nickel-powered luminosity starts to emerge is +3.85 days after the first light in the Kepler data, suggesting other origins of the early-time emission, e.g., mixing of 56Ni to outer layers of the ejecta or interaction between the ejecta and nearby circumstellar material or a nondegenerate companion star. The spectral evolution of SN 2018oh is similar to that of a normal SN Ia but is characterized by prominent and persistent carbon absorption features. The C ii features can be detected from the early phases to about 3 weeks after the maximum light, representing the latest detection of carbon ever recorded in an SN Ia. This indicates that a considerable amount of unburned carbon exists in the ejecta of SN 2018oh and may mix into deeper layers.
We present an exquisite 30 minute cadence Kepler (K2) light curve of the Type Ia supernova (SN Ia) 2018oh (ASASSN-18bt), starting weeks before explosion, covering the moment of explosion and the ...subsequent rise, and continuing past peak brightness. These data are supplemented by multi-color Panoramic Survey Telescope (Pan-STARRS1) and Rapid Response System 1 and Cerro Tololo Inter-American Observatory 4 m Dark Energy Camera (CTIO 4-m DECam) observations obtained within hours of explosion. The K2 light curve has an unusual two-component shape, where the flux rises with a steep linear gradient for the first few days, followed by a quadratic rise as seen for typical supernovae (SNe) Ia. This "flux excess" relative to canonical SN Ia behavior is confirmed in our i-band light curve, and furthermore, SN 2018oh is especially blue during the early epochs. The flux excess peaks 2.14 0.04 days after explosion, has a FWHM of 3.12 0.04 days, a blackbody temperature of K, a peak luminosity of , and a total integrated energy of . We compare SN 2018oh to several models that may provide additional heating at early times, including collision with a companion and a shallow concentration of radioactive nickel. While all of these models generally reproduce the early K2 light curve shape, we slightly favor a companion interaction, at a distance of ∼ based on our early color measurements, although the exact distance depends on the uncertain viewing angle. Additional confirmation of a companion interaction in future modeling and observations of SN 2018oh would provide strong support for a single-degenerate progenitor system.
Taiga ecosystems in interior Alaska van Cleve, K.; Dyrness, C. T.; Viereck, L. A. ...
Bioscience,
1983, 19830101, 1983-01-00, Letnik:
33, Številka:
1
Journal Article
Recenzirano
For several years the University of Alaska and the Institute of Northern Forestry (USDA Forest Service) have conducted a multidisciplinary study of interior-Alaska forest ecosystems, especially the ...black spruce type. Black spruce forests are wide-spread in interior Alaska and are the most fire-prone forest type. They are also the most nutrient-limited and least productive forest type, especially in the late stages of succession. Ecosystem differences in productivity and degree of nutrient limitation are controlled mainly by soil and forest-floor temperatures.
The excitation functions for quasielastic scattering of ^{22}Ne+^{248}Cm, ^{26}Mg+^{248}Cm, and ^{48}Ca+^{238}U are measured using a gas-filled recoil ion separator. The quasielastic barrier ...distributions are extracted for these systems and are compared with coupled-channel calculations. The results indicate that the barrier distribution is affected dominantly by deformation of the actinide target nuclei, but also by vibrational or rotational excitations of the projectile nuclei, as well as neutron transfer processes before capture. From a comparison between the experimental barrier distributions and the evaporation residue cross sections for Sg (Z=106), Hs (108), Cn (112), and Lv (116), it is suggested that the hot fusion reactions take advantage of a compact collision, where the projectile approaches along the short axis of a prolately deformed nucleus. A new method is proposed to estimate the optimum incident energy to synthesize unknown superheavy nuclei using the barrier distribution.
Polymer electrolyte membrane fuel cell (PEMFC) electrodes with a 0.07 mgPt cm–2 Pt/Vulcan electrocatalyst loading, containing only a sulfonated poly(ionic liquid) block copolymer (SPILBCP) ionomer, ...were fabricated and achieved a ca. 2× enhancement of kinetic performance through the suppression of Pt surface oxidation. However, SPILBCP electrodes lost over 70% of their electrochemical active area at 30% RH because of poor ionomer network connectivity. To combat these effects, electrodes made with a mix of Nafion/SPILBCP ionomers were developed. Mixed Nafion/SPILBCP electrodes resulted in a substantial improvement in MEA performance across the kinetic and mass transport-limited regions. Notably, this is the first time that specific activity values determined from an MEA were observed to be on par with prior half-cell results for Nafion-free Pt/Vulcan systems. These findings present a prospective strategy to improve the overall performance of MEAs fabricated with surface accessible electrocatalysts, providing a pathway to tailor the local electrocatalyst/ionomer interface.
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