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11.
Factors influencing ammonia volatilization from urea in soils of the shortgrass steppe 总被引：2，自引：0，他引：2
D. G. Milchunas W. J. Parton D. S. Bigelow D. S. Schimel 《Journal of Atmospheric Chemistry》1988,6(4):323-340
The effects of soil moisture, temperature, and humidity treatments on urea hydrolysis and NH3 volatilization were assessed in the laboratory. Field studies were conducted to determine seasonal NH3 losses from simulated urine patches applied to contrasting soils of a representative hillslope of the shortgrass steppe region in the North American Great Plains.Losses of NH3–N were most influenced by soil moisture. The effects of temperature and humidity on total, or temporal, losses of NH3 were dependent on soil moisture. Losses ranged from 18.5% under conditions of low-temperature/high-humidity/wet soil to 7.7% under conditions of high-temperature/low-humidity/dry soil. In contrast, urea hydrolysis was not affected by soil moisture.Losses of NH3–N from simulated urine applied to field plots ranged from 1.5% on footslope soils in summer to 14.1% on backslope (midslope) soils in summer, whereas losses were 8.1% on back-slope soils in winter. Factors such as soil texture, microbial activity, and plant productivity along a toposequence had larger effects than climatic variables on variation in the volatile losses of NH3–N from this grassland.This paper is a report on the work presented at the international symposium Influence of marine and terrestrial biosphere on the chemical composition of the atmosphere, held in Mainz, F.R.G., on 16–22 March 1986. 相似文献
12.
C. M. Finlayson J. A. Davis P. A. Gell R. T. Kingsford K. A. Parton 《Aquatic Sciences - Research Across Boundaries》2013,75(1):73-93
The condition of many wetlands across Australia has deteriorated due to increased water regulation and the expansion and intensification of agriculture and increased urban and industrial expansion. Despite this situation, a comprehensive overview of the distribution and condition of wetlands across Australia is not available. Regional analyses exist and several exemplary mapping and monitoring exercises have been maintained to complement the more general information sets. It is expected that global climate change will exacerbate the pressures on inland wetlands, while sea level rises will adversely affect coastal wetlands. It is also expected that the exacerbation of these pressures will increase the potential for near-irreversible changes in the ecological state of some wetlands. Concerted institutional responses to such pressures have in the past proven difficult to sustain, although there is some evidence that a more balanced approach to water use and agriculture is being developed with the provision of increasing funds to purchase water for environmental flows being one example. We identify examples from around Australia that illustrate the impacts on wetlands of long-term climate change from palaeoecological records (south-eastern Australia); water allocation (Murray-Darling Basin); dryland salinisation (south-western Australia); and coastal salinisation (northern Australia). These are provided to illustrate both the extent of change in wetlands and the complexity of differentiating the specific effects of climate change. An appraisal of the main policy responses by government to climate change is provided as a basis for further considering the opportunities for mitigation and adaptation to climate change. 相似文献
13.
In this article we attend to an emergent practice of visualising GIS data in physical geography using the graphics engine of a videogame, Crysis. We suggest these modes of image-making aid the possibility of imagining and disseminating complex geographical data differently by re-contextualising seemingly abstract mathematical information within a human horizon of embodied meaning. Furthermore we argue these ways of imagining are closely linked to the technology and phenomenology of screens which make the presentation of these images possible. We close by reflecting on the possibility that these technologies are shifting the grounds of vision and the geographical imagination of users. 相似文献
14.
Snow avalanches are a significant natural hazard in alpine regions and their flow dynamics have similarities to pyroclastic flows and other geological mass movements. However, the potential for artificial release and the temporary nature of their deposits makes them somewhat easier to study. This article explains recent developments in radar technology for imaging these flows. These new data mean that, for the first time, we are seeing the whole flow averaged over spatial scales that are dynamically relevant. This provides an opportunity to properly test existing models for the dynamics used in risk applications and to gain knowledge of the flow physics, which will guide the next generation of model formulations. 相似文献
15.
T.J. McCoy R.J. Walker J. Yang D. Rumble R.D. Ash J.R. Michael 《Geochimica et cosmochimica acta》2011,75(22):6821-6843
We report analyses of 14 group IVA iron meteorites, and the ungrouped but possibly related, Elephant Moraine (EET) 83230, for siderophile elements by laser ablation ICP-MS and isotope dilution. EET was also analyzed for oxygen isotopic composition and metallographic structure, and Fuzzy Creek, currently the IVA with the highest Ni concentration, was analyzed for metallographic structure. Highly siderophile elements (HSE) Re, Os and Ir concentrations vary by nearly three orders of magnitude over the entire range of IVA irons, while Ru, Pt and Pd vary by less than factors of five. Chondrite normalized abundances of HSE form nested patterns consistent with progressive crystal-liquid fractionation. Attempts to collectively model the HSE abundances resulting from fractional crystallization achieved best results for 3 wt.% S, compared to 0.5 or 9 wt.% S. Consistent with prior studies, concentrations of HSE and other refractory siderophile elements estimated for the bulk IVA core and its parent body are in generally chondritic proportions. Projected abundances of Pd and Au, relative to more refractory HSE, are slightly elevated and modestly differ from L/LL chondrites, which some have linked with group IVA, based on oxygen isotope similarities.Abundance trends for the moderately volatile and siderophile element Ga cannot be adequately modeled for any S concentration, the cause of which remains enigmatic. Further, concentrations of some moderately volatile and siderophile elements indicate marked, progressive depletions in the IVA system. However, if the IVA core began crystallization with ∼3 wt.% S, depletions of more volatile elements cannot be explained as a result of prior volatilization/condensation processes. The initial IVA core had an approximately chondritic Ni/Co ratio, but a fractionated Fe/Ni ratio of ∼10, indicates an Fe-depleted core. This composition is most easily accounted for by assuming that the surrounding silicate shell was enriched in iron, consistent with an oxidized parent body. The depletions in Ga may reflect decreased siderophilic behavior in a relatively oxidized body, and more favorable partitioning into the silicate portion of the parent body.Phosphate inclusions in EET show Δ17O values within the range measured for silicates in IVA iron meteorites. EET has a typical ataxitic microstructure with precipitates of kamacite within a matrix of plessite. Chemical and isotopic evidence for a genetic relation between EET and group IVA is strong, but the high Ni content and the newly determined, rapid cooling rate of this meteorite show that it should continue to be classified as ungrouped. Previously reported metallographic cooling rates for IVA iron meteorites have been interpreted to indicate an inwardly crystallizing, ∼150 km radius metallic body with little or no silicate mantle. Hence, the IVA group was likely formed as a mass of molten metal separated from a much larger parent body that was broken apart by a large impact. Given the apparent genetic relation with IVA, EET was most likely generated via crystal-liquid fractionation in another, smaller body spawned from the same initial liquid during the impact event that generated the IVA body. 相似文献
16.
S. D. Crossley N. G. Lunning R. G. Mayne T. J. McCoy S. Yang M. Humayun R. D. Ash J. M. Sunshine R. C. Greenwood I. A. Franchi 《Meteoritics & planetary science》2018,53(10):2122-2137
The incompatible trace element-enriched Stannern-trend eucrites have long been recognized as requiring a distinct petrogenesis from the Main Group-Nuevo Laredo (MGNL) eucrites. Barrat et al. ( 2007 ) proposed that Stannern-trend eucrites formed via assimilation of crustal partial melts by a MGNL-trend magma. Previous experimental studies of low-degree partial melting of eucrites did not produce sufficiently large melt pools for both major and trace element analyses. Low-degree partial melts produced near the solidus are potentially the best analog to the assimilated crustal melts. We partially melted the unbrecciated, unequilibrated MGNL-trend eucrite NWA 8562 in a 1 atm gas-mixing furnace, at IW-0.5, and at temperatures between 1050 and 1200 °C. We found that low-degree partial melts formed at 1050 °C are incompatible trace element enriched, although the experimental melts did not reach equilibrium at all temperatures. Using our experimental melt compositions and binary mixing modeling, the FeO/MgO trend of the resultant magmas coincides with the range of known Stannern-trend eucrites when a primary magma is contaminated by crustal partial melts. When experimental major element compositions for eucritic crustal partial melts are combined with trace element concentrations determined by previous modeling (Barrat et al. 2007 ), the Stannern-trend can be replicated with respect to both major, minor, and trace element concentrations. 相似文献
17.
N. Thompson Hobbs Kathleen A. Galvin Chris J. Stokes Jill M. Lackett Andrew J. Ash Randall B. Boone Robin S. Reid Philip K. Thornton 《Global Environmental Change》2008,18(4):776
Fragmentation of the ecosystems of the earth into spatially isolated units has emerged as a primary component of global change. Often, fragmentation results from actions that are intended to enhance human livelihoods and well-being; however, there are often costs to ecosystems and human economies that are not considered. We describe the three general categories of processes causing fragmentation of rangelands worldwide: dissection, decoupling, and compression. We show that access to heterogeneity of landscapes is an important attribute of grazing ecosystems worldwide, and that fragmentation of these systems, even when it proceeds in the absence of habitat loss, can limit options of people and animals, options that are particularly important in temporally heterogeneous environments. We discuss the consequences of fragmentation for people, livestock, wildlife, and landscapes and describe potential adaptations that can mitigate its harmful outcomes. We close by reviewing policy options that promote re-aggregation of landscapes and adaptation to fragmentation. 相似文献
18.
Initial 129I/127I values (I-Xe ages) have been obtained for individual mineralogically characterized chondrules and interchondrule matrix from the enstatite chondrites Qingzhen (EH3) and Kota Kota (EH3). In view of the absence of aqueous alteration and the low-peak metamorphic temperatures experienced by these meteorites, we suggest that the I-Xe ages for the chondrules record the event in which they were formed. These ages are within the range recorded for chondrules from ordinary chondrites, demonstrating that chondrules formed during the same time interval in the source regions of both ordinary chondrites and enstatite chondrites. The timing of this chondrule-forming episode or episodes brackets the I-Xe closure age of planetesimal bodies such as the Shallowater aubrite parent body. Although chondrule formation need not have occurred close to planetesimals, the existence of planetesimals at the same time as chondrule formation provides constraints on models of this process. Whichever mechanisms are proposed to form and transport chondrules, they must be compatible with models of the protosolar nebula which predict the formation of differentiated bodies on the same timescale at the same heliocentric distance. 相似文献
19.
T. D. C. Ash A. P. Reynolds P. Roche A. J. Norton M. D. Still L. Morales-Rueda 《Monthly notices of the Royal Astronomical Society》1999,307(2):357-364
We report new radial velocity observations of V779 Cen, the optical companion to the X-ray pulsar Cen X-3. Two sets of results at two epochs yield very different radial velocity amplitudes. We demonstrate there are problems with the first set, not least that they are incompatible with the observed duration of the X-ray eclipse for all inclination angles. The anomalously high radial velocities are probably a result of changes in the outflow behaviour of the companion star. Although there is no reason to doubt the results from the second epoch when viewed in isolation, given the anomalous radial velocities of the first epoch, they must be treated with caution. Using these data, the semi-amplitude of the resulting radial velocity curve is found to be 24.4±4.1 km s−1 . Given the accurately measured semi-amplitude of the orbit of the pulsar, 414.3±0.9 km s−1 , the mass ratio of the system is 0.059±0.010. The inclination of the system is found to be 702±27, assuming that the optical component fills its Roche lobe, and that the system is in synchronous rotation. Hence the mass of the neutron star is 1.21±0.21 M⊙ , and the mass of the optical companion is 20.5±0.7 M⊙ . This is a smaller uncertainty than previously reported values, and is consistent with the canonical neutron star mass of 1.4 M⊙ .
In addition, we use our spectra to determine the spectral class of V779 Cen to be O6-7II-III. 相似文献
In addition, we use our spectra to determine the spectral class of V779 Cen to be O6-7II-III. 相似文献
20.
We performed dense single mobile microtremor measurements at 1181 sites throughout the city of Port of Spain, Trinidad. The results yielded fundamental periods of the soil of 1.2–1.7 s in the coastal areas, 0.3–1.2 s in the downtown area, and 0.0–0.3 s in the surrounding areas of the city. We employed the variogram plot to retrieve the spatial variability of fundamental periods. The results suggest that the variability of the period yields about 0.010 s2 at short distances of 100 m and it can be modeled as a Gaussian distribution which is directionally dependent. Finally, we propose an elastic design response spectra for different zones in the city in accordance with the observed periods of vibration and compatible with the International Building Code (IBC) provisions. 相似文献