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21.
T. C. Feijtel Y. Salingar C. A. Hordijk J. P. R. A. Sweerts N. Van Breemen TH. E. Cappenberg 《Water, air, and soil pollution》1989,44(3-4):215-234
Atmospheric S deposition, sulfate reduction and the accumulation of reduced S components were investigated in sediments of a Dutch moorland pool. Laboratory 35SO42? tracer experiments indicated that the variation in reduction rate with depth in the sediment could be described by an exponential fit, suggesting first order kinetics. Sulfate reduction rates calculated from a kinetic transport model were highest in June and July (>0.6 mmol m?2 day?1), and decreased towards the end of the summer (<0.2 mmol m?2 day?1). Winter reduction rates were low, but increased towards the beginning of the spring. Concentrations of total, organic and inorganic S decreased with depth, consistent with an increasing depositional flux of S over the last century. Increased atmospheric sulfate deposition rates and consumption in the upper sedimentary layers caused a continuous increase in subsurface S concentrations. Present day S-immobilization rates were estimated between 25 to 30 mmol S in?2 yr?1, in the order of the annual input of S from wet atmospheric deposition. Sulfate metabolism in anaerobic moorland pool sediments may be the most important mechanism of long-term S-accumulation. 相似文献
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The solid phases and the precipitation boundary characterizing the system H+-Al3+-oxalic acid-silicic acid-Na+ are discussed. Model experiments have been used to throw more light on two environmental problems: the formation of sparingly soluble aluminium silicates in oceans and alkaline lakes, which could be determining aluminium and silicate concentrations in pore waters of sediments, and the validity of inorganic and organic mechanisms of podzolization and their significance for soil science. pH and Tyndallometric measurements were performed at constant ionic strength of 0.6 M NaCl at 25°C. Three phases Al(OH)4, H4SiO4 (phase Via), Al2, (OH)6.H4SiO4 (phase VIb) and NaAl(OH)4.(H4SiO4), (phase VIII) determine the precipitation boundary. Phase NaAl(OH)4.H4SiO4 (phase VII precipitates at 0.4pH units above NaAl(OH)4.(H4SiO4)2. Using a set of previously determined binary and ternary complexes, and phases of the subsystems, the following formation constants were deduced: Phases VIa and VIb are described as end-members of the allophane series with Si: Al ratios of 1:1 and 1.2. Phase VIb was identified with protoimogolite allophane. These two phases are good model clays for podzolic soils and are extremely soluble at pH < 4. Sodium phases could be hydrous feldspathoids. These phases are possible in sediments of seawater or saline lakes. It is suggested that organic and inorganic mechanisms of podzolization operate sequentially and that neither of them alone can completely describe the process. 相似文献
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Cryptococcus species are environmental yeasts, with a worldwide distribution and remarkable environmental adaptation. Although many species do not cause disease, C. neoformans and C. gattii are causative agents of cryptococcosis, a life threatening infection and a significant public health problem worldwide. Infection especially affects immunocompromised animals and humans. In wildlife, cryptococcosis appears to be more prevalent in captive populations. The objective of this study was to assess whether apparently healthy quokkas (Setonix brachyurus) harbor Cryptococcus spp. Using cultural and molecular methods, we studied yeasts isolated from nasal swabs collected from 130 free-ranging quokkas on Rottnest Island (RI, n = 97) and the mainland (n = 33) of Western Australia. Unspeciated Cryptococcus spp. (from four quokkas), C. neoformans var. grubii (serotype A) (two quokkas) and C. magnus (one quokka) were isolated from the nasal lining of apparently healthy quokkas from RI. Cryptococcus neoformans var. grubii was isolated from animals captured in a human-populated area on RI. There was no significant effect of the presence of Cryptococcus on the results of haematology, blood chemistry, peripheral blood cell morphology or clinical examination. To the best of our knowledge, this is the first documented isolation of C. neoformans var. grubii (serotype A) and C. magnus in a free-ranging macropod in Western Australia. The public health implications of this finding should be further explored. 相似文献
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TH Jones LJ Thompson JH Lawton TM Bezemer RD Bardgett TM Blackburn KD Bruce PF Cannon GS Hall SE Hartley G Howson CG Jones C Kampichler E Kandeler DA Ritchie 《Science (New York, N.Y.)》1998,280(5362):441-443
In model terrestrial ecosystems maintained for three plant generations at elevated concentrations of atmospheric carbon dioxide, increases in photosynthetically fixed carbon were allocated below ground, raising concentrations of dissolved organic carbon in soil. These effects were then transmitted up the decomposer food chain. Soil microbial biomass was unaffected, but the composition of soil fungal species changed, with increases in rates of cellulose decomposition. There were also changes in the abundance and species composition of Collembola, fungal-feeding arthropods. These results have implications for long-term feedback processes in soil ecosystems that are subject to rising global atmospheric carbon dioxide concentrations. 相似文献
27.
T Fujisawa TH Oosterkamp van der Wiel WG BW Broer R Aguado S Tarucha LP Kouwenhoven 《Science (New York, N.Y.)》1998,282(5390):932-935
A double quantum dot device is a tunable two-level system for electronic energy states. A dc electron current was used to directly measure the rates for elastic and inelastic transitions between the two levels. For inelastic transitions, energy is exchanged with bosonic degrees of freedom in the environment. The inelastic transition rates are well described by the Einstein coefficients, relating absorption with stimulated and spontaneous emission. The most effectively coupled bosons in the specific environment of the semiconductor device used here were acoustic phonons. The experiments demonstrate the importance of vacuum fluctuations in the environment for quantum dot devices and potential design constraints for their use for preparing long-lived quantum states. 相似文献
28.
The source parameters of the 1994 Bolivian earthquake (magnitude Mw = 8.3) suggest that the maximum seismic efficiency eta was 0.036 and the minimum frictional stress was 550 bars. Thus, the source process was dissipative, which is consistent with the observed slow rupture speed, only 20% of the local S-wave velocity. The amount of nonradiated energy produced during the Bolivian rupture was comparable to, or larger than, the thermal energy of the 1980 Mount St. Helens eruption and was sufficient to have melted a layer as thick as 31 centimeters. Once rupture was initiated, melting could occur, which reduces friction and promotes fault slip. 相似文献
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LP Kouwenhoven TH Oosterkamp MWS Danoesastro M Eto DG Austing T Honda S Tarucha 《Science (New York, N.Y.)》1997,278(5344):1788-1792
Studies of the ground and excited states in semiconductor quantum dots containing 1 to 12 electrons showed that the quantum numbers of the states in the excitation spectra can be identified and compared with exact calculations. A magnetic field induces transitions between the ground and excited states. These transitions were analyzed in terms of crossings between single-particle states, singlet-triplet transitions, spin polarization, and Hund's rule. These impurity-free quantum dots allow "atomic physics" experiments to be performed in magnetic field regimes not accessible for atoms. 相似文献