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41.
The release of CO2 by soil microorganisms after the addition of nitrogen and glucose in excess and calibration additions of phosphorus has successfully
been used to assess microbial available P, assuming the native soil P pool is then limiting respiration. However, in P-fixing
soils and soils with high P content, carbon can be exhausted before the available soil P pool. It is not possible to simply
increase the amount of glucose as then the glucose concentration would be lethal for microorganisms. A modified method was
tested where soil is mixed with perlite. It was hypothesised that perlite, having a high water holding capacity, would dilute
the concentration of glucose, while maintaining the bioavailability of added nutrients, thus avoiding carbon limitation. Factorial
combinations of amount of soil and perlite (both adjusted to −25 kPa water potential) were tested to examine if perlite as
such had any effect on the respiration. Five tropical soil samples with a sharp gradient in P availability and one N-limited
compost material were used. The method successfully reduced the risk of carbon limitation. Microbial indices, such as basal
respiration, substrate-induced respiration and maximum P-limited respiration, were directly proportional to the amount of
soil in the experiments but unrelated to the amount of perlite, showing that perlite did not affect microbial measurements. 相似文献
42.
Sander Bruun Bo Stenberg Jon Gudmundsson Lars S. Jensen Jesper Luxhøi Anders Pedersen 《Soil biology & biochemistry》2005,37(12):2283-2296
Prediction of carbon (C) and nitrogen (N) mineralization patterns of plant litter is desirable for both agronomic and environmental reasons. Near infrared reflectance (NIR) spectroscopy has recently been introduced in decomposition studies to characterize biochemical composition. The purpose of the current study was to use empirical techniques to predict C and N mineralization patterns of a wide range of plant materials incubated under controlled temperature and moisture conditions. We hypothesized that the richness of information in the NIR spectra would considerably improve predictions compared to traditional stepwise chemical digestion (SCD) or C/N ratios. Initially, we fitted a number of empirical functions to the observed C and N mineralization patterns. The best functions fitted with R2=0.990 and 0.949 to C and N, respectively. The fractions of C and N mineralized at different points in time were then either predicted directly with regression functions or indirectly by prediction of the parameters of the empirical functions fitted to incubation data. In both cases, partial least squares (PLS) regressions were used and predictions were validated by cross-validations. We found that the NIR spectra (best R2=0.925) were able to predict C mineralization patterns marginally better than the SCD fractions (best R2=0.911), but considerably better than the C/N ratios (best R2=0.851). In contrast, N mineralization was better predicted by SCD fractions (best R2=0.533) than the C/N ratio (best R2=0.497), which was better than NIR predictions (best R2=0.446). Although the predictions with the NIR spectra were only slightly better for C and worse for N mineralization compared to SCD fractions, NIR spectroscopy still holds advantages, as it is a much less laborious and cheaper analytical method. Furthermore, exploration of the applications of NIR spectroscopy in decomposition studies has only just begun, and offers new ways to gain insights into the decomposition process. 相似文献
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45.
The glassy coating of lunar rock 12017 is enriched in 15 trace elements relative to the crystalline interior. It apparently consists chiefly of shock-melted rock, somewhat richer in rare earth elements and alkali metals than rock 12017 itself. The glass has been contaminated by about 0.5 percent carbonaceous-chondrite-like material or, alternatively, by a mixture of 0.06 to 0.3 percent fractionated meteoritic material and approximately 10 to 15 percent local soil. The glazing seems to represent molten material splashed from a nearby meteorite impact and not in situ melting by a sudden increase in solar luminosity. 相似文献
46.
The discovery of the first definitively correlated earliest Jurassic (200 million years before present) tetrapod assemblage (Fundy basin, Newark Supergroup, Nova Scotia) allows reevaluation of the duration of the Triassic-Jurassic tetrapod extinction event. Present are tritheledont and mammal-like reptiles, prosauropod, theropod, and ornithischian dinosaurs, protosuchian and sphenosuchian crocodylomorphs, sphenodontids, and hybodont, semionotid, and palaeonisciform fishes. All of the families are known from Late Triassic and Jurassic strata from elsewhere; however, pollen and spore, radiometric, and geochemical correlation indicate an early Hettangian age for these assemblages. Because all "typical Triassic" forms are absent from these assemblages, most Triassic-Jurassic tetrapod extinctions occurred before this time and without the introduction of new families. As was previously suggested by studies of marine invertebrates, this pattern is consistent with a global extinction event at the Triassic-Jurassic boundary. The Manicouagan impact structure of Quebec provides dates broadly compatible with the Triassic-Jurassic boundary and, following the impact theory of mass extinctions, may be implicated in the cause. 相似文献
47.
If the unexplained fission xenon component in meteorites is due to a volatile superheavy element, this element must have had a heat of vaporization of 54 +/- 3 kilocalories per mole and a normal boiling point of 2500 degrees +/- 400 degrees K. The prime candidates are elements 111 and 115, followed by 113, 114, 112, and 116. Elements 105 to 110 are not volatile enough and can therefore be excluded. 相似文献
48.
Thorleifsson G Magnusson KP Sulem P Walters GB Gudbjartsson DF Stefansson H Jonsson T Jonasdottir A Jonasdottir A Stefansdottir G Masson G Hardarson GA Petursson H Arnarsson A Motallebipour M Wallerman O Wadelius C Gulcher JR Thorsteinsdottir U Kong A Jonasson F Stefansson K 《Science (New York, N.Y.)》2007,317(5843):1397-1400
Glaucoma is a leading cause of irreversible blindness. A genome-wide search yielded multiple single-nucleotide polymorphisms (SNPs) in the 15q24.1 region associated with glaucoma. Further investigation revealed that the association is confined to exfoliation glaucoma (XFG). Two nonsynonymous SNPs in exon 1 of the gene LOXL1 explain the association, and the data suggest that they confer risk of XFG mainly through exfoliation syndrome (XFS). About 25% of the general population is homozygous for the highest-risk haplotype, and their risk of suffering from XFG is more than 100 times that of individuals carrying only low-risk haplotypes. The population-attributable risk is more than 99%. The product of LOXL1 catalyzes the formation of elastin fibers found to be a major component of the lesions in XFG. 相似文献
49.
Four core and soil samples from Apollo 12 are enriched in a number of trace elements of meteoritic origin to virtually the same degree as Apollo 11 soil. An average meteoritic influx rate of about 4 x 10(-9) gram per square centimeter per year thus seems to be valid for the entire moon. A sample from a light gray, coarse-grained layer in the core resembles lunar basalts in composition, but is enriched by factors of 10(4) to 10(5) in bismuth and cadmium. 相似文献
50.
Willerslev E Hansen AJ Binladen J Brand TB Gilbert MT Shapiro B Bunce M Wiuf C Gilichinsky DA Cooper A 《Science (New York, N.Y.)》2003,300(5620):791-795
Genetic analyses of permafrost and temperate sediments reveal that plant and animal DNA may be preserved for long periods, even in the absence of obvious macrofossils. In Siberia, five permafrost cores ranging from 400,000 to 10,000 years old contained at least 19 different plant taxa, including the oldest authenticated ancient DNA sequences known, and megafaunal sequences including mammoth, bison, and horse. The genetic data record a number of dramatic changes in the taxonomic diversity and composition of Beringian vegetation and fauna. Temperate cave sediments in New Zealand also yielded DNA sequences of extinct biota, including two species of ratite moa, and 29 plant taxa characteristic of the prehuman environment. Therefore, many sedimentary deposits may contain unique, and widespread, genetic records of paleoenvironments. 相似文献