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Small Zone (small + zone)

Distribution by Scientific Domains

Medical Sciences	50%

Selected Abstracts

Modelling approach to analyse the effects of nitrification inhibition on primary production

FUNCTIONAL ECOLOGY, Issue 1 2009
S. Boudsocq
Summary 1Wet tropical savannas have high grass productivity despite the fact that nitrogen is generally limiting for primary production and soil nutrient content is typically very low. Nitrogen recycling, and especially nitrification, is supposed to be a strong determinant of the balance between conservation and loss of nutrients at the ecosystem level. The important primary production observed in wet tropical savannas might be due to a tight nutrient cycling and the fact that some grass species inhibit soil nitrification. 2Using a general theoretical ecosystem model taking both nitrate and ammonium into account, we investigate analytically, using a four,compartment-differential-equation system the general conditions under which nitrification inhibition enhances primary production. We then estimate the quantitative impact of such a mechanism on the dynamics and budget of nitrogen in a well-documented ecosystem, the Lamto savanna (Ivory Coast). This ecosystem is dominated by the grass Hyparrhenia diplandra, which drastically reduces nitrification in the whole savanna except for a small zone. While this small zone supports a lower grass primary production, nitrification is higher, most likely due to the presence of another genotype of H. diplandra, which has no effect on nitrification processes. Ultimately, we test whether differences in nitrification fluxes can alone explain this variation in primary production. 3Model analysis shows that nitrification inhibition enhances primary production only if the recycling efficiency , that is, the fraction of nitrogen passing through a compartment that stays inside the ecosystem , of ammonium is higher than the recycling efficiency of nitrate. This condition probably manifests itself in most soils as ammonium is less mobile than nitrate and is not touched by denitrification. It also depends partially on the relative affinity of plants for ammonium or nitrate. The numerical predictions for this model in the Lamto savanna show that variations in nitrification inhibition capacity may explain observed differences in primary production. 4In conclusion we find that nitrification inhibition is a process which probably enhances ecosystem fertility in a sustainable way, particularly in situations of high nitrate leaching and denitrification fluxes. This mechanism could explain the ecological advantage exhibited by native African grasses over indigenous grasses in South-American pastures. [source]

Criterion for the Avoidance of Edge Cracking in Layered Systems

JOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 8 2004
M. Y. He
When fabricating multilayers with brittle constituents, a prevalent design strategy is to choose fabrication conditions and thermal expansion coefficients that impose in-plane compression on the brittle layers. In such designs, a small zone of out-of-plane tension is induced at the edges that can cause cracks to form and extend, especially along the midplane. The associated stresses and energy release rates have been analyzed, revealing a fail-safe criterion, attributed to the existence of a maximum possible energy release rate, Gmax. Equating this maximum to the toughness defines a fail-safe parameter expressing the influence of the layer thickness, the misfit stress, and the toughness. When fail-safe designs cannot be realized, thin interlayers can be interposed in a manner that diminishes Gmax, broadening accessibility. The roles of misfit stress and interlayer thickness in attaining this condition are derived. [source]

Micro-fabrication and monitoring of three-dimensional microstructures based on laser-induced thermoplastic formation

MICROSCOPY RESEARCH AND TECHNIQUE, Issue 10 2009
Leyan Wang
Abstract This article reports a novel laser-induced micro-fabrication method and its monitoring system for three-dimensional (3D) microstructures. The mechanism of the method is that a small zone of thermoplastic material melted by laser heating grows in liquid surrounding environment, solidifying into a convex microstructure, such as micro-dot or micro-pillar. A laser diode (808 nm) with maximum power output of 130 mW is used as power source, and a kind of paraffin mixed with stearic acid and paint serves as the thermoplastic material for 3D microstructure formation experiments. A light microscope system consisting of a charge-coupled device (CCD) and a computer is utilized to realize real-time observation of the micro-fabricating process. The distribution of local temperature rise on material surface created by laser irradiation is simulated. The effects of liquid environment on microstructure formation have been theoretically analyzed and experimentally studied. Experiments are further carried out to investigate the relationship between laser spot and fabricated microstructures. The results indicate that the widths of micro-dots or micro-pillars are mostly determined by the size of focal spot, and their heights increase with the enlargement of laser power density. With this method, a micro-dot array of Chinese characters meaning "China" has been successfully fabricated through computer programming. This method has the advantages of implementing direct, mask-less, real-time and inexpensive 3D microstructure fabrication. Therefore, it would be widely applied in the fields of micro/nano-technology for practical fabrication of different kinds of 3D microstructures. Microsc. Res. Tech., 2009. © 2009 Wiley-Liss, Inc. [source]

Structure of the lid margin in laboratory animals

ACTA OPHTHALMOLOGICA, Issue 2008
N KNOP
Purpose The eye lid margin is of great importance for the spreading and the limitation of the tears. The so called lid wiper which is a specialized zone at the inner lid border directly apposed to the corneal surface for spreading the preocular tear film has, at present, only been described for the human conjunctiva. We have investigated common laboratory animals (rat and rabbit) for the presence of such a zone. Methods Conjunctival whole-mount specimens and total bulbi were investigated by serial section histology in ten rats (DA and Lewis) and in ten rabbits (NZW and Chinchilla). Results The stratified squamous keratinised epidermis of the free lid margin showed a sharp transition with loss of the keratin layer at the level of the meibomian glands. Close to the inner lid border it was replaced by a small zone of an optically denser epithelium covered by para-keratinised cells. This area represented the mucocutaneous junction (MCJ) equivalent to the line of Marx in the human. The MCJ rapidly transformed into a thickened 8-12 cell layered stratified epithelium of that formed a cushion-like epithelial elevation, reclined sharply towards the inner lid border and hence formed a typically relatively sharp lip-like edge. The lid-wiper epithelium showed species-specific differences in morphology (cuboidal with goblet cells in the rabbit versus squamous without goblet cells in the rat) but it extended all along the lid margin in both species. Conclusion At the inner border of the upper and lower lid of rat and rabbit, several zones of different morphology occur similar to the human including a lid-wiper structure. Since lid wiper epitheliopathy was shown as a sensitive early indicator for human dry eye syndrome its investigation may be useful for future research in dry eye models of laboratory animals. [source]