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Zero Group Velocity (ZGV) modes are peculiar guided waves that can exist in elastic plates or cylinders, and have proved to be very sensitive tools in characterizing materials or thickness variations with sub-percent accuracy at space resolutions of about the plate thickness. In this article we show theoretically and experimentally how such a mode can be generated as the sum-frequency interaction of two high amplitude primary waves, and then serve as a local probe of material non-linearity. The solutions to the phase matching condition, i.e. condition for a constructive non-linear effect, are obtained numerically in the mark of classical, quadratic non-linearity. The coupling coefficients that measure the transfer rate of energy as a function of time from primary to secondary modes are derived. Experiments are conducted on an aluminum plate using piezo-electric transducers and a laser interferometer, and explore the interaction for incident symmetric and anti-symmetric fundamental Lamb modes. In an experiment operated without voltage amplifier we demonstrate that the resonant nature of these ZGV modes can be leveraged to accumulate energy from long excitations and produce detectable effects at extraordinarily low input power even in such weakly non-linear material.

International audience; Question: How do functional types respond to contrasting levels of herbage use in temperate and fertile grasslands? Location: Central France (3 degrees 1' E, 45 degrees 43' N), 870 m a.s.l. Methods: Community structure and the traits of dominant plant species were evaluated after 12 years of contrasted grazing and mowing regimes in a grazing trial, comparing three levels of herbage use (high, medium and low). Results and Conclusions: Of 22 measured traits (including leaf traits, shoot morphology and composition, phenology), seven were significantly affected by the herbage use treatment. A decline in herbage use reduced individual leaf mass, specific leaf area and shoot digestibility, but increased leaf C and dry matter contents. Plants were taller, produced larger seeds and flowered later under low than high herbage use. Nine plant functional response types were identified by multivariate optimization analysis; they were based on four optimal traits: leaf dry matter content, individual leaf area, mature plant height and time of flowering. In the high-use plots, two short and early flowering types were co-dominant, one competitive. grazing-tolerant and moderately grazing-avoiding, and one grazing-avoiding but not -tolerant. Low-use plots were dominated by one type, neither hardly grazing-avoiding C 0 nor grazing-tolerant, but strongly competitive for light.

From Leibniz's Meditationes de cognitione, veritatis et ideis (1684), to Kant's Kritik der Urteilskraft (1790), the concept of symbol undergoes a radical and fundamental change : first viewed only as an arbitrary sign, as a conventional substitute for an idea, and as an instrument of intellectual knowledge, the symbol gradually becomes the intuitive presentation of the idea. This evolution allows the symbol to play a central part in post-Kantian aesthetics, and explains today's prevailing meaning as expressive or " iconic " sign. The article first deals with Leibniz's and Kant's definitions of the symbol. It then underlines some of the major stages of this semantic change which takes place over a century, and involves epistemology, semiotics and aesthetics. The article particularly highlights the contributions offered by Wolff, Baumgarten, Mendelssohn, Lessing and Herder, within this time period., Entre les Méditations sur la connaissance, la vérité et les idées de Leibniz (1684) et la Critique de la faculté de juger de Kant (1790) prend place une transformation essentielle et radicale de la notion de symbole : de simple signe arbitraire se substituant conventionnellement à une idée, et compris comme instrument de la connaissance intellectuelle, le symbole devient la présentation intuitive de l'idée. Par là est rendu possible son rôle central dans les philosophies de l'art postkantiennes, mais aussi son sens de signe " iconique " ou expressif aujourd'hui prédominant. Après avoir reconstitué les définitions leibnizienne et kantienne du symbole, l'article retrace plusieurs étapes majeures de ce changement de sens qui se produit, sur un siècle, à la croisée de la théorie de la connaissance, de la théorie des signes et de la réflexion sur l'art ; sont soulignées les contributions de Wolff et de Baumgarten, de Mendelssohn et de Lessing, ainsi que de Herder à cette évolution.

48 ref.; International audience; Nitrous oxide (N2O) emissions of three different soils – a rendzina on cryoturbed soil, a hydromorphic leached brown soil and a superficial soil on a calcareous plateau – were measured using the chamber method. Each site included four types of land management: bare soil, seeded unfertilized soil, a suboptimally fertilized rapeseed crop and an overfertilized rapeseed crop. Fluxes varied from –1g to 100g N2O-nitrogen ha–1 day–1. The highest rates of N2O emissions were measured during spring on the hydromorphic leached brown soil which had been fertilized with nitrogen (N); the total emissions during a 5-month period exceeded 3500gNha–1. Significant fluxes were also observed during the summer. Very marked effects of soil type and management were observed. Two factors – the soil hydraulic behaviour and the ability of the microbial population to reduce N2O – appear to be essential in determining emissions of N2O by soils. In fact, the hydromorphic leached brown soil showed the highest emissions, despite having the lowest denitrification potential because of its water-filled pore space and low N2O reductase activity. Soil management also appears to affect both soil nitrate content and N2O emissions.