Long time correlations in Lagrangian dynamics: a key to intermittency in turbulence

New aspects of turbulence are uncovered if one considers flow motion from the perspective of a fluid particle (known as the Lagrangian approach) rather than in terms of a velocity field (the Eulerian viewpoint). Using a new experimental technique, based on the scattering of ultrasounds, we have obtained a direct measurement of particle velocities, resolved at all scales, in a fully turbulent flow. It enables us to approach intermittency in turbulence from a dynamical point of view and to analyze the Lagrangian velocity fluctuations in the framework of random walks. We find experimentally that the elementary steps in the 'walk' have random uncorrelated directions but a magnitude that is extremely long-range correlated in time. Theoretically, we study a Langevin equation that incorporates these features and we show that the resulting dynamics accounts for the observed one- and two-point statistical properties of the Lagrangian velocity fluctuations. Our approach connects the intermittent statistical nature of turbulence to the dynamics of the flow.Comment: 4 pages, 4 figure

Land-use and other biogeochemical constraints modulate priming effect at the landscape scale: a fingerprinting approach using analytical pyrolysis

International audienceSoil has the potential to sequester or to emit great amounts of CO2, since soil carbon stocks constitute the largest terrestrial reservoir of carbon. Land-use is among the main factors affecting the net storage or the net mineralization of soil organic carbon (SOC) and land-use policies are currently aiming to increase soil organic carbon stocks.However, adding labile sources of carbon to soil (e.g. more crop residues or manure) may alter the carbon cycle, increasing the microbial degradation of the existing SOC pools, a process called “priming effect” (PE). PE could significantly reduce the net storage of carbon and taking into account this effect into carbon storage models may alter their predictions.Land-use is one of the numerous constraints that influence PE, together with soil chemical, biological and physical properties, all of them are contributing to shape the chemical composition of the SOC pools. The objectives of this study are:- to conduct a fingerprint characterization of the chemical composition of soil organic matter using analytical pyrolysis coupled to gas chromatography and mass spectrometry;- to use pyrograms to decipher which soil chemical markers are the most relevant descriptors of the PE;- to relate those chemical markers to the different types of crop rotations.This approach will help to decipher the complex interactions of the PE descriptors and to relate PE intensity to specific land-uses for a determined area, identifying which land management is capable to reduce soil susceptibility to priming effect and to be more efficient in term of net SOC storage.With this purpose, soil samples from 88 agricultural farms within a 12 km2 water catchment in Brittany (France) were incubated in presence of a 13C labelled wheat litter to measure the PE. Data regarding soil chemistry, soil microbial diversity, crop rotation, land-use, and manuring obtained for the studied farms were coupled to pyrograms to select the most relevant descriptors of the priming effect.Our results showed that:- the inclusion of grassland into crop rotation reduce the susceptibility of soils to PE, but only if grassland is maintained for an adequate period of the rotation time;- although PE is a microbial process, only a marginal part of its total variance is assigned to both richness and evenness of microbial communities;- the intensity of PE is correlated with the concentration of condensed aromatic compounds in soil water extracts, identified with excitation-emission fluorescence spectroscopy;- the intensity of the PE decreases with the molecular complexity of the soil organic matter assessed with pyrolysis coupled to gas chromatography and mass spectrometry.Disclosure of Interest: None declare

Land-use and other biogeochemical constraints modulate priming effect at the landscape scale: a fingerprinting approach using analytical pyrolysis

International audienceSoil has the potential to sequester or to emit great amounts of CO2, since soil carbon stocks constitute the largest terrestrial reservoir of carbon. Land-use is among the main factors affecting the net storage or the net mineralization of soil organic carbon (SOC) and land-use policies are currently aiming to increase soil organic carbon stocks.However, adding labile sources of carbon to soil (e.g. more crop residues or manure) may alter the carbon cycle, increasing the microbial degradation of the existing SOC pools, a process called “priming effect” (PE). PE could significantly reduce the net storage of carbon and taking into account this effect into carbon storage models may alter their predictions.Land-use is one of the numerous constraints that influence PE, together with soil chemical, biological and physical properties, all of them are contributing to shape the chemical composition of the SOC pools. The objectives of this study are:- to conduct a fingerprint characterization of the chemical composition of soil organic matter using analytical pyrolysis coupled to gas chromatography and mass spectrometry;- to use pyrograms to decipher which soil chemical markers are the most relevant descriptors of the PE;- to relate those chemical markers to the different types of crop rotations.This approach will help to decipher the complex interactions of the PE descriptors and to relate PE intensity to specific land-uses for a determined area, identifying which land management is capable to reduce soil susceptibility to priming effect and to be more efficient in term of net SOC storage.With this purpose, soil samples from 88 agricultural farms within a 12 km2 water catchment in Brittany (France) were incubated in presence of a 13C labelled wheat litter to measure the PE. Data regarding soil chemistry, soil microbial diversity, crop rotation, land-use, and manuring obtained for the studied farms were coupled to pyrograms to select the most relevant descriptors of the priming effect.Our results showed that:- the inclusion of grassland into crop rotation reduce the susceptibility of soils to PE, but only if grassland is maintained for an adequate period of the rotation time;- although PE is a microbial process, only a marginal part of its total variance is assigned to both richness and evenness of microbial communities;- the intensity of PE is correlated with the concentration of condensed aromatic compounds in soil water extracts, identified with excitation-emission fluorescence spectroscopy;- the intensity of the PE decreases with the molecular complexity of the soil organic matter assessed with pyrolysis coupled to gas chromatography and mass spectrometry.Disclosure of Interest: None declare

Guided Bloch Surface Waves on Ultrathin Polymeric Ridges

We present a direct evidence of Bloch surface waves (BSWs) waveguiding on ultrathin polymeric ridges, supported by near-field measurements. It is demonstrated that near-infrared BSWs sustained by a silicon-based multilayer can be locally coupled and guided through dielectric ridges of nanometric thickness with low propagation losses. Using a conventional prism-based configuration, we demonstrate a wavelength-selective BSW coupling inside and outside the ridge. Such a result can open interesting opportunities in surface wave-mediated sensing applications, where light could be selectively coupled in specific regions defined by nanometric reliefs