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International audience; We evaluated whether and how rapidly temperate forest tree species able to stimulate or inhibit nitrification (through biological nitrification inhibition, BNI) also influence denitrifier activity and abundance in soil and identified the main determinants of changes in denitrification. A reciprocal soil core transfer approach was implemented at a long-term experimental site between Douglas fir, a species assumed to stimulate nitrification, and stands of tree species with (spruce and Nordmann fir) or without (Corsican pine and beech) BNI capacity. Sixteen months after soil transfer, potential denitrification decreased in soils transferred from Douglas fir to another tree species and increased in soil cores transferred from any species to Douglas fir. The change in denitrification was not related to the BNI capacity of the tree species 'receiving' the soil cores. Structural equation modelling revealed that in soil cores transferred between Douglas fir and BNI species, changes in denitrification were mainly correlated to nitrifier abundances, i.e. AOA and Nitrobacter, nitrification rates, and soil NO3- availability, whereas in soil cores transferred between Douglas fir and non-BNI species denitrification changes were well correlated to C availability and the abundance of nirK-harbouring denitrifiers. Overall, our results indicate that denitrification rates can change strongly and quickly following soil transfers between tree stands. Particularly, when Douglas fir replaces BNI tree species, this quickly results in both increased nitrification and denitrification rates, thus exacerbating the ensuing risk of increased NO3- leaching and N2O emission from soil.

International audience; Some forest tree species are able to carry out a process known as biological nitrification inhibition, BNI, i.e. they inhibit nitrifiers through the production of specific compounds. We tested the hypothesis that, by restricting N supply to NO2-- and N2O--reducers, BNI would decrease potential N2O production and consumption and in situ N2O emissions. as compared to soils under trees without BNI capacity. Soils were collected from long-term monocultures (>43 ys) of three tree species without BNI capacity (Fagus sylvatica, Pinus nigra and Pseudotsuga menziesii) and two tree species with BNI capacity (Abies nordmanniana and Picea abies). The level of limitation of denitrification by NO3 was high for species with BNI capacity and low for species without BNI capacity, and was correlated with potential nitrification rates and the abundances of genes specifically harboured by ammonia oxidizing archaea and Nitrobacter. However, potential denitrification and actual N2O emissions did not reflect the tree BNI status, and denitrification limitation by soil carbon was higher than limitation by N under three tree species. Structural equation modelling revealed that the ratio between the gene copy abundances of nitritereducers and N2O-reducers was the microbial variable that best explained N2O emissions, along with soil pH and moisture. In addition, the NO3- concentration in the soil solution at 60 cm depth increased with the potential nitrification-to-denitrification ratio, suggesting a higher risk of NO3 leaching under some tree species like Douglas fir.

International audience; Although many environments like soils are constantly subjected to invasion by alien microbes, invaders usually fail to succeed, succumbing to the robust diversity often found in nature. So far, only successful invasions have been explored, and it remains unknown to what extent an unsuccessful invasion can impact resident communities. Here we hypothesized that unsuccessful invasions can cause impacts to soil functioning by decreasing the diversity and niche breadth of resident bacterial communities, which could cause shifts to community composition and niche structure\textemdashan effect that is likely exacerbated when diversity is compromised. To examine this question, diversity gradients of soil microbial communities were subjected to invasion by the frequent, yet oft-unsuccessful soil invader, Escherichia coli, and evaluated for changes to diversity, bacterial community composition, niche breadth, and niche structure. Contrary to expectations, diversity and niche breadth increased across treatments upon invasion. Community composition and niche structure were also altered, with shifts of niche structure revealing an escape by the resident community away from the invader's resources. Importantly, the extent of the escape varied in response to the community's diversity, where less diverse communities experienced larger shifts. Thus, although transient and unsuccessful, the invader competed for resources with resident species and caused tangible impacts that modified both the diversity and functioning of resident communities, which can likely generate a legacy effect that influences future invasion attempts.

Optical microresonators with high quality factors are key in photonic circuits requiring fine spectral filtering or resonant storage of optical power. Silicon (Si) photonics provides high-performance optoelectronic circuits but yields planar Si microresonators with rather low quality factors ( Q < 10 5 ). On the other hand, bulk resonators achieve exceptionally high quality factors, Q > 10 7 . Si photonic waveguides and bulk resonators have very different sizes and refractive indices that preclude efficient coupling. Here, we show an efficient method to couple bulk resonators and Si waveguides based on subwavelength metamaterial engineering. We demonstrate up to 99% light coupling efficiency for microspheres and microdisks made of silica, lithium niobate, and calcium fluoride, with 0.3 − 3.6 m m diameter. This achievement could enable the heterogeneous integration of bulk resonators and silicon photonic circuits, with potential applications in sensing, communications, and quantum information.

Uniform grating couplers based on versatile L-shaped waveguides are experimentally demonstrated, with coupling loss of -2.7 dB and back-reflections of -20 dB. Apodized couplers with subwavelength-grating metamaterials predict improved fiber-chip coupling down to -0.46 dB within device layouts compatible with lithographic technologies available in nanophotonic foundries., 2019 IEEE 16th International Conference on Group IV Photonics (GFP), August 28-30, 2019, Singapore, Singapore

International audience; Mid-infrared racetrack resonators are demonstrated working at 8µm wavelength. The devices are based on a graded SiGe platform providing low propagation loss on a large wavelength range in the mid-IR. Different resonators designs have been fabricated, with varying gap distances in the directional coupler. Q factors of more than 3000 have been experimentally demonstrated. These results pave the way towards compact mid-IR sensors or efficient active devices.

International audience; Hetero-structured silicon-germanium-silicon photodetectors operating under low-reverse-voltages with high responsivity, fast response, and low dark-current levels are reported. A bit-error-rate of 10-9 is experimentally achieved for conventional data rates of 10, 20, and 25 Gbps, providing optical power sensitivities of-13.9,-12.7, and-11.3 dBm.

International audience; In this paper, the recent progress on a new Ge-rich SiGe platform for mid-IR integrated photonics is presented. Low-loss spiral waveguides working over a broadband wavelength range are discussed, followed by a sensing proof-of-concept using a standalone photoresist with a known spectral absorption pattern. In addition, the development of new mid-IR interferometric devices for wavelength filtering and enhancement of the light-matter interaction are presented. Finally, efficient designs to exploit the third-order nonlinearities in these Ge-rich SiGe waveguides at mid-IR wavelengths are shown. The demonstration of these key building blocks will pave the way towards the implementation of new mid-IR photonic integrated systems with multiple functionalities.

International audience; Mid-infrared (mid-IR) silicon photonics is expected to lead key advances in different areas including spectroscopy, remote sensing, nonlinear optics or free-space communications, among others. Still, the inherent limitations of the silicon-on-insulator (SOI) technology, namely the early mid-IR absorption of silicon oxide and silicon at λ~3.6 µm and at λ ~8.5 µm respectively, remain the main stumbling blocks that prevent this platform to fully exploit the mid-IR spectrum (λ ~2-20 µm). Here, we propose using a compact Ge-rich graded-index Si1-xGex platform to overcome this constraint. A flat propagation loss characteristic as low as 2-3 dB/cm over a wavelength span from λ = 5.5 µm to 8.5 µm is demonstrated in Ge-rich Si1-xGex waveguides of only 6 µm thick. The comparison of three different waveguides design with different vertical index profiles demonstrates the benefit of reducing the fraction of the guided mode that overlaps with the Si substrate to obtain such flat low loss behavior. Such Ge-rich Si1-xGex platforms may open the route towards the implementation of mid-IR photonic integrated circuits with low-loss beyond the Si multi-phonon absorption band onset, hence truly exploiting the full Ge transparency window up to λ ~15 µm.

Recent works towards the development of Ge-rich SiGe photonic integrated circuits will be presented, such as the demonstration of low-loss waveguides and ultra-wideband Mach Zehnder interferometer from 5.5 to 8.6 μm wavelength, as well as the first steps towards the realization of efficient wideband optical sources.

This work explores the use of Ge-rich graded-index Si

International audience; This work explores the use of Ge-rich graded-index Si 1-x Ge x rib waveguides as building blocks to develop integrated nonlinear optical devices for broadband operation in the mid-IR. The vertical Ge gradient concentration in the waveguide core renders unique properties to the guided optical mode, providing tight mode confinement over a broadband mid-IR wavelength range from λ = 3 µm to 8 µm. Additionally, the gradual vertical confinement pulls the optical mode upwards in the waveguide core, overlapping with the Ge-rich area where the nonlinear refractive index is larger. Moreover, the Ge-rich graded-index Si 1-x Ge x waveguides allow efficient tailoring of the chromatic dispersion curves, achieving flat anomalous dispersion for the quasi-TM optical mode with D ≤ 14 ps/nm/km over a ~ 1.4 octave span while retaining an optimum third-order nonlinear parameter, γ eff. These results confirm the potential of Ge-rich graded-index Si 1-x Ge x waveguides as an attractive platform to develop mid-IR nonlinear approaches requiring broadband dispersion engineering.

We report an efficient energy-time entangled photon-pair source based on four-wave mixing in a CMOS-compatible silicon photonics ring resonator. Thanks to suitable optimization, the source shows a large spectral brightness of 400 pairs of entangled photons /s/MHz for 500 μW pump power, compatible with standard telecom dense wavelength division multiplexers. We demonstrate high-purity energy-time entanglement, i.e., free of photonic noise, with near perfect raw visibilities (98%) between various channel pairs in the telecom C-band. Such a compact source stands as a path towards more complex quantum photonic circuits dedicated to quantum communication systems.

International audience; We report an efficient energy-time entangled photon-pair source based on four-wave mixing in a CMOS-compatible silicon photonics ring resonator. Thanks to suitable optimization, the source shows a large spectral brightness of 400 pairs of entangled photons /s/MHz for 500 µW pump power. Additionally, the resonator has been engineered so as to generate a frequency comb structure compatible with standard telecom dense wavelength division multiplexers. We demonstrate high-purity energy-time entanglement, i.e., free of photonic noise, with near perfect raw visibilities (> 98%) between various channel pairs in the telecom C-band. Such a compact source stands as a path towards more complex quantum photonic circuits dedicated to quantum communication systems.

International audience; Optical interconnects are expected to overcome the copper link bottlenecks. Amongthe different developed technologies, silicon photonics becomes one of the preferred solutions. Indeed,due to its compatibility with the CMOS platform, silicon photonics is considered as aviable solution for the development of the future generation photonic interconnects. However,the on-chip integration of all photonic and optoelectronic building blocks (sources, modulatorsand detectors. . . ) on the silicon platform is very complex and is not cost-effective due to thevarious materials used: Ge for detection, Si for waveguiding and modulators and III-V for lasing.We propose here to use carbon nanotubes (CNTs) integrated into silicon photonics for the developmentof all optoelectronic devices. Indeed, CNTs are nanomaterials of particular interest inphotonics thanks to their fundamental optical properties including near-IR luminescence, Starkeffect, Kerr effect and absorption. In consequence, CNTs have the ability to emit, modulateand detect light in the wavelength range of the silicon transparency. Furthermore, they are alsoconsidered as an electronic compatible material thanks to the recent advances in nanoelectronicsbased on CNTs.Here, we report on the study of the light emission coupling from CNTs into optical resonatorsimplemented on the silicon-on-insulator (SOI) platform including ring resonators, nano-beamcavities, micro-disks. . . . A theoretical and experimental analysis of the light interaction of CNTswith micro-ring resonators based on slot waveguides and slot photonic crystal heterostructurecavities will be presented.

International audience; Silicon is the mainstream material in the electronic industry and it is rapidly expanding its dominance into the field of photonics. Indeed, silicon photonics has been the subject of intense research activities to pave the way for next generation of energy-efficient high-speed computing, information processing and communications systems. The trend is to use optics in intimate proximity to the electronic circuit, which implies a high level of optoelectronic integration. Over the last decade, the field of silicon photonics has advanced at a remarkable pace. Recent advances in integrated germanium receiver will be presented including the developments of a new Ge-based detector integration and avalanche Ge structures.

International audience; Hybrid systems based on carbon nanotubes emitting in the telecom wavelength range and Si-photonic platforms are promising candidates for developing integrated photonic circuits. Here, we consider semiconducting single walled carbon nanotubes (s-SWNTs) emitting around 1300 nm or 1550 nm wavelength. The nanotubes are deposited on quartz substrate for mapping their photoluminescence in hyperspectral near-field microscopy. This method allows for a sub-wavelength resolution in detecting the spatial distribution of the emission of single s-SWNTs at room temperature. Optical signature delocalized over several micrometers is observed, thus denoting the high quality of the produced carbon nanotubes on a wide range of tube diameters. Noteworthy, the presence of both nanotube bundles and distinct s-SWNT chiralities is uncovered.

International audience; Recent advances in the theoretical and experimental studies of Ge/SiGe quantum wells structures will be presented with a focus on optical telecommunication applications. Firstly, high-speed stand-alone Ge/SiGe QW electro-absorption modulators and photodetectors will be reported. It will be followed by the presentation of different methods for engineering Ge/SiGe QW to tune the operating wavelength to 1.3 µm. Then the demonstration of a strong electro-refraction Ge/SiGe QW will be reported. This effect could be used to achieve optical modulation but requires the embedding of the QWs in a Mach-Zehnder interferometer. Finally it will be shown that these active devices can be combined with advanced passive structures using Ge-rich SiGe virtual substrate on graded buffer as a waveguide.

Polarization controllers are key elements in many fields of optics, including coherent communications, optical imaging, and quantum applications. Here we present a technology-independent polarization controller scheme based on electrically tunable phase shifters and polarization rotators with largely relaxed fabrication tolerances. Using this scheme, we experimentally demonstrate a fully integrated polarization controller in the silicon-on-insulator platform that is tunable over the complete C-band and achieves a polarization extinction range of 40 dB (±20 dB ). These results constitute, to the best of our knowledge, the highest polarization extinction range achieved in a fully integrated device, and overcome the existing limitation in the trade-off between integration and performance in polarization management circuits.

International audience; This paper reports on recent results on the integration of carbon nanotubes on the silicon photonic platform. Light coupling from carbon nanotubes in a fully integrated silicon resonator will be presented.

International audience; Optical antennas based on noble metal nanoparticles can increase the photoluminescence of quantum dots,but the exact strength of this enhancement depends on the brightness (i.e., the intrinsic quantum yield ηi) ofthe emitters. Here we perform temperature-dependent measurements on a system of PbS colloidal quantumdots coupled with Au ring arrays that bring quantitative insight into this phenomenon. We show that althoughthe boost in photoluminescence is lower at cryogenic temperatures where the nanocrystals become very bright emitters, the spectral signature of this enhancement is remarkably independent of ηi . These observations remain true even at wavelengths where the losses by absorption in the metal nanoparticles considerably increase due to the excitation of localized plasmon resonances, in contradiction with standard theory that treats the emitters as a collection of two-level systems. We propose a mechanism in which the quantum dots are modeled as multilevel and inhomogeneously broadened emitters to account for these findings.

International audience; Ge/SiGe Quantum well structures have a strong potential to revolutionize silicon photonics. This paper reviews recent works including high speed modulator and photodetector, QW engineering to tune the wavelength and waveguide integration. OCIS codes: (130.3120) Integrated optics devices; (230.4205) Multiple quantum well (MQW) modulators; (130.0250) Optoelectronics.

It has been suggested that grazing could stimulate the aboveground dry mass and/or nitrogen yields available to grazers (i.e., the grazing optimization hypothesis, GOH), but the actual importance of this effect is still controversial. The GOH has only been tested for a few grasslands and one savanna ecosystem, but not for the moistest and warmest grasslands and savannas of the world. The objectives of this study were to test the GOH in the humid savanna of Lamto (West Africa) by analyzing the growth of grasses in response to a field trial with three levels of clipping × two levels of fertilization. We quantified the effects of clipping and fertilization on the dry matter and nitrogen yields to grazers (i.e., mass or nitrogen amount in clipped-off tissues during the experiment) and on the remaining yield (i.e., mass or nitrogen amount in residual phytomass at the end of the experiment) over a three-month period. Total phytomass yield, the sum of yield to grazers, and the remaining yield was maintained under moderate clipping frequency and fertilization as compared to control conditions. Both clipping frequencies decreased the remaining phytomass yield as compared to control plots. Clipping frequency significantly increased nitrogen concentrations in the total yield, in the remaining yield, and in the yield to grazers. Total nitrogen yield and nitrogen yield to grazers were 65% and 91% higher on the plots experiencing moderate clipping frequency with fertilization as compared to control plots. The study shows that grazers in this humid savanna system could potentially modify ecosystem processes in such a way as to partly alleviate nutritional deficiencies, but only in the presence of increased nitrogen availability.

Summary 1The rate of CO2 emission by two major termite species (Ancistrotermes cavithorax and Odontotermes n. pauperans) was studied in a West African savanna (Lamto, Cote d’Ivoire). First, in three major savanna types (grassy, shrubby and woody savannas), CO2 emission from the soil surface was measured using a closed container system. Control soil without termite fungus-comb chambers, and soil of eroded termite mound with or without Odontotermes fungus-comb chambers was sampled in each savanna type. Second, the mass-specific respiration rate of the different components of termite fungus-comb chambers (i.e. workers, soldiers, fungus comb and chamber walls) was measured under laboratory conditions. CO2 emission by termites at the landscape-scale was computed from both field biomass data and laboratory measurements. 2Whatever the savanna type, CO2 emission from the soil surface was not different between control soil and soil of eroded termite mound without termite fungus-comb chambers, but was significantly higher in areas with fungus-comb chambers than in areas without fungus-comb chambers (10–19 µmol CO2 m−2 s−1vs 5–10 µmol CO2 m−2 s−1). 3The mass-specific respiration rates were higher for individuals of O. pauperans than for individuals of A. cavithorax. Total respiration rate from an individual fungus-comb chamber was around 56 and 143 µmol CO2 h−1 for Ancistrotermes and Odontotermes, respectively. 4Despite a low mass-specific respiration rate, fungus comb accounted for 51% of the total respiration flux from whole chambers in Odontotermes and for 82% in Ancistrotermes. The laboratory-derived respiration rate from individual Odontotermes chambers was consistent with the field estimates. 5At the landscape-scale, the CO2 emission due to A. cavithorax and O. pauperans was 0·022 and 0·050 µmol CO2 m−2 s−1, respectively. This total (27·2 g C m−2 years−1) represented 4·9% of the total above-ground net primary production in this ecosystem and 11·3% of the carbon not mineralized by annual fires.

The temperature dependence of C 3 photosynthesis is known to vary with growth environment and with species. In an attempt to quantify this variability, a commonly used biochemically based photosynthesis model was parameterized from 19 gas exchange studies on tree and crop species. The parameter values obtained described the shape and amplitude of the temperature responses of the maximum rate of Rubisco activity ( V cmax ) and the potential rate of electron transport ( J max ). Original data sets were used for this review, as it is shown that derived values of V cmax and its temperature response depend strongly on assumptions made in derivation. Values of J max and V cmax at 25 ° C varied considerably among species but were strongly correlated, with an average J max : V cmax ratio of 1·67. Two species grown in cold climates, however, had lower ratios. In all studies, the J max : V cmax ratio declined strongly with measurement temperature. The relative temperature responses of J max and V cmax were relatively constant among tree species. Activation energies averaged 50 kJ mol − 1 for J max and 65 kJ mol − − − 1 for V cmax , and for most species temperature optima averaged 33 ° ° ° C for J max and 40 ° C for V cmax . However, the cold climate tree species had low temperature optima for both J max ( 19 ° C) and V cmax (29 ° ° ° C), suggesting acclimation of both processes to growth temperature. Crop species had somewhat different temperature responses, with higher activation energies for both J max and V cmax , implying narrower peaks in the temperature response for these species. The results thus suggest that both growth environment and

Comprehensive studies on the processes involved in photosynthetic acclimation after a sudden change in light regime are scarce, particularly for trees. We tested (i) the ability of photosynthetic acclimation in the foliage of walnut trees growing outdoors after low-to-high and high-to-low light transfers made early or late in the vegetation cycle, and (ii) the relative importance of changes in total leaf nitrogen versus changes in the partitioning of leaf nitrogen between the different photosynthetic functions during a 2 month period after transfer. Changes in maximum carboxylation rate, light-saturated electron transport rate, respiration rate, total leaf nitrogen, ribulose 1.5-bisphosphate carboxylase/oxygenase (Rubisco) and total chlorophylls were surveyed before and after the change in light regime. Respiration rate acclimated fully within 1 week of transfer, and full acclimation was observed 1 month after transfer for the amount of Rubisco. In contrast, total nitrogen and photosynthetic capacity acclimated only partially during the 2 mouth period. Changes in photosynthetic capacity i were driven by changes in both total leaf nitrogen and leaf nitrogen partitioning. The extent of acclimation also depended strongly on leaf age at the time of the change in light regime.

The model RATP (radiation absorption, transpiration and photosynthesis) is presented. The model was designed to simulate the spatial distribution of radiation and leaf-gas exchanges within vegetation canopies as a function of canopy structure, canopy microclimate within the canopy and physical and physiological leaf properties. The model uses a three-dimensional (3D) representation of the canopy (i.e. an array of 3D cells, each characterized by a leaf area density). Radiation transfer is computed by a turbid medium analogy, transpiration by the leaf energy budget approach, and photosynthesis by the Farquhar model, each applied for sunlit and shaded leaves at the individual 3D cell-scale. The model typically operates at a 20‐30 min time step. The RATP model was applied to an isolated, 20-yearold walnut tree grown in the field. The spatial distribution of wind speed, stomatal response to environmental variables, and light acclimation of leaf photosynthetic properties were taken into account. Model outputs were compared with data acquired in the field. The model was shown to simulate satisfactorily the intracrown distribution of radiation regime, transpiration and photosynthetic rates, at shoot or branch scales.

Termitaria are major sites of functional heterogeneity in tropical ecosystems, through their strong influence on soil characteristics, in particular soil physico-chemical properties and water status. These factors have important consequences on nutrient availability for plants, plant spatial distribution, and vegetation dynamics. However, comprehensive information about the influence of termite-rehandled soil on soil water regime is lacking. In a humid shrubby savanna, we characterized the spatial variations in soil texture, soil structure and maximum soil water content available for plants (AWC max) induced by a large termite mound, at three deepths (0–0.10, 0.20–0.30 and 0.50–0.60 m). In addition, during a three month period at the end of the rainy season, soil water potential was surveyed by matrix sensors located on the termite mound and in the surrounding soil at the same depths and for the 80–90 cm layer. Concurrently, the leaf shedding patterns of two coexisting deciduous shrub species exhibiting contrasted soil water uptake patterns were compared for individuals located on termite mounds and in undisturbed control areas. For all the soil layers studied, clay and silt contents were higher for the mound soil. Total soil clods porosity was higher on the mound than in control areas, particularly in the 0.20-0.60 m layer, and mound soil exhibited a high shrinking/swelling capacity. AWCmax of the 0-0.60 m soil layer was substantially higher on the termite mound (112 mm) than in the surroundings (84 mm). Furthermore, during the beginning of the dry season, soil water potential measured in situ for the 0.20-0.90 m soil layer was higher on the mound than in the control soil. In contrast, soil water potential of the 0-0.10 m soil layer was similar on the mound and in the control soil. In the middle of the dry season, the leaf shedding pattern of Crossopteryx febrifuga shrubs (which have limited access to soil layers below 0.60 m) located on mounds was less pronounced than that of individuals located on control soil. In contrast, the leaf shedding pattern of the shrub Cussonia barteri (which has a good access to deep soil layers) was not influenced by the termite mound. We conclude that in this savanna ecosystem, termite mounds appear as peculiar sites which exhibit improved soil water availability for plants in upper soil layers, and significantly influence aspects of plant function. Implications of these results for understanding and modelling savanna function and dynamics, and particularly competitive interactions between plant species, are discussed.

Integrated optical affinity sensors based on Mach-Zehnder interferometers (MZIs) enable sensitive and label-free detection of chemical and biological molecules. However, since MZIs transduce optical phase changes into sinusoidal intensity variations, they exhibit a variable sensitivity that vanishes at the extrema of the transmittance function. Moreover, the direction of phase change cannot be unambiguously determined with these sensors. Here, we present a coherent detection scheme based on integrated optics that enables unambiguous readout of the optical phase with a constant sensitivity. Our approach furthermore cancels the effect of imperfections in the sensor hardware using a blind calibration scheme. We experimentally show completely linear readout of the optical phase, with a fourfold enhancement of average sensitivity compared to conventional detection.

Leaf and grass canopy photosynthetic rates were measured in a West African humid savanna during several stages of the early growing season. The results obtained on the dominant grass speciesHyparrhenia diplandraand data published previously show that C4savanna grasses exhibit a remarkably high leaf photosynthetic capacity despite their low nitrogen content. A variation of leaf photosynthetic capacity in relation to leaf rank on stems is observed which is interpreted by ageing and shading effects within the canopy. Seasonal variations of the canopy CO2assimilation rate is explained in relation to variations of leaf area index and canopy nitrogen content. Despite low nitrogen content or low leaf area index, maximum canopy net photosynthesis was high (24 μmol CO2m-3s-1for LAI = 1.5). The high photosynthetic nitrogen use efficiency exhibited by leaves of humid savanna grass species is a major attribute explaining high photosynthetic rates of the grass canopy in this environment. This result sustains the emerging opinion that tropical savannas could be highly productive despite the generally low nutrient status they experience.

Photonic metamaterials made of graded photonic crystals operating near the bandgap frequency region are proposed for field manipulation around l=1.5μm. Proof-of-concept structures have been studied using Hamiltonian optics and FDTD simulation, fabricated, and characterized using farfield optical measurements. Experimental results are in good agreement with predictions, showing the interest of graded photonic crystals as an (ultra-low loss) alternative solution to the use of metamaterials combining dielectric and metallic materials with sub-wavelength unit cells.

International audience; Responses of soil nitrogen (N) cycling to simultaneous and potentially interacting global environmental changes are uncertain. Here, we investigated the combined effects of elevated CO2, warming, increased precipitation and enhanced N supply on soil N cycling in an annual grassland ecosystem as part of the Jasper Ridge Global Change Experiment (CA, USA). This field experiment included four treatments—CO2, temperature, precipitation, nitrogen—with two levels per treatment (ambient and elevated), and all their factorial combinations replicated six times. We collected soil samples after 7 and 8 years of treatments, and measured gross rates of N mineralization, N immobilization and nitrification, along with potential rates of ammonia oxidation, nitrite oxidation and denitrification. We also determined the main drivers of these microbial activities (soil ammonium and nitrate concentrations, soil moisture, soil temperature, soil pH, and soil CO2 efflux, as an indicator of soil heterotrophic activity). We found that gross N mineralization responded to the interactive effects of the CO2, precipitation and N treatments: N addition increased gross N mineralization when CO2 and precipitation were either both at ambient or both at elevated levels. However, we found limited evidence for interactions among elevated CO2, warming, increased precipitation, and enhanced N supply on the other N cycling processes examined: statistically significant interactions, when found, tended not to persist across multiple dates. Soil N cycling responded mainly to single-factor effects: long-term N addition increased gross N immobilization, potential ammonia oxidation and potential denitrification, while increased precipitation depressed potential nitrite oxidation and increased potential ammonia oxidation and potential denitrification. In contrast, elevated CO2 and modest warming did not significantly affect any of these microbial N transformations. These findings suggest that global change effects on soil N cycling are primarily additive, and therefore generally predictable from single factor studies.

International audience

Urine deposition on grassland causes significant N2O losses, which in some cases may result from increased denitrification stimulated by labile compounds released from scorched plant roots. Two 12-day experiments were conducted in 13C- labelled grassland monoliths to investigate the link between N2O production and carbon mineralization following application of low rates of urine-N. Measurements of N2O and CO2 emissions from the monoliths as well as \(\delta^{13}\)C signal of evolved CO2 were done on day −4, −1, 0, 1, 2, 4, 5, 6 and 7 after application of urine corresponding to 3.1 and 5.5 g N m−2 in the first and second experiment, respectively. The \(\delta^{13}\)C signal was also determined for soil organic matter, dissolved organic C and CO2 evolved by microbial respiration. In addition, denitrifying enzyme activity (DEA) and nitrifying enzyme activity (NEA) were measured on day −1, 2 and 7 after the first urine application event. Urine did not affect DEA, whereas NEA was enhanced 2 days after urine application. In the first experiment, urine had no significant effect on the N2O flux, which was generally low (−8 to \(14\,\upmu\)g N2O-N m−2 h−1). After the second application event, the N2O emission increased significantly to \(87\,\upmu\)g N2O-N m−2 h−1 and the N2O emission factor for the added urine-N was 0.18%. However, the associated 13C signal of soil respiration was unaffected by urine. Consequently, the increased N2O emission from the simulated low N-urine patches was not caused by enhanced denitrification stimulated by labile compounds released from scorched plant roots.

International audience; The effects of global change on below-ground processes of the nitrogen (N) cycle have repercussions for plant communities, productivity and trace gas effluxes. However, the interacting effects of different components of global change on nitrification or denitrification have rarely been studied in situ. 2. We measured responses of nitrifying enzyme activity (NEA) and denitrifying enzyme activity (DEA) to over 4 years of exposure to several components of global change and their interaction (increased atmospheric CO2 concentration, temperature, precipitation and N addition) at peak biomass period in an annual grassland ecosystem. In order to provide insight into the mechanisms controlling the response of NEA and DEA to global change, we examined the relationships between these activities and soil moisture, microbial biomass C and N, and soil extractable N. 3. Across all treatment combinations, NEA was decreased by elevated CO2 and increased by N addition. While elevated CO2 had no effect on NEA when not combined with other treatments, it suppressed the positive effect of N addition on NEA in all the treatments that included N addition. We found a significant CO2-N interaction for DEA, with a positive effect of elevated CO2 on DEA only in the treatments that included N addition, suggesting that N limitation of denitrifiers may have occurred in our system. Soil water content, extractable N concentrations and their interaction explained 74% of the variation in DEA. 4. Our results show that the potentially large and interacting effects of different components of global change should be considered in predicting below-ground N responses of Mediterranean grasslands to future climate changes.

A 3 dB bandwidth of 35 GHz at 1.31 /spl mu/m and 1.55 /spl mu/m wavelengths is reported for inter-digited germanium on silicon-on-insulator metal-semiconductor-metal photodetector with finger spacing of 500 nm.

A three-dimensional model of radiative transfer and leaf gas exchange was used to quantify daily carbon (C) assimilation of all fruit-bearing shoots (FBS) in an early maturing 6-year-old peach tree (Prunus persica L. Batsch) with a heavy crop load. For a sample of FBS (n=36), growth of fruit and leafy shoots was measured every 1-2 weeks from 24 days after bloom (DAB) until harvest, between 93-101 DAB. The objective was to relate shoot C assimilation with harvested fruit mass for each shoot to test the hypothesis that variation in C supply contributes significantly to variation in fruit growth within and among FBS. Mean C assimilation of the sampled shoots was 0.07 g C fruit(-1) day(-1), but varied between 0.014 and 0.32 g C fruit(-1) day(-1). This indicates that C availability for fruit growth would have varied significantly among individual FBS if they were autonomous. Mean fruit dry mass on each FBS varied between 0.716 and 7.68 g C at harvest, and most of the variation originated among, not within, individual FBS. However, there were no correlations between the mean and standard deviation of fruit mass and fruit relative growth rate when each was plotted against shoot C assimilation, indicating that factors such as those regulating C demand of fruit, or C transfer among individual FBS, may be more important in controlling variability in fruit growth than intra-crown variability in shoot C assimilation. Under the study conditions, FBS were non-autonomous for C, because a model of fruit and leafy shoot growth was unable to reproduce the observed growth without supplementary contribution of C from shoots without fruit.

Summary Variations in leaf nitrogen concentration per unit mass (Nm) and per unit area (Na), mass-to-area ratio (Ma), total nonstructural carbohydrates (Ta), and photosynthetic capacity (maximum carboxylation rate, electron transport capacity, rate of phosphate release in triose phosphate utilization and dark respiration rate) were studied within the digitized crowns of two 3-year-old mango trees (#Mangifera indica# L.) on La Reunion Island. Additional measurements of Nm, Na, Ma, Ta, and photosynthetic capacities were performed on young, fully expanded leaves of 11-year-old mango trees. Leaves of similar gap fractions were taken far from and close to developing fruits. Unlike Nm, both Na, and Ta were linearly correlated to gap fraction. Similar relationships were found for all leaves whatever their age and origin, except for Ta, for which we found a significant tree effect. Photosynthetic capacity was nonlinearly correlated to Na, and a unique relationship was obtained for all types of leaves. Photosynthetic acclimation to light was mainly driven by changes in Ma, but allocation of total leaf N between the different photosynthetic functions also played a substantial role in acclimation to the lowest irradiances. Leaves close to developing fruits exhibited a higher photosynthetic capacity than other leaves, but similar Ta. Our data suggest that Ta does not control photosynthetic capacity in mango leaves. We used the data to parameterize a biochemically based model of photosynthesis and an empirical stomatal conductance model, allowing accurate predictions of net photosynthesis of leaves in field-grown mango trees. (Resume d'auteur)

Stimulation of nitrification and denitrification by long term (from years to decades) grazing has commonly been reported in different grassland ecosystems. However, grazing generally induces important changes in plant species composition, and whether changes in nitrification and denitrification are primarily due to changes in vegetation composition has never been tested. We compared soil nitrification- and denitrification-enzyme activities (NEA and DEA, respectively) between semi-natural grassland sites experiencing intensive (IG) and light (LG) grazing/mowing regimes for 13 years. Mean NEA and DEA (i.e. observed from random soil sampling) were higher in IG than LG sites. The NEA/DEA ratio was higher in IG than LG sites, indicating a higher stimulation of nitrification. Marked changes in plant species composition were observed in response to the grazing/mowing regime. In particular, the specific phytomass volume of Elymus repens was lower in IG than LG sites, whereas the specific volume of Lolium perenne was higher in IG than LG sites. In contrast, the specific volume of Holcus lanatus, Poa trivialis and Arrhenatherum elatius were not significantly different between treatments. Soils sampled beneath grass tussocks of the last three species exhibited higher DEA, NEA and NEA/DEA ratio in IG than LG sites. For a given grazing regime, plant species did not affect significantly soil DEA, NEA and NEA/DEA ratio. The modification of plant species composition is thus not the primary factor driving changes in nitrification and denitrification in semi-natural grassland ecosystems experiencing long term intensive grazing. Factors such as trampling, N returned in animal excreta, and/or modification of N uptake and C exudation by frequently defoliated plants could be responsible for the enhanced microbial activities.

Photosynthetic light acclimation of leaves can result from (i) changes in mass-based leaf nitrogen concentration, Nm, (ii) changes in leaf mass:area ratio, Ma, and (iii) partitioning of total leaf nitrogen among different pools of the photosynthetic machinery. We studied variations in Nm and Ma within the crowns of two peach (Prunus persica L. Batsch) trees grown in an orchard in Portugal, and one peach tree grown in an orchard in France. Each crown was digitized and a 3-D radiation transfer model was used to quantify the intra-crown variations in time-integrated leaf irradiance,PARi. Nitrogen concentration, leaf mass:area ratio, chlorophyll concentration, and photosynthetic capacity were also measured on leaves sampled on five additional peach trees in the orchard in Portugal. The data were used to compute the coefficients of leaf nitrogen partitioning among carboxylation, bioenergetics, and light harvesting pools. Leaf mass:area ratio and area-based leaf nitrogen concentration, Na, were nonlinearly related toPARi, and photosynthetic capacity was linearly related to Na. Photosynthetic light acclimation resulted mainly from changes in Ma and leaf nitrogen partitioning, and to a lesser extent from changes in Nm. This behavior contrasts with photosynthetic light acclimation observed in other tree species like walnut (Juglans regia L.) in which acclimation results primarily from changes in Ma.

To assess the spatial distribution of photosynthetic capacity within an isolated 20-year-old walnut tree (Juglans regia L.) crown, the distribution of relevant leaf characteristics was measured. Variations in leaf dry weight per area (W a ), and nitrogen content on a weight (N w ) and area basis (N a ) were studied along two horizontal and one vertical gradients of leaf irradiance, at two dates (July 30 and September 3). In addition, the content of total nonstructural carbon on a weight (TNC w ) and area basis (TNC a ) was measured on July 30. Concurrently, the spatial distribution of daily integrated leaf irradiance within the crown was simulated by a three-dimensional radiation transfer model over a one week period before sampling at each date. High spatial heterogeneity was observed for W a (from 50 to 140 g m −2 ), TNC a (from 4 to 17 g m −2 ) and N a (from 1.2 to 3.6 g m −2 ) among the foliage. Although TNC w and N w were not correlated and only weakly correlated to daily leaf irradiance, respectively, W a , TNC a and N a were strongly correlated to daily leaf irradiance. The relationship between observed N a and simulated daily leaf irradiance was used to assess the spatial distribution of N a within the crown at each date. Total leaf nitrogen in the foliage was estimated to be 339 g in late July and 317g in early September. For the whole crown (i.e., 1729 current-year shoots), N a increased strongly with basal shoot diameter (an index of “shoot vigor”), highlighting the fact that large shoots were mainly located in sunlit locations and exhibited high photosynthetic capacity.

International audience; NO emission rates from soils were measured for twelve major African ecosystems in four countries (Congo, Niger, Ivory Coast, and South Africa) and within four major phytogeographic domains: the Guineo-Congolese, Guinean, Sahelian, and Zambezian domains. Measurements were performed during wet and/or dry seasons. All the measurements were made with the same dynamic chamber device, which allowed true comparisons to be made. This study showed that emission rates strongly differed between ecosystems and exhibited a marked temporal variability. Ecosystem effect was highly significant during both the dry and wet seasons. Emission rates were low (7 ng NO-N m(-2) s(-1)) in a seasonally wetted grassland (site 2) and in particular sites subjected to various disturbances, for example soil fauna activity (termite mounds) or past human disturbance (Acacia patches-settlement site). Microbial activity potentials (i.e., carbon mineralization, nitrification, denitrification, and total net N mineralization) were determined for most of the soils where NO fluxes were measured. In some sites, these potential activities were useful to identify the major processes controlling NO emission rates. Denitrification potential was very low and could not explain substantial NO fluxes from broad-and fine-leafed savannas and Hyperthelia savannas of the Zambezian domain. Very low potentials of both nitrification and denitrification could be related to the low NO fluxes for the three Guinean savanna sites studied. NO fluxes were significantly higher during the wet season than the dry season in both savanna and forest ecosystems. Emission rates in savanna ecosystems were significantly increased within a few hours after fire. The measurements presented here provide a unique, consistent database which can be used to further analyze the processes involved in the spatial and temporal variations of NO emissions.

Despite their overwhelming influence on (1) variations in the physiological activity of vegetation (2) plant phenology, and (3) potential competitive interactions between coexisting species, comprehensive information on water relations and soil water uptake patterns is still lacking for grasses and shrubs in African humid savannas. Over 2 years, seasonal variations in plant water status were measured with the pressure bomb technique for two deciduous shrub species (Cussonia barteri and Crossopteryx febrifuga) and a perennial grass species (Hyparrhenia diplandra) in a humid savanna. Concurrently, soil moisture was surveyed with the neutron probe technique in grassy areas and under shrub clumps. Absence of transpiration at dawn was assessed by surveys of the leaf water isotopic signal, and plant water relations were characterized during rainy and dry periods with the pressure-volume curve technique. The seasonal changes in predawn and minimum shoot water potentials were pronounced for the grass species and the shrub C. febrifuga, but weak for the shrub C. barteri. Relationships between plant and soil water status showed that both the grass species and the shrub C. febrifuga did not maintain high water potentials when soil moisture in the upper soil layers decreased. In contrast, the shrub C. barteri exhibited a predawn water potential around −0.5 MPa when the 0 to 60-cm soil layer was at the permanent wilting point. During the early dry season, leaf dehydration was observed for C. febrifuga, but not for C. barteri. We conclude that the grass H. diplandra and the shrub C. febrifuga had very poor and limited access to deep soil layers (below 60 cm), respectively while the shrub C. barteri was able to access this water resource. The relatively low osmotic potential and tissue elasticity measured for C. febrifuga as compared to C. barteri were consistent with the use of water from upper soil layers by this species, since these characteristics could help water withdrawal and promote turgor maintenance at lower shoot water potentials. Different water uptake patterns could explain the contrasted leaf shedding patterns reported for the two shrub species. These results emphasize that the water economy of deciduous shrub species of African humid savannas can differ significantly and that both grasses and some shrub species acquire water from the upper soil layers even during dry spells. These findings will aid understanding of interspecific competition and modelling ecosystem function in this kind of humid savanna.

We demonstrate high-speed silicon modulators based on carrier depletion in interleaved pn junctions fabricated on 300 mm-SOI wafers using CMOS foundry facilities. 950 µm-long Mach Zehnder (MZ) and ring resonator (RR) modulator with a 100 µm radius, were designed, fabricated and characterized. 40 Gbit/s data transmission has been demonstrated for both devices. The MZ modulator exhibited a high extinction ratio of 7.9 dB with only 4 dB on-chip losses at the operating point.

Most savanna water balance models assume water partitioning between grasses and shrubs in a two-layer hypothesis, but this hypothesis has not been tested for humid savanna environments. Spatial partitioning of soil water between grasses and shrubs was investigated in a West African humid savanna by comparing the isotopic composition (oxygen-18 and deuterium) of soil water and plant stem water during rainy and dry conditions. Both grass and shrub species acquire most of their water from the top soil layer during both rainy and dry periods. A shift of water uptake pattern towards deeper horizons was observed only at the end of the dry season after shrub defoliation. The mean depth of water uptake, as determined by the isotopic signature of stem water, was consistent with grass and shrub root profiles and with changes in soil water content profiles as surveyed by a neutron probe. This provides evidence for potentially strong competition between shrubs and grasses for soil water in these humid savannas. Limited nutrient availability may explain these competitive interactions. These results enhance our understanding of shrub-grass interactions, and will contribute to models of ecosystem functioning in humid savannas.

Electroabsorption spectroscopy of well-identified index-defined semiconducting carbon nanotubes is reported. The measurement of high definition electroabsorption spectra allows direct indexation with unique nanotube chirality. Results show that at least for a limited range of diameters, electroabsorption is directly proportional to the exciton binding energy of nanotubes. Electroabsorption is a powerful technique which directly probes into carbon nanotube excitonic states, and may become a useful tool for in situ study of excitons in future nanotube-based photonic devices such as electroabsorption modulators.

International audience; Enhancement of soil nitrogen (N) cycling by grazing has been observed in many grassland ecosystems. However, whether grazing affects the activity only of the key microbial functional groups driving soil N dynamics or also affects the size (cell number) and/or composition of these groups remains largely unknown. We studied the enzyme activity, size, and composition of five soil microbial communities (total microbial and total bacterial communities, and three functional groups driving N dynamics: nitrifiers, denitrifiers, and free N-2 fixers) in grassland sites experiencing contrasting sheep grazing regimes (one light grazing [LG] site and one intensive grazing [IG] site) at two topographical locations. Enzyme activity was determined by potential carbon mineralization, nitrification, denitrification, and N-2 fixation assays. The size of each community (except N-2 fixers) was measured by the most-probable-number technique. The composition of the total soil microbial community was characterized by phospholipid fatty acid analysis (PLFA), and the genetic structure of the total bacterial community was assessed by ribosomal intergenic spacer analysis. The genetic structures of the ammonia-oxidizing, nitrate-reducing, and N-2- fixing communities were characterized by polymerase chain reaction and restriction fragment length polymorphism (PCR-RFLP) or by polymerase chain reaction and denaturing gradient gel electrophoresis (PCR-DGGE) targeting group-specific genes. Greater enzyme activities, particularly for nitrification, were observed in IG than in LG sites at both topographical locations. The numbers of heterotrophs, nitrifiers, and denitrifiers were higher in IG than in LG sites at both topographical locations. The amplitude of changes in community size was higher than that of community enzyme activity. Phospholipid and nucleic acid analyses showed that the composition/structure of all the communities, except nitrate reducers, differed between IG and LG sites at both locations. For each community, changes in activity were correlated with changes in the occurrence of a few individual PLFAs or DNA fragments. Our results thus indicate that grazing enhances the activity of soil microbial communities but also concurrently induces changes in the size and composition/structure of these communities on the sites studied. Although the generality of our conclusions should be tested in other systems, these results are of major importance for predicting the effects of future disturbances or changed grazing regimes on the functioning of grazed ecosystems.