Processus de stockage du carbone dans les sols mésiques en pessière à mousses au Québec

Les forêts boréales renferment d'importantes quantités de carbone organique, principalement dans leurs sols. Elles constituent ainsi une composante critique du cycle du carbone terrestre et jouent, par là-même, un rôle de premier plan dans la régulation du climat planétaire. Accroître les stocks de carbone dans les écosystèmes terrestres, et particulièrement dans les sols des forêts boréales, est une des stratégies proposées pour atténuer le réchauffement climatique à moindre coût. L'amplitude et la direction du développement des réservoirs de carbone du sol qui surviendra en lien avec des modifications environnementales, induits par le réchauffement climatique, sont néanmoins incertaines. Les sols pourraient accumuler davantage de carbone avec l'accroissement de la productivité de l'écosystème, ce qui améliorerait la chimie de l'atmosphère et atténuerait le réchauffement climatique. A l'inverse, si la décomposition du carbone par les microorganismes du sol est stimulée avec l'augmentation des températures, les sols pourraient accélérer le réchauffement climatique en retournant du carbone vers l'atmosphère. Comprendre les variabilités spatiale et temporelle des stocks de carbone dans les sols, ainsi que les processus qui régissent sa séquestration à long terme apparaissent ainsi être des clés essentielles pour adapter des pratiques d'aménagement du territoire raisonnées pour participer aux efforts d'atténuation du réchauffement climatique. L'objectif général de nos travaux était ainsi d'approfondir notre compréhension des processus de stockage du carbone dans les sols forestiers, avec l'étude de cas des sols à drainage modéré de la pessière à mousses au Québec. En se basant sur l'échantillonnage des sols dans 72 sites d'étude, répartis le long d'une chronoséquence après feu (de 2 à 314 ans) et dans un vaste territoire caractérisé par une variabilité climatique, nos travaux visaient à déterminer les principaux facteurs de contrôle et les mécanismes rattachés aux processus de stockage du carbone dans les sols. Pour chacun des sites étudiés, nous avons analysé la quantité et la qualité des réservoirs de carbone du sol. La quantité de carbone du sol à 1 'échelle du peuplement a été calculée à partir des données récoltées au terrain et de la concentration des échantillons en carbone déterminée en laboratoire. La qualité du carbone a été déterminée avec une méthode d'incubation des sols en atmosphère contrôlée pour évaluer la réactivité biologique du carbone, et avec l'hydrolyse acide des échantillons pour évaluer la récalcitrance du carbone. Nous avons aussi déterminé certaines propriétés physico-chimiques des sols par des analyses standardisées. D'abord, nos résultats ont montré que seules les variables climatiques liées à la disponibilité en eau, et non pas aux températures, n'exerçaient qu'un contrôle indirect sur l'accumulation du carbone dans le sol. L'effet indirect du climat s'exprimait au travers de l'influence directe des variables liées à la disponibilité en eau sur la strate des mousses, qui elle-même avait une influence directe sur l'accumulation ou indirecte sur la réactivité biologique du carbone dans le sol. Le climat influençait aussi la complexation organométallique. Le temps écoulé depuis le dernier feu était le principal déterminant de l'accumulation de carbone dans l'horizon organique développé en surface du sol. Le temps depuis le dernier feu déterminait aussi le stock de carbone dans le sol minéral et la réactivité biologique du carbone dans l'horizon organique au travers de son influence sur le pH du sol. En limitant l'activité microbienne, le pH du sol a été identifié comme étant un facteur majeur de contrôle direct du stock de carbone dans le sol minéral, de la réactivité biologique du carbone dans 1 'horizon organique et dans le sol minéral. Le pH du sol modulait aussi les processus de complexation organométallique et influençait la quantité d'aluminium échangeable. Or la complexation organométallique influençait directement le stock et la réactivité biologique du carbone et l'aluminium échangeable influençait directement la réactivité biologique du carbone dans le sol minéral. Les résultats de la thèse remettent en question le postulat de base utilisé dans les exercices de modélisation pour projeter 1 'évolution du climat futur, fondé sur la dépendance de la décomposition microbienne aux températures. Dans leur ensemble, nos travaux montrent que la dynamique du carbone organique du sol est contrôlée par des interactions complexes entre plusieurs facteurs bio-physicochimiques et que les conditions physico-chimiques du sol, tel que l'acidité, propres aux forêts anciennes sont favorables à l'accumulation du carbone dans le sol. Sur la base de nos résultats et pour séquestrer davantage de carbone en forêt boréale, nous préconisons de maintenir des attributs de forêts anciennes au sein du territoire, tout en conservant les mécanismes responsables de la séquestration de carbone dans les sols. Pour ce faire, il faudrait créer davantage d'aires naturelles protégées, augmenter la durée des rotations et adopter les coupes partielles dans les pratiques d'aménagement sylvicoles

Tubulin tyrosination regulates synaptic function and is disrupted in Alzheimer's disease

: Microtubules play fundamental roles in the maintenance of neuronal processes and in synaptic function and plasticity. While dynamic microtubules are mainly composed of tyrosinated tubulin, long-lived microtubules contain detyrosinated tubulin, suggesting that the tubulin tyrosination/detyrosination cycle is a key player in the maintenance of microtubule dynamics and neuronal homeostasis, conditions which go awry in neurodegenerative diseases. In the tyrosination/detyrosination cycle, the C-terminal tyrosine of α-tubulin is removed by tubulin carboxypeptidases and re-added by tubulin tyrosine ligase. Here we show that tubulin tyrosine ligase hemizygous mice exhibit decreased tyrosinated microtubules, reduced dendritic spine density, and both synaptic plasticity and memory deficits. We further report decreased tubulin tyrosine ligase expression in sporadic and familial Alzheimer's disease, and reduced microtubule dynamics in human neurons harboring the familial APP-V717I mutation. Finally, we show that synapses visited by dynamic microtubules are more resistant to oligomeric amyloid β peptide toxicity and that expression of tubulin tyrosine ligase, by restoring microtubule entry into spines, suppresses the loss of synapses induced by amyloid β peptide. Together, our results demonstrate that a balanced tyrosination/detyrosination tubulin cycle is necessary for the maintenance of synaptic plasticity, is protective against amyloid β peptide-induced synaptic damage, and that this balance is lost in Alzheimer's disease, providing evidence that defective tubulin retyrosination may contribute to circuit dysfunction during neurodegeneration in Alzheimer's disease

Mutations in TUBG1, DYNC1H1, KIF5C and KIF2A cause malformations of cortical development and microcephaly.

International audienceThe genetic causes of malformations of cortical development (MCD) remain largely unknown. Here we report the discovery of multiple pathogenic missense mutations in TUBG1, DYNC1H1 and KIF2A, as well as a single germline mosaic mutation in KIF5C, in subjects with MCD. We found a frequent recurrence of mutations in DYNC1H1, implying that this gene is a major locus for unexplained MCD. We further show that the mutations in KIF5C, KIF2A and DYNC1H1 affect ATP hydrolysis, productive protein folding and microtubule binding, respectively. In addition, we show that suppression of mouse Tubg1 expression in vivo interferes with proper neuronal migration, whereas expression of altered γ-tubulin proteins in Saccharomyces cerevisiae disrupts normal microtubule behavior. Our data reinforce the importance of centrosomal and microtubule-related proteins in cortical development and strongly suggest that microtubule-dependent mitotic and postmitotic processes are major contributors to the pathogenesis of MCD

Jet energy measurement with the ATLAS detector in proton-proton collisions at root s=7 TeV

The jet energy scale and its systematic uncertainty are determined for jets measured with the ATLAS detector at the LHC in proton-proton collision data at a centre-of-mass energy of √s = 7TeV corresponding to an integrated luminosity of 38 pb-1. Jets are reconstructed with the anti-kt algorithm with distance parameters R=0. 4 or R=0. 6. Jet energy and angle corrections are determined from Monte Carlo simulations to calibrate jets with transverse momenta pT≥20 GeV and pseudorapidities {pipe}η{pipe}<4. 5. The jet energy systematic uncertainty is estimated using the single isolated hadron response measured in situ and in test-beams, exploiting the transverse momentum balance between central and forward jets in events with dijet topologies and studying systematic variations in Monte Carlo simulations. The jet energy uncertainty is less than 2. 5 % in the central calorimeter region ({pipe}η{pipe}<0. 8) for jets with 60≤pT<800 GeV, and is maximally 14 % for pT<30 GeV in the most forward region 3. 2≤{pipe}η{pipe}<4. 5. The jet energy is validated for jet transverse momenta up to 1 TeV to the level of a few percent using several in situ techniques by comparing a well-known reference such as the recoiling photon pT, the sum of the transverse momenta of tracks associated to the jet, or a system of low-pT jets recoiling against a high-pT jet. More sophisticated jet calibration schemes are presented based on calorimeter cell energy density weighting or hadronic properties of jets, aiming for an improved jet energy resolution and a reduced flavour dependence of the jet response. The systematic uncertainty of the jet energy determined from a combination of in situ techniques is consistent with the one derived from single hadron response measurements over a wide kinematic range. The nominal corrections and uncertainties are derived for isolated jets in an inclusive sample of high-pT jets. Special cases such as event topologies with close-by jets, or selections of samples with an enhanced content of jets originating from light quarks, heavy quarks or gluons are also discussed and the corresponding uncertainties are determined. © 2013 CERN for the benefit of the ATLAS collaboration

Measurement of the inclusive and dijet cross-sections of b-jets in pp collisions at sqrt(s) = 7 TeV with the ATLAS detector

The inclusive and dijet production cross-sections have been measured for jets containing b-hadrons (b-jets) in proton-proton collisions at a centre-of-mass energy of sqrt(s) = 7 TeV, using the ATLAS detector at the LHC. The measurements use data corresponding to an integrated luminosity of 34 pb^-1. The b-jets are identified using either a lifetime-based method, where secondary decay vertices of b-hadrons in jets are reconstructed using information from the tracking detectors, or a muon-based method where the presence of a muon is used to identify semileptonic decays of b-hadrons inside jets. The inclusive b-jet cross-section is measured as a function of transverse momentum in the range 20 < pT < 400 GeV and rapidity in the range |y| < 2.1. The bbbar-dijet cross-section is measured as a function of the dijet invariant mass in the range 110 < m_jj < 760 GeV, the azimuthal angle difference between the two jets and the angular variable chi in two dijet mass regions. The results are compared with next-to-leading-order QCD predictions. Good agreement is observed between the measured cross-sections and the predictions obtained using POWHEG + Pythia. MC@NLO + Herwig shows good agreement with the measured bbbar-dijet cross-section. However, it does not reproduce the measured inclusive cross-section well, particularly for central b-jets with large transverse momenta.Comment: 10 pages plus author list (21 pages total), 8 figures, 1 table, final version published in European Physical Journal

Measurement of the polarisation of W bosons produced with large transverse momentum in pp collisions at sqrt(s) = 7 TeV with the ATLAS experiment

This paper describes an analysis of the angular distribution of W->enu and W->munu decays, using data from pp collisions at sqrt(s) = 7 TeV recorded with the ATLAS detector at the LHC in 2010, corresponding to an integrated luminosity of about 35 pb^-1. Using the decay lepton transverse momentum and the missing transverse energy, the W decay angular distribution projected onto the transverse plane is obtained and analysed in terms of helicity fractions f0, fL and fR over two ranges of W transverse momentum (ptw): 35 < ptw < 50 GeV and ptw > 50 GeV. Good agreement is found with theoretical predictions. For ptw > 50 GeV, the values of f0 and fL-fR, averaged over charge and lepton flavour, are measured to be : f0 = 0.127 +/- 0.030 +/- 0.108 and fL-fR = 0.252 +/- 0.017 +/- 0.030, where the first uncertainties are statistical, and the second include all systematic effects.Comment: 19 pages plus author list (34 pages total), 9 figures, 11 tables, revised author list, matches European Journal of Physics C versio

Observation of a new chi_b state in radiative transitions to Upsilon(1S) and Upsilon(2S) at ATLAS

The chi_b(nP) quarkonium states are produced in proton-proton collisions at the Large Hadron Collider (LHC) at sqrt(s) = 7 TeV and recorded by the ATLAS detector. Using a data sample corresponding to an integrated luminosity of 4.4 fb^-1, these states are reconstructed through their radiative decays to Upsilon(1S,2S) with Upsilon->mu+mu-. In addition to the mass peaks corresponding to the decay modes chi_b(1P,2P)->Upsilon(1S)gamma, a new structure centered at a mass of 10.530+/-0.005 (stat.)+/-0.009 (syst.) GeV is also observed, in both the Upsilon(1S)gamma and Upsilon(2S)gamma decay modes. This is interpreted as the chi_b(3P) system.Comment: 5 pages plus author list (18 pages total), 2 figures, 1 table, corrected author list, matches final version in Physical Review Letter

Search for displaced vertices arising from decays of new heavy particles in 7 TeV pp collisions at ATLAS

We present the results of a search for new, heavy particles that decay at a significant distance from their production point into a final state containing charged hadrons in association with a high-momentum muon. The search is conducted in a pp-collision data sample with a center-of-mass energy of 7 TeV and an integrated luminosity of 33 pb^-1 collected in 2010 by the ATLAS detector operating at the Large Hadron Collider. Production of such particles is expected in various scenarios of physics beyond the standard model. We observe no signal and place limits on the production cross-section of supersymmetric particles in an R-parity-violating scenario as a function of the neutralino lifetime. Limits are presented for different squark and neutralino masses, enabling extension of the limits to a variety of other models.Comment: 8 pages plus author list (20 pages total), 8 figures, 1 table, final version to appear in Physics Letters

Single hadron response measurement and calorimeter jet energy scale uncertainty with the ATLAS detector at the LHC

The uncertainty on the calorimeter energy response to jets of particles is derived for the ATLAS experiment at the Large Hadron Collider (LHC). First, the calorimeter response to single isolated charged hadrons is measured and compared to the Monte Carlo simulation using proton-proton collisions at centre-of-mass energies of sqrt(s) = 900 GeV and 7 TeV collected during 2009 and 2010. Then, using the decay of K_s and Lambda particles, the calorimeter response to specific types of particles (positively and negatively charged pions, protons, and anti-protons) is measured and compared to the Monte Carlo predictions. Finally, the jet energy scale uncertainty is determined by propagating the response uncertainty for single charged and neutral particles to jets. The response uncertainty is 2-5% for central isolated hadrons and 1-3% for the final calorimeter jet energy scale.Comment: 24 pages plus author list (36 pages total), 23 figures, 1 table, submitted to European Physical Journal

Measurement of the top quark-pair production cross section with ATLAS in pp collisions at \sqrt{s}=7\TeV

A measurement of the production cross-section for top quark pairs(\ttbar) in $pp$ collisions at \sqrt{s}=7 \TeV is presented using data recorded with the ATLAS detector at the Large Hadron Collider. Events are selected in two different topologies: single lepton (electron $e$ or muon $\mu$) with large missing transverse energy and at least four jets, and dilepton ($ee$, $\mu\mu$ or $e\mu$) with large missing transverse energy and at least two jets. In a data sample of 2.9 pb-1, 37 candidate events are observed in the single-lepton topology and 9 events in the dilepton topology. The corresponding expected backgrounds from non-\ttbar Standard Model processes are estimated using data-driven methods and determined to be $12.2 \pm 3.9$ events and $2.5 \pm 0.6$ events, respectively. The kinematic properties of the selected events are consistent with SM \ttbar production. The inclusive top quark pair production cross-section is measured to be \sigmattbar=145 \pm 31 ^{+42}_{-27} pb where the first uncertainty is statistical and the second systematic. The measurement agrees with perturbative QCD calculations.Comment: 30 pages plus author list (50 pages total), 9 figures, 11 tables, CERN-PH number and final journal adde