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Amazonian forests continuously accumulate carbon (C) in biomass and in soil, representing a carbon sink of 0.420.65 GtC yr(-1). In recent decades, more than 15% of Amazonian forests have been converted into pastures, resulting in net C emissions (similar to 200 tC ha(-1)) due to biomass burning and litter mineralization in the first years after deforestation. However, little is known about the capacity of tropical pastures to restore a C sink. Our study shows in French Amazonia that the C storage observed in native forest can be partly restored in old (>= 24 year) tropical pastures managed with a low stocking rate (+/- 1 LSU ha(-1)) and without the use of fire since their establishment. A unique combination of a large chronosequence study and eddy covariance measurements showed that pastures stored between >= 1.27 +/- 0.37 and -5.31 +/- 2.08 tC ha(-1) yr(-1) while the nearby native forest stored -3.31 +/- 0.44 tC ha(-1) yr(-1). This carbon is mainly sequestered in the humus of deep soil layers (20-100 cm), whereas no C storage was observed in the 0- to 20-cm layer. C storage in C4 tropical pasture is associated with the installation and development of C3 species, which increase either the input of N to the ecosystem or the C: N ratio of soil organic matter. Efforts to curb deforestation remain an obvious priority to preserve forest C stocks and biodiversity. However, our results show that if sustainable management is applied in tropical pastures coming from deforestation (avoiding fires and overgrazing, using a grazing rotation plan and a mixture of C3 and C4 species), they can ensure a continuous C storage, thereby adding to the current C sink of Amazonian forests.

The respiratory release of CO2 from soils is a major determinant of the global carbon cycle. It is traditionally considered that this respiration is an intracellular metabolism consisting of complex biochemical reactions carried out by numerous enzymes and co-factors. Here we show that the endoenzymes released from dead organisms are stabilised in soils and have access to suitable substrates and co-factors to permit function. These enzymes reconstitute an extracellular oxidative metabolism (EXOMET) that may substantially contribute to soil respiration (16 to 48% of CO2 released from soils in the present study). EXOMET and respiration from living organisms should be considered separately when studying effects of environmental factors on the C cycle because EXOMET shows specific properties such as resistance to high temperature and toxic compounds.

The financial transaction tax (FTT) is a policy idea with a long history that, in the wake of the global financial crisis, has attracted renewed interest in some quarters. This article examines the evidence of the impact of an FTT on market quality and explores a few of the practical issues surrounding the implementation of an FTT. Proponents argue that an FTT will generate substantial tax revenues and reduce market volatility. The majority of the empirical evidence, however, supports the arguments of opponents of the tax who assert that an FTT reduces volume and liquidity and increases volatility. In addition, there are numerous challenges in implementing an FTT, which may reduce the intended revenues. Whether an FTT is beneficial hinges on its effect on market quality and its ability to raise revenues. However, there are many unanswered questions regarding its design.

International audience; OBJECTIVE AND METHODS: This study deals with cardiovascular autonomic neuropathy (CAN) in type 1 diabetic patients and its association with other complications. We searched for CAN in 684 patients (age, 47 +/- 12 years; diabetes duration, 22 +/- 11 years) by cardiovascular responses to deep breathing and standing. Patients considered as positive had laboratory evaluation: "Ewing" tests (deep breathing, Valsalva, stand test, hand grip); heart rate variability (HRV) [low frequency (LF) and high frequency (HF) power] and spontaneous baroreflex slope (SBS). Logistic regression was used to identify the combination of patient characteristics, including other complications, most associated with CAN severity according to Ewing Score (ES 0-5). RESULTS: 66.2% presented no significant abnormality (ES 0-0.5), 21.5 % had mild abnormalities (ES 1-2), and 12.3% had confirmed autonomic failure (ES > 2). Decrease in LF, HF and SBS was highly correlated to CAN severity. In the stepwise regression, age, retinopathy, nephropathy, bladder dysfunction, erectile dysfunction, peripheral neuropathy and hypertension remained correlated with CAN, whereas digestive neuropathy, BMI and HbA1c were excluded. Despite a small number of events, we found a significant association between coronary disorders and CAN severity. CONCLUSIONS: Simple bedside tests can detect CAN. HRV and SBS provide additional elements on CAN severity. Diabetes duration did not discriminate sufficiently patients with CAN. The association with retinopathy is in favor of the role of poor glycemic control in CAN development. This study shows the interest of CAN detection and the need to look for extracardiac autonomic neuropathy and silent myocardial ischemia in patients with confirmed CAN.

We find strong evidence of a funding risk premium in the cross-section of asset returns. Our estimate for the price of funding risk is robust across Treasury bonds, corporate bonds, equities, and hedge funds. Funding shocks pose a risk to investors because they exacerbate the illiquidity and volatility of securities, increase the dispersions of asset illiquidity and volatility, and decrease contemporaneous returns. Our price-of-risk estimates are also robust to using mimicking portfolio returns, alternative portfolio sorts, traditional test assets, monthly returns or quarterly returns. Funding shocks are not subsumed by common proxies for market-wide illiquidity or dealers' balance sheet risk.

Purpose: Germline mutations in genes encoding mitochondrial succinate dehydrogenase (SDH) are found in patients with paragangliomas, pheochromocytomas, gastrointestinal stromal tumors, and renal cancers. SDH inactivation leads to a massive accumulation of succinate, acting as an oncometabolite and which levels, assessed on surgically resected tissue are a highly specific biomarker of SDHx-mutated tumors. The aim of this study was to address the feasibility of detecting succinate in vivo by magnetic resonance spectroscopy. Experimental Design: A pulsed proton magnetic resonance spectroscopy (1H-MRS) sequence was developed, optimized, and applied to image nude mice grafted with Sdhb−/− or wild-type chromaffin cells. The method was then applied to patients with paraganglioma carrying (n = 5) or not (n = 4) an SDHx gene mutation. Following surgery, succinate was measured using gas chromatography/mass spectrometry, and SDH protein expression was assessed by immunohistochemistry in resected tumors. Results: A succinate peak was observed at 2.44 ppm by 1H-MRS in all Sdhb−/−-derived tumors in mice and in all paragangliomas of patients carrying an SDHx gene mutation, but neither in wild-type mouse tumors nor in patients exempt of SDHx mutation. In one patient, 1H-MRS results led to the identification of an unsuspected SDHA gene mutation. In another case, it helped define the pathogenicity of a variant of unknown significance in the SDHB gene. Conclusions: Detection of succinate by 1H-MRS is a highly specific and sensitive hallmark of SDHx mutations. This noninvasive approach is a simple and robust method allowing in vivo detection of the major biomarker of SDHx-mutated tumors. Clin Cancer Res; 22(5); 1120–9. ©2015 AACR.

Soil organic matter (SOM) models are based on the equation dC/dt = −kC which states that the decomposition rate of a particular carbon (C) pool is proportional to the size of the pool and the decomposition constant k. However, this equation does not adequately describe the decomposition of recalcitrant SOM compounds. We present an alternative theory of SOM dynamics in which SOM decay rate is controlled by the size and the diversity of microbe populations and by the supply of energy-rich litter compounds. We show that the SOM pool does not necessarily reach equilibrium and may increase continuously, which explains how SOM can accumulate over thousands of years. However, the simulated SOM accumulation involves the sequestration of available nutrients. How can plants persist? This question is explored with two models that couple the C cycle with a limiting nutrient. The first model considers a single type of microbe whereas the second includes two functional types in competition for energy and nutrient acquisition. The condition for plant persistence is the presence of these two competing microbial types.

It is commonly predicted that the intensity of primary production and soil carbon (C) content are positively linked. Paradoxically, many long-term field observations show that although plant litter is incorporated to soil in large quantities, soil C content does not necessarily increase. These results suggest that a negative relationship between C input and soil C conservation exists. Here, we demonstrate in controlled conditions that the supply of fresh C may accelerate the decomposition of soil C and induce a negative C balance. We show that soil C losses increase when soil microbes are nutrient limited. Results highlight the need for a better understanding of microbial mechanisms involved in the complex relationship between C input and soil C sequestration. We conclude that energy available to soil microbes and microbial competition are important determinants of soil C decomposition.

Two mechanisms have been hypothesized to explain the priming effect (PE), that is, the acceleration of soil C decomposition by fresh C input to soil. First, extracellular enzymes that are produced to decompose fresh C by fresh C specialized microbes may be partly efficient in degrading soil C. Second, depending on the competition with fresh C specialized microbes, part of the fresh C may be absorbed by soil C decomposing microbes. This absorption increases the populations of soil C decomposing microbe and hence the decomposition rate of soil C. The PE was quantified in a savannah soil amended with 13 C-labeled cellulose at a rate of 495 mg C kg -1 . Cellulase was applied to the soil at a rate of 30 000 units kg -1 soil to quantify the contribution of cellulase to the PE of cellulose. The rate of soil C decomposition increased by 55% with cellulose addition leading to PE of 234 mg C kg -1 . Cellulose released 32 mg C-glucose from soil cellulose, representing only 14% of the PE. This indicated that the decomposition of soil C required the production of specific enzymes, and that the PE resulted from the stimulation of microbes able to provide soil C decomposing enzymes. Our results also showed that cellulose stimulated at least two types of microbes: soil C decomposing microbes that may also use cellulose and cellulose specialized microbes that exclusively decompose cellulose. These results indicate that the PE depends on microbial competition. We estimated that, following cellulose addition, soil humus stock was depleted by 174 mg C kg -1 .

It is generally accepted that the low quality of soil carbon limits the amount of energy available for soil microorganisms, and in turn the rate of soil carbon mineralization. The priming effect, i.e. the increase in soil organic matter (SOM) decomposition rate after fresh organic matter input to soil, is often supposed to result from a global increase in microbial activity due to the higher availability of energy released from the decomposition of fresh organic matter. Work to date, however, suggests that supply of available energy induces no effect on SOM mineralization. The mechanisms of the priming effect are much more complex than commonly believed. The objective of this review was to build a conceptual model of the priming effect based on the contradictory results available in the literature adopting the concept of nutritional competition. After fresh organic matter input to soils, many specialized microorganisms grow quickly and only decompose the fresh organic matter. We postulated that the priming effect results from the competition for energy and nutrient acquisition between the microorganisms specialized in the decomposition of fresh organic matter and those feeding on polymerised SOM.

We greatly expand the space of tractable term-structure models. We consider one example that combines positive yields with rich volatility and correlation dynamics. Bond prices are expressed in closed form and estimation is straightforward.

WOS:000346545800018; International audience; The presence of plants induces strong accelerations in soil organic matter (SOM) mineralization by stimulating soil microbial activity a phenomenon known as the rhizosphere priming effect (RPE). The RPE could be induced by several mechanisms including root exudates, arbuscular mycorrhizal fungi (AMF) and root litter. However the contribution of each of these to rhizosphere priming is unknown due to the complexity involved in studying rhizospheric processes. In order to determine the role of each of these mechanisms, we incubated soils enclosed in nylon meshes that were permeable to exudates, or exudates & AMF or exudates, AMP and roots under three grassland plant species grown on sand. Plants were continuously labeled with C-13 depleted CO2 that allowed distinguishing plant-derived CO2 from soil-derived CO2. We show that root exudation was the main way by which plants induced RPE (58-96% of total RPE) followed by root litter. AMP did not contribute to rhizosphere priming under the two species that were significantly colonized by them i.e. Poa trivialis and Trifolium repens. Root exudates and root litter differed with respect to their mechanism of inducing RPE. Exudates induced RPE without increasing microbial biomass whereas root litter increased microbial biomass and raised the RPE mediating saprophytic fungi. The RPE efficiency (RPE/unit plant-C assimilated into microbes) was 3-7 times higher for exudates than for root litter. This efficiency of exudates is explained by a microbial allocation of fresh carbon to mineralization activity rather than to growth. These results suggest that root exudation is the main way by which plants stimulated mineralization of soil organic matter. Moreover, the plants through their exudates not only provide energy to soil microorganisms but also seem to control the way the energy is used in order to maximize soil organic matter mineralization and drive their own nutrient supply. (C) 2014 Elsevier Ltd. All rights reserved.

International audience; Integration of the priming effect (PE) in ecosystem models is crucial to better predict the consequences of global change on ecosystem carbon (C) dynamics and its feedbacks on climate. Over the last decade, many attempts have been made to model PE in soil and its underlying mechanisms. However, PE has not yet been incorporated into any ecosystem models. Here we build plant-soil models to explore how PE and microbial diversity influence soil-plant interactions and ecosystem C and nitrogen (N) dynamics in response to global change (elevated CO2 and increased atmospheric N deposition). Our results show that plant persistence, soil organic matter (SOM) accumulation and low N leaching in undisturbed ecosystems relies on a fine adjustment of microbial N mineralization to plant N uptake. This adjustment can be modeled in the SYMPHONY model by considering the destruction of SOM through PE and the interactions between two distinct microbial functional groups: SOM-decomposers and SOM-builders. After estimation of parameters, SYMPHONYprovided realistic predictions on forage production, soil C storage and N leaching for a permanent grassland. Consistent with recent observations, SYMPHONY predicted a CO2-induced modification of soil microbial communities leading to an intensification of SOM mineralization and a decrease in the soil C stock. SYMPHONY also indicated that atmospheric N deposition may stimulate SOM accumulation via changes in the structure and metabolic activities of microbial communities. Collectively, these results suggest that PE and the functional role of microbial diversity may be incorporated in current ecosystem models with a few additional parameters, improving accuracy of predictions.

Most central banks effect changes to their target or policy rate in discrete increments (e.g., multiples of 0.25%) following public announcements on scheduled dates. Still, for most applications, researchers rely on the assumption that the policy rate changes linearly with economic conditions and they do not distinguish between dates with and without scheduled announcements.

Theoretical risk factors underlying time-variations of risk premium across asset classes are typically unobservable or hard to measure by construction. Important examples include risk factors in Long Run Risk [LRR] structural models (Bansal and Yaron 2004) as well as stochastic volatility or jump intensities in reduced-form affine representations of stock returns (Duffie, Pan, and Singleton 2000). Still, we show that both classes of models predict that the term structure of risk-neutral variance should reveal these risk factors. Empirically, we use model-free measures and construct the ex-ante variance term structure from option prices. This reveals (spans) two risk factors that predict the bond premium and the equity premium, jointly. Moreover, we find that the same risk factors also predict the variance premium. This important contribution is consistent with theory and confirms that a small number of factors underlies common time-variations in the bond premium, the equity premium and the variance premium. Theory predicts that the term structure of higher-order risks can reveal the same factors. This is confirmed in the data. Strikingly, combining the information from the variance, skewness and kurtosis term structure can be summarized by two risk factors and yields similar level of predictability (i.e., R2s). This bodes well for our ability to bridge the gap between the macro-finance literature, which uses very few state variables, and valuations in option markets.

International audience; 1. It is important to study how evolution impacts on plant functional traits and to determine how this subsequently determines ecosystem functioning. We tackle this general issue by studying the evolution of plant strategies that affect mineralization through the chemical quality of their own litter and their position on the leaf economic spectrum. This spectrum allows us to classify all plants on single axis ranging from resource-acquisitive to resource-conservative strategies. 2. We build a spatially explicit and individual-based simulation model: individual plants grow in the cells of a lattice and the limiting nutrient is recycled locally in these cells. Individual plants may die and produce seeds that are dispersed. Mutants with different mineralization strategies appear stochastically. A trade-off is implemented between the rate of nutrient loss from plants and litter mineralization. 3. In the spatial-explicit model, plant capacity to increase mineralization evolves and reaches an evolutionary equilibrium in most cases. The evolved mineralization decreases with plant longevity, seed dispersal efficiency, spatial homogenization of mineral nutrient availability and inputs of mineral nutrient to the ecosystem. 4. The evolved mineralization strategies neither maximize plant biomass, nor minimize the availability of mineral nutrient or the stock of dead organic matter. The evolutionary and ecological impacts of nutrient enrichment on the stock of organic matter are different. 5. Synthesis. Our results suggest that plant mineralization strategy may evolve provided that the mineral resource is not fully shared by all individuals. Such an evolution modifies soil capacity to store organic carbon thereby being relevant in the context of the current climate change and global nutrient enrichment. Indeed, our model shows that evolutionary feedbacks of plants to nutrient enrichment are likely to differ from purely ecological feedbacks.

This article presents the results of the first survey conducted in Belgium about the interest and knowledge in astronomy. Two samples were studied, the public at large (667 questionnaires) and students (2589 questionnaires), but the results are generally similar in both samples. We evaluated people's interest, main information source, and attitudes towards astronomy, as well as their supposed and actual knowledge of the subject. The main conclusion is that, despite a poor self-confidence, people do know the basic astronomical concepts. However, that knowledge is not deeply rooted, as reasoning questions show well-spread misconceptions and/or misunderstandings., accepted for publication by Physics Education

13 pages; International audience; The turnover of organic matter in soil depends on the activity of microbial decomposers. However, little is known about how modifications of the diversity of soil microbial communities induced by fresh organic matter (FOM) inputs can regulate carbon cycling. Here, we investigated the decomposition of two 13C labeled crop residues (wheat and alfalfa) and the dynamics of the genetic structure and taxonomic composition of the soil bacterial communities decomposing 13C labeled FOM and native unlabeled soil organic matter (SOM), respectively. It was achieved by combining the stable isotope probing method with molecular tools (DNA genotyping and pyrosequencing of 16S rDNA). Although a priming effect (PE) was always induced by residue addition, its intensity increased with the degradability of the plant residue. The input of both wheat and alfalfa residues induced a rapid dynamics of FOM-degrading communities, corresponding to the stimulation of bacterial phyla which have been previously described as copiotrophic organisms. However, the dynamics and the identity of the bacterial groups stimulated depended on the residue added, with Firmicutes dominating in the wheat treatment and Proteobacteria dominating in the alfalfa treatment after 3 days of incubation. In both treatments, SOMdegrading communities were dominated by Acidobacteria, Verrucomicrobia, and Gemmatimonadetes phyla which have been previously described as oligotrophic organisms. An early stimulation of SOM-degrading populations mainly belonging to Firmicutes and Bacteroidetes groups was observed in the alfalfa treatment whereas no change occurred in the wheat treatment. Our findings support the hypothesis that the succession of bacterial taxonomic groups occurring in SOM- and FOMdegrading communities during the degradation process may be an important driver of the PE, and consequently of carbon dynamics in soil.

Background Diastolic dysfunction evaluation may be relevant for early diagnosis of hypertrophic cardiomyopathy (HCM) and subsequent risk assessment. The aim of our study was to assess phase contrast cardiac magnetic resonance (PCCMR) diastolic parameters obtained with a semi-automated method in relation to left ventricular (LV) remodeling and late gadolinium enhancement (LGE) in patients with HCM. Methods We studied 48 patients with HCM and 23 healthy volunteers matched for age, sex and body surface area (BSA). Mitral inflow and myocardial velocities were assessed using through plane 2D PC-CMR (VENC=150cm/s and 20cm/s respectively). Transmitral peak flow-rates (Ef and Af) and early E’ peak myocardial longitudinal velocity were obtained semi-automatically using CardFlow (INSERM U678). LV volumes, segmental thickness and mass were obtained from SSFP images. LGE volume was quantified semi-automatically using a 6 SD threshold. Results Peak myocardial longitudinal velocity E’ was significantly lower and E/E’ was significantly higher in patients with HCM compared with controls (Table). We found a linear relationship between decreased E’ and increased LV mass index (p

Cochrane and Piazzesi (2005) show that (i) lagged forward rates help predict bond returns and that (ii) modern Markovian dynamic term structure models (DTSMs) cannot match the evidence. We develop the family of Conditional Mean DTSMs where the dynamics depend on current yields and their history through a moving-average component. Our preferred Conditional Mean model combines one moving-average with the usual three Gaussian risk factors, closely matches the bond risk premium measured from predictive regressions and provides better forecasts of bond returns. Our framework nests Duffee (2011) models with a small “hidden” factor and our results compare favorably with his 5-factor model. Conditional Mean models are easier to estimate than state-space term structure based on Kalman estimates of latent factors.

WOS:000305771900009; International audience; Grazing or mowing is central to the management of grasslands and may alter mineralization of soil organic matter (SUM) and soil carbon (C) stocks. Some studies have shown grazing-induced reductions in total soil respiration suggesting decreases in SUM mineralization. However, it has also been suggested that grazing may increase SUM mineralization, based on observations of increased soluble C, microbial biomass and mineral nitrogen (N) in soil after clipping. No studies to date have directly measured SUM mineralization to determine the effects of grazing on SUM mineralization and the underlying mechanisms. We examined the effect of clipping on soil-derived CO2 efflux (Rs) for six gramineae and one leguminous species typical of temperate grasslands. Continuous C-13 labeling of monocultures coupled with a new method of directly measuring Rs and the rhizosphere priming effect (RPE i.e. Rs from planted soils minus respiration from bare soils) in perennial herbaceous plants was used. For a model species, Lolium perenne, the clipping effects on aboveground biomass production, mineral N and soluble C in soil, microbial biomass and microbial community composition were also quantified. We found that clipping decreased the RPE and Rs (SUM mineralization) within 48 hours for all the studied species. For Lolium perenne, this reduced SUM mineralization persisted for one month after clipping. Moreover, clipping reduced the production of aboveground biomass and the total N assimilated by the plants. These changes increased N availability in soil and induced shifts in the soil microbial community structure favoring gram positive bacteria (i16:0) over saprophytic fungi (18:2 omega 6). The strong correlation of fungi (18:2 omega 6) with Rs across treatments suggests that saprophytic fungi play a key role in SUM mineralization. In conclusion, our study shows that plant clipping decelerates SUM mineralization and induces shifts in microbial community structure, most likely as an indirect effect of clipping on plant N uptake. (C) 2012 Elsevier Ltd. All rights reserved.

The respiratory release of CO2 from soils is a major determinant of the global carbon cycle. It is traditionally considered that this respiration is an intracellular metabolism consisting of complex biochemical reactions carried out by numerous enzymes and co-factors. Here we show that the intracellular enzymes released from dead organisms are stabilized in soils and have access to suitable substrates and co-factors to permit function. These enzymes reconstitute an extracellular oxidative metabolism (Exomet) that may substantially contribute to soil respiration (16 to 48% of CO2 released from soils in the present study). Exomet and respiration from living organisms should be considered separately when studying effects of environmental factors on the C cycle because Exomet shows specific properties such as resistance to high temperature and toxics.

Gastroparesis is a chronic complication of diabetes the appearance of which is linked to prolonged exposure to chronic hyperglycaemia. Its physiopathology, its clinical expression, its diagnosis (particularly through gastric scintigraphy) and its treatment, notably by gastric electrical stimulation are specific.

Standard Gaussian macro-finance term structure models impose the Markov property: the conditional mean is a function of the risk factors. We relax this assumption parsimoniously, and consider models where yields are linear in the conditional mean (but not in the risk factors). To illustrate, if inflation is one of the factors, then yields should span expected inflation but not inflation. We confirm that model forecasts match the out-of-sample accuracy of survey forecasts. Second, expected and surprise yield changes can have opposite contemporaneous effects on expected inflation. We confirm the difference empirically. Third, the inflation survey forecasts and the inflation rate can be used consistently within the state equation. These three features are inconsistent with the Markov assumption. Our results hold for the US and for Canada, and the decomposition of nominal yields differs from that of the standard specification.

International audience; It is increasingly recognized that soil microbes have the ability to decompose old recalcitrant soil organic matter (SOM) by using fresh carbon as a source of energy, a phenomena called priming effect (PE). However, efforts to determine the consequences of this PE for soil carbon and nitrogen dynamics are in their early stage. Moreover, little is known about the microbial populations involved. Here we explore the consequences of PE for SOM dynamics and mineral nitrogen availability in a soil incubation experiment (161 days), combining the supply of dual-labeled (13C and 14C) cellulose and mineral nutrients. The microbial groups involved in PE were investigated using molecular fingerprinting techniques (FAMEs and B- and F-ARISA). We show that mean residence time of SOM pool controlled by the PE decreased from 3130 years in the subsoil, where the availability of fresh carbon is very low, to 17–39 years in the surface layer. This result suggests that the decomposition of this recalcitrant soil C pool is strictly dependent on the presence of fresh C and is not an energetically viable mean of accessing C for soil microbes. We also suggest that fungi are the predominant actors of cellulose decomposition and induced PE and they adjust their degradation activity to nutrient availability. The predominant role of fungi can be explained by their ability to grow as mycelium which allows them to explore soil space and mine large reserve of SOM. Finally, our results support the existence of a bank mechanism that regulates nutrient and carbon sequestration in soil: PE is low when nutrient availability is high, allowing sequestration of nutrients and carbon; in contrast, microbes release nutrients from SOM when nutrient availability is low. This bank mechanism may help to synchronize the availability of soluble nutrients to plant requirement and contribute to long-term SOM accumulation in ecosystems.

National audience; La connaissance de la rhizodéposition, transfert de carbone des racines vivantes des plantes vers le sol, est un verrou pour la prévision et la modélisation du stockage de carbone dans les sols. D’une part les flux souterrains entrant dans le sol sont difficiles à quantifier, d’autre part le devenir à long terme du carbone rhizodéposé est peu connu. Notre analyse des données bibliographiques obtenues par traçages isotopiques par le 14C indique que le flux de rhizodéposition s'élève en moyenne à 0,57 x la production nette de racines, avec une grande variabilité et une part d'incertitude assez élevée. Nous avons quantifié le flux de rhizodéposition dans quelques systèmes sols-plantes (sur des systèmes en blé, colza, maïs, tournesol, ray-grass, fétuque) au moyen de traçages isotopiques par le 13C des photoassimilats et enregistré des flux du même ordre, entre 0,30 et 0,55 x la production racinaire nette. Dans le cas du blé, les rhizodépôts sont en majorité des sucres, où prédomine le glucose. Ces sucres sont principalement engagés dans des polymères excrétés ou pariétaux. Les sucres simples (exsudats) libérés dans le sol sont biodégradés très vite, avec un rendement de conversion en carbone microbien est élevé. In fine, les rhizodépôts ont une rémanence à long terme dans les sols au moins égale à celle des autres produits végétaux. L'expression du priming effect (stimulation de la biodégradation des matières organiques du sol par l'apport de carbone dégradable) 202 J. Balesdent et al.Etude et Gestion des Sols, 18, 3, 2011 par les exsudats d'une part et par les litières racinaires d'autre part a été évaluée par des expériences de double marquage 13C 14C. Les composés complexes comme la cellulose induisent un "priming effect", au contraire des exsudats de bas poids moléculaire. Ce priming par les apports racinaires concerne les matières organiques profondes, dont la dégradation est limitée par la faible densité de microorganismes. Concernant la modélisation du carbone à long terme, la stratégie de modélisation proposée est, en l'absence d'informations plus précises, d'estimer le flux de rhizodéposition entre 0,30 et 0,60 (0,45 ± 0,15) x la production racinaire nette telle que mesurée par extraction et lavage des racines, et de l'allouer aux compartiments végétaux les plus biodégradables des modèles (compartiment DPM pour Roth C et META pour Century). La stratégie de modélisation de l'impact de second ordre des apports de carbone souterrain sur les matières organiques est plus complexe, non linéaire et nécessitera une modélisation plus prospective, de nouvelle génération, sur la base de stratégies écologiques. La contribution quantitative importante des racines aux matières organiques du sol nous amène à vivement déconseiller la récolte des parties souterraines des cultures, qui réduirait considérablement la qualité des sols à terme.

Specifications of the Federal Reserve target rate that have more realistic features mitigate in-sample over-fitting and are favored in the data.

International audience; The humid tropics have seen a considerable expansion in ruminant farming since the 1970s (at present 25% of the world’s ruminant stock). This situation is often criticized for its negative environmental impacts: deforestation, loss of biodiversity, greenhouse gas (GHG) production (carbon dioxide, methane, nitrous oxide). The increase of direct emissions from the agriculture sector (17%) mostly occur in emerging countries like Brazil (IPCC, 2007). Although ruminant production systems are viewed as a major source of GHG emissions (18% of global emissions according to FAO soil carbon (C) sequestration (SOC) the world’s permanent pastures could potentially offset up to 4% of the global GHG emissions (Soussana et al., 2009; Steinfeld et al., 2010). This research, started in 2010, aims at identifying the carbon dynamics in pastures and define the management practices likely to compensate a part of the C losses resulting from their establishment

pièce joint : uncorrected proof; International audience; The question of long-term carbon uptake by old-growth forests has lead to extensive debate between modelling and experimental communities in the past. Based on so-called “carbon pool models”, where the soil carbon is assumed to be distributed among different pools, and to decay according to a first-order kinetic with pool-specific turnover constants, large factions of the modelling community have put forward a strong case that there cannot be long-term uptake of carbon by ecosystems because there is a limit defined by the steady state where total input equals total efflux of carbon. However, this theoretical deduction from first-order kinetic pool models seems to contradict a number of observations where long-term carbon uptake has been perceived. In this chapter, however, we will show that the modelling view stated here is completely contingent on the first-order reaction kinetics paradigm, and that there exist both old and recent alternative model formulations predicting that, under certain conditions, soil carbon pools never reach a steady state. Hence, from a modelling point of view, there is no justification for excluding the possibility of long-term old-growth forest carbon uptake. Since several plausible model formulations currently exist, in particular new models that include the role of soil microbial limitations, we need initiatives and experimental designs that can distinguish between, and potentially exclude, the respective decomposition modelling paradigms. From the perspective of scientific theory, this example reminds us that models should never be confounded with the truth and that they must be critically examined and tested again and again. Otherwise models can turn into fairy tales.

P>1. Depending on grazing intensity, grasslands tend towards two contrasting systems that differ in terms of species diversity and soil carbon (C) storage. To date, effects of grazing on C cycling have mainly been studied in grasslands subject to constant grazing regimes, whereas little is known for grasslands experiencing a change in grazing intensity. Analysing the transition between C-storing and C-releasing grasslands under low- and high-grazing regimes, respectively, will help to identify key plant-soil interactions for C cycling. 2. The transition was studied in a mesocosm experiment with grassland monoliths submitted to a change in grazing after 14 years of constant high and low grazing. Plant-soil interactions were analysed by following the dynamics of plant and microbial communities, roots and soil organic matter fractions over 2 years. After disturbance change, mesocosms were continuously exposed to C-13-labelled CO2, which allowed us to trace both the incorporation of new litter C produced by a modified plant community in soil and the fate of old unlabelled litter C. 3. Changing disturbance intensity led to a cascade of events. After shift to high disturbance, photosynthesis decreased followed by a decline in root biomass and a change in plant community structure 1.5 months later. Those changes led to a decrease of soil fungi, a proliferation of Gram(+) bacteria and accelerated decomposition of old particulate organic C (< 6 months). At last, accelerated decomposition released plant available nitrogen and decreased soil C storage. Our results indicate that intensified grazing triggers proliferation of Gram(+) bacteria and subsequent faster decomposition by reducing roots adapted to low disturbance. 4. Synthesis. Plant communities exert control on microbial communities and decomposition through the activity of their living roots: slow-growing plants adapted to low disturbance reduce Gram(+) bacteria, decomposition of low and high quality litter, nitrogen availability and, thus, ingress of fast-growing plants. Our results indicate that grazing impacts on soil carbon storage by altering plant roots and their control on the soil microbial community and decomposition, and that these processes will foster decomposition and soil C loss in more productive and disturbed grassland systems. [References: 49]

The uncertainty around future changes to the Federal Reserve target rate varies over time. In our results, the main driver of uncertainty is a “path” factor signaling information about future policy actions, which is filtered from federal funds futures data.

Les auteurs presentent le modele « homoscedastique gamma », ou modele HG, ou la distribution des rendements est caracterisee par sa moyenne, sa variance et un parametre d'asymetrie. Dans le modele HG, l'ecart entre les volatilites observee et neutre a l'egard du risque est fonction de la prime de risque et du degre d'asymetrie : la prime de risque appliquee aux actions est en fait le double du ratio de l'ecart de volatilite a la mesure de l'asymetrie.

International audience; BACKGROUND: Soil ecology has produced a huge corpus of results on relations between soil organisms, ecosystem processes controlled by these organisms and links between belowground and aboveground processes. However, some soil scientists think that soil ecology is short of modelling and evolutionary approaches and has developed too independently from general ecology. We have tested quantitatively these hypotheses through a bibliographic study (about 23000 articles) comparing soil ecology journals, generalist ecology journals, evolutionary ecology journals and theoretical ecology journals. FINDINGS: We have shown that soil ecology is not well represented in generalist ecology journals and that soil ecologists poorly use modelling and evolutionary approaches. Moreover, the articles published by a typical soil ecology journal (Soil Biology and Biochemistry) are cited by and cite low percentages of articles published in generalist ecology journals, evolutionary ecology journals and theoretical ecology journals. CONCLUSION: This confirms our hypotheses and suggests that soil ecology would benefit from an effort towards modelling and evolutionary approaches. This effort should promote the building of a general conceptual framework for soil ecology and bridges between soil ecology and general ecology. We give some historical reasons for the parsimonious use of modelling and evolutionary approaches by soil ecologists. We finally suggest that a publication system that classifies journals according to their Impact Factors and their level of generality is probably inadequate to integrate "particularity" (empirical observations) and "generality" (general theories), which is the goal of all natural sciences. Such a system might also be particularly detrimental to the development of a science such as ecology that is intrinsically multidisciplinary.

The world's soils store more carbon than is present in biomass and in the atmosphere. New experimental evidence suggests that the delivery of fresh plant-derived carbon to the subsoil stimulates microbial activity and results in mineralization of thousand-year-old carbon. This supports the recent proposal that the conservation of organic carbon at depth results from a lack of energy for decomposers. This large pool of deep carbon is unlikely to respond to future changes in temperature if no fresh carbon is supplied, limiting the predicted positive feedback between global warming and soil organic carbon decomposition. The results imply that management practices that increase the distribution of fresh carbon along the soil profile (such as deep ploughing and the use of drought-resistant crops with extensive root systems) will stimulate loss of this ancient buried carbon. It is shown that the supply of fresh plant-derived carbon to deep soil layers stimulated the microbial mineralization of carbon that is thousands of years old, and is suggested that a lack of supply of fresh-carbon may prevent the decomposition of the organic carbon pool in deep soil layers in response to future changes in temperature. The world’s soils store more carbon than is present in biomass and in the atmosphere1. Little is known, however, about the factors controlling the stability of soil organic carbon stocks2,3,4 and the response of the soil carbon pool to climate change remains uncertain5,6. We investigated the stability of carbon in deep soil layers in one soil profile by combining physical and chemical characterization of organic carbon, soil incubations and radiocarbon dating. Here we show that the supply of fresh plant-derived carbon to the subsoil (0.6–0.8 m depth) stimulated the microbial mineralization of 2,567 ± 226-year-old carbon. Our results support the previously suggested idea7 that in the absence of fresh organic carbon, an essential source of energy for soil microbes, the stability of organic carbon in deep soil layers is maintained. We propose that a lack of supply of fresh carbon may prevent the decomposition of the organic carbon pool in deep soil layers in response to future changes in temperature. Any change in land use and agricultural practice that increases the distribution of fresh carbon along the soil profile1,8,9 could however stimulate the loss of ancient buried carbon.

The use of flight simulation tools to reduce the risk and required amount of flight testing for complex aerospace systems is a well recognized benefit of these tools. However, some special challenges arise when one attempts to extrapolate these benefits to very low-cost Uninhabited Aerial Vehicle (UAV) flight testing. These types of vehicles are characterized by a lack of payload capacity (and therefore limited in their capacity for additional flight test instrumentation or telemetry), limited baseline capabilities (processing, instrumentation, and telemetry), and by a lower marginal cost of added flight tests (less fuel, personnel, etc. is required per flight test, and a lower cost associated with a failure). In this paper, flight simulation architectures for system design and integration of low-cost UAVs are explored, and the development of a simulation tool for a low cost UAV described.

International audience; Laboratory incubations and a field experiment were carried out to determine the factors controlling N mineralization and nitrification, and to estimate the N losses (leaching and volatilization) in a sewage-sludge-amended Oxisol. Aerobically digested sludge was applied at a rate equivalent to 625 kg N/ha. The incubations were conducted as a factorial experiment of temperature (20 degreesC, 30 degreesC, and 40 degreesC) x soil water (-30 kPa and -1500 kPa) x sludge type [fresh (FS) water content 6230 g/kg; dry (DS) water content 50 g/kg]. The amount of nitrifiers was determined at the beginning and at the end of the experiment. The incubation lasted 24 weeks. The field study was conducted using bare microplots (4 m(2)) and consisted of a factorial experiment of sludge type (FS and DS) x sludge placement (subsurface, I+; surface, I-). Ammonia volatilization and the profile (0-0.90 m) of mineral N concentration were measured during 6 and 29 weeks after sludge application, respectively. After 24 weeks of incubation at 40 degreesC and -30 kPa, net N mineralization represented 52% (FS) and 71% (DS) of the applied N. The difference between sludges was due to an initial period of N immobilization in FS. Nitrification was more sensitive than N mineralization to changes in water potential and it was fully inhibited at -1500 kPa. The introduction of a large amount of nitrifiers with FS did not modify the rate of nitrification, which was principally limited by soil acidity (pH 4.9). Although N mineralization was greatest at 30 degreesC, nitrification increased continuously with temperature. Nitrogen mineralization from DS was well described by the double-exponential equation. For FS, the equation was modified to take into account an immobilization-remineralization period. Sludge placement significantly affected the soil NO3-/NH4+ ratio in the field: 16 for I+ and 1.5 for I-, after 11 weeks. In the I- treatment, nitrification of the released NH4+ was limited by soil moisture because of the dry soil mulch formed a few hours after rain. At the end of the field experiment, the estimated losses of N by leaching were 432 kg N/ha for I+ and 356 kg N/ha for I-. Volatilization was not detectable in the I+ microplots and it represented only 0.5% of the applied N in the I- microplots. The results showed that placement of sludge may be a valuable tool to decrease NO3- leaching by placing the sludge under unfavourable conditions for nitrification.

openaire: EC/H2020/819202/EU//SOS.aquaterra As global temperatures continue to rise, a key uncertainty of climate projections is the microbial decomposition of vast organic carbon stocks in thawing permafrost soils. Decomposition rates can accelerate up to fourfold in the presence of plant roots, and this mechanism—termed the rhizosphere priming effect—may be especially relevant to thawing permafrost soils as rising temperatures also stimulate plant productivity in the Arctic. However, priming is currently not explicitly included in any model projections of future carbon losses from the permafrost area. Here, we combine high-resolution spatial and depth-resolved datasets of key plant and permafrost properties with empirical relationships of priming effects from living plants on microbial respiration. We show that rhizosphere priming amplifies overall soil respiration in permafrost-affected ecosystems by ~12%, which translates to a priming-induced absolute loss of ~40 Pg soil carbon from the northern permafrost area by 2100. Our findings highlight the need to include fine-scale ecological interactions in order to accurately predict large-scale greenhouse gas emissions, and suggest even tighter restrictions on the estimated 200 Pg anthropogenic carbon emission budget to keep global warming below 1.5 °C.

It has been recently demonstrated that soil organic carbon (SOC) mineralization is supported by intracellular respiration of heterotrophic microorganisms and by non-cellular oxidative processes. However, little is known about the prevalence and drivers of non-cellular SOC mineralization among soils. In this study, untreated and gamma-irradiated soils sampled along a latitudinal gradient and exhibiting contrasted physicochemical properties were incubated in order to quantify potential non-cellular SOC mineralization and to identify its sensibility to soil properties. In sterilized and unsterilized soils, CO2 emission mirrored O2 consumption signifying the presence of several coupled redox reactions transferring electrons from organic C to intermediate acceptors and to O2. This supports the idea that non-cellular mineralization results from extracellular oxidative metabolisms catalyzed by soil enzymes and/or abiotic catalysts. Our findings also show that non-cellular SOC mineralization is ubiquitous and contributes to 24% of soil respiration on average. Cellular and non-cellular SOC mineralization are positively linked but the contribution of non-cellular processes to soil CO2 emissions increases with dissolved organic carbon concentration.

The priming effect (PE) is a key mechanism contributing to the carbon balance of the soil ecosystem. Almost 100 years of research since its discovery in 1926 have led to a rich body of scientific publications to identify the drivers and mechanisms involved. A few review articles have summarised the acquired knowledge; the last major one was published in 2010. Since then, knowledge on the soil microbial communities involved in PE and in PE + C sequestration mechanisms has been considerably renewed.This article reviews current knowledge on soil PE to state to what extent new insights may improve our ability to understand and predict the evolution of soil C stocks. We propose a framework to unify the different concepts and terms that have emerged from the international scientific community on this topic, report recent discoveries, and identify key research needs.Seventy percent of the studies on the soil PE were published in the last 10 years, illustrating a renewed interest for PE, probably linked to the increased concern about the importance of soil carbon for climate change and food security issues. Among all the drivers and mechanisms proposed along with the different studies to explain PE, some are named differently but actually refer to the same object. This overall introduces “artificial” complexity for the mechanistic understanding of PE, and we propose a common, shared terminology. Despite the remaining knowledge gaps, consistent progress has been achieved to decipher the abiotic mechanisms underlying PE, together with the role of enzymes and the identity of the microbial actors involved. However, including PE into mechanistic models of SOM dynamics remains challenging as long as the mechanisms are not fully understood. In the meantime, empirical alternatives are available that reproduce observations accurately when calibration is robust.Based on the current state of knowledge, we propose different scenarios depicting to what extent PE may impact ecosystem services under climate change conditions., Le « priming effect » (PE) est un mécanisme clé contribuant au bilan de carbone de l'écosystème sol. Depuis sa découverte en 1926, près de 100 ans de recherche ont conduit à un riche corpus de publications scientifiques pour identifier les déterminants et les mécanismes impliqués. Quelques articles de synthèse ont résumé les connaissances acquises, le dernier datant de 2010. Depuis, les connaissances sur les communautés microbiennes du sol impliquées dans le PE ainsi que dans les mécanismes de séquestration du C se sont considérablement renouvelées. En conséquence, il est aujourd’hui nécessaire de faire le point et de déterminer dans quelle mesure ces nouvelles connaissances peuvent améliorer notre capacité à comprendre et à prédire l'évolution des stocks de C du sol. Cet article passe en revue les connaissances actuelles sur le PE du sol. Nous rappelons l'historique de la recherche sur le PE et nous proposons un cadre pour unifier les différents concepts et termes qui ont émergé de la communauté scientifique internationale sur ce sujet. Enfin nous rapportons les découvertes récentes et identifions les principaux besoins de recherche. Pas moins de 70 % des études sur le PE du sol ont été publiées au cours des 10 dernières années, illustrant un regain d'intérêt pour le sujet, probablement lié à la prise de conscience croissante du rôle du carbone du sol dans le changement climatique et la sécurité alimentaire. Parmi tous les déterminants et mécanismes proposés au fil des études destinées à expliquer le PE, certains ont été nommés différemment mais font en réalité référence au même objet. Ceci introduit une complexité « artificielle » pour la compréhension mécaniste du PE et nous préconisons d’utiliser par la suite une terminologie commune et partagée. D’importants progrès ont été réalisés pour déchiffrer les mécanismes abiotiques sous-jacents au PE, ainsi que le rôle des enzymes et l'identité des acteurs microbiens impliqués. Cependant, la prise en compte du PE dans les modèles mécanistes de dynamique des SOM reste un défi tant que les mécanismes ne sont pas entièrement compris. En attendant, des alternatives empiriques sont disponibles qui reproduisent les observations avec précision lorsque le calibrage est robuste. Sur la base de l'état actuel des connaissances, nous proposons différents scénarios décrivant dans quelle mesure le PE peut avoir un impact sur les services écosystémiques dans des conditions de changement climatique.

Description du sujet. L\'article présente une estimation du renouvellement à moyen terme (dizaines d\'années) du carbone profond des sols cultivés.Objectifs. Le stockage de carbone sous forme organique dans les sols est un moyen de limiter l\'augmentation du CO2 global et le réchauffement climatique. L\'objectif est de proposer une première quantification du renouvellement des horizons profonds (> 30 cm) qui contiennent environ la moitié du carbone organique des sols. Méthode. Nous avons effectué une synthèse d\'études de la dynamique du carbone de 41 profils où le renouvellement du carbone est quantifié grâce au traçage isotopique par les abondances naturelles en carbone-13.Résultats. Le renouvellement du carbone profond est en moyenne quatre fois plus lent qu\'en surface, mais le flux de renouvellement n\'est pas négligeable. La distribution moyenne du flux annuel d\'apport de carbone aux matières organiques est de 81 % dans l\'horizon 0 - 30 cm et 19 % dans la couche 30 - 100 cm, avec une erreur standard de ± 4 %. À plus long terme (20 ans), le sous-sol (30 - 100 cm) contient en moyenne 23 % du carbone récent du sol.Conclusions. Les matières organiques profondes ne doivent pas être négligées dans les bilans de C et N en réponse à différentes pratiques agricoles, et nous proposons une approximation simple pour ces bilans. Dans le futur, les estimations des évolutions temporelles du stock de carbone profond des sols et de leurs facteurs de variation devront s\'appuyer sur les essais agronomiques de longue durée ou la modélisation calée sur des sites fortement instrumentés, et être accompagnées de descriptions pédologiques détaillées., Turnover of deep organic carbon in cultivated soils: an estimate from a review of isotope dataDescription of the subject. This study is an estimate of medium-term renewal (decades) of deep carbon in cultivated soils. Objectives. Sequestration of organic carbon in soils is a way to mitigate the global rise in CO2 and warming. Deep horizons (> 30 cm) contain about half of the soil organic carbon but, in agriculture, deep organic carbon and nitrogen are often neglected when the balance of these elements is calculated. Our goal in this study was to propose a quantification of deep C turnover.Method. We collected data from 41 profiles, where C turnover was studied using the natural 13C labeling technique. The studied cropping systems were basically tropical and temperate C4 plant monocultures.Results. Deep carbon (30 – 100 cm) turnover was on average four times slower than in topsoil (0 – 30 cm), but renewal was significant. The average depth-distribution of the carbon flux to organic matter build-up was 81% in the layer at the 0 – 30 cm level and 19% in the layer at 30 – 100 cm, with a standard error of ± 4%. Over the longer term (20 years), subsoil at 30 – 100 cm contained on average 23% of the recent soil carbon accumulated in the first meter.Conclusions. Deep organic matter should not be overlooked when considering the balance of C and N and we therefore propose a simple method for an initial first assessment. In the future, further estimates of the temporal evolution of the deep carbon stock and its factors of variation should be based on long-term agronomic experiments or modeling parameterized on heavily instrumented sites, and should be accompanied by detailed pedological descriptions.

International audience

International audience

Dans un contexte de changements globaux, il est primordial de comprendre les cycles du Carbone (C) et de l’Azote (N) dans les agrosystèmes. Dans les écosystèmes prairiaux (puits de C et faible lixiviation), une synchronisation entre l’offre du sol et la demande de la plante en N semble exister. Cette synchronisation repose sur différents processus microbiens, tel que le « rhizosphere priming effect » (RPE), défini comme l’accélération de la minéralisation des matières organiques du sol (MOS) induite par la plante. L’influence de la disponibilité en CO2 (400 et 700 ppm) sous prairie d’une part, et en N (40 et 200 kg N/ha) à travers trois modes de gestion : (i) prairie, (ii) blé conventionnel, et (iii) blé sur couverture permanente de prairie d’autre part, a été étudiée. Un dispositif de marquage continu au 13CO2 a permis de distinguer la respiration du sol de celle des plantes dans les mésocosmes. L’augmentation de la disponibilité en CO2 de l’atmosphère a permis une hausse significative de la biomasse végétale totale, de l’appareil racinaire, donc de la demande en N et de l’offre du sol via l’augmentation du RPE. En outre, l’augmentation de la biomasse, en prairie et association blé-prairie sous fertilisation forte, a appauvri le sol en N et conduit à une hausse du RPE. Ces résultats préliminaires valident l’existence d’une synchronisation entre l’offre du sol et la demande de la plante en N. En revanche, le remaniement récent du sol prairial riche en N ne permet pas d’observer une influence nette du mode de gestion sur cette synchronisation. Cependant, l’association blé-prairie suggère une meilleure synchronisation sol-plante, notamment par la réduction significative du potentiel de lixiviation. Finalement, la poursuite du dispositif à plus long terme semble essentielle afin d’observer l’évolution de cette synchronisation au travers des divers modes de gestion., In the framework of global changes, it is crucial to better comprehend the carbon (C) and nitrogen (N) cycles involved in agrosystems. Within grassland ecosystems (C sink and low N lixiviation), there seems to be a synchronization between plant N-demand and soil N-offer. This synchronization relies on different microbial processes, such as the “rhizosphere priming effect” (RPE), which is defined as an enhancement of mineralization of soil organic matter (SOM) induced by plants. The influence of CO2 availability (400 and 700 ppm) in grassland on one hand, and N availability (40 et 200 kg N/ha) through three land-uses (i) grassland, (ii) wheat cropping, (iii) wheat on permanent grassland cover on the other hand, have been studied. A continuous 13CO2 labeling device permitted to distinguish soil respiration from plant respiration in the mesocosms. The rising CO2 availability enabled a significant increase of total plant biomass, root systems, N-demand and thereafter of soil offer via a stronger RPE. Besides, the increased plant biomass in grassland and mixed cropping, under a high N-fertilization, led to soil-N depletion and higher RPE. These preliminary results validate the existence of synchronization between plant N-demand and soil N-offer. However, the recent disturbance of the N-rich grassland soil, did not allow the clear observation of land-uses impact on this synchronization. Mixed cropping suggests better soil-plant synchronization, particularly thanks to the significant reduction of the lixiviation potential. Finally, a long-term follow-up appears essential in order to observe how this synchronization evolves, from one land-use to another.

Integration of the priming effect* (PE) in ecosystem models is crucial to better predict the consequences of global change on ecosystem carbon (C) dynamics and its feedbacks on climate. Over the last decade, many attempts have been made to model PE in soil. However, some basic knowledge to model the PE is lacking such as the relationship between decomposition rate of soil organic matter (SOM) and microbial biomass (MB). Moreover, the PE has never been inserted in a plant-soil model to analyze its role on plant-soil interactions. The main objectives of this thesis were to 1) integrate the activity, biomass and diversity of soil microorganisms in models of ecosystem C and nitrogen (N) dynamics in order to simulate the PE, and 2) determine the consequence of this integration for ecosystem functioning and response to global change. These objectives were achieved thanks to the combination of diverse approaches such as modeling, experimentation and statistical. In a lab experiment, I show that the rate of SOM decomposition increases 1) linearly with MB, and 2) with a saturating effect with SOM content. The linear response of SOM decomposition to MB is explained by the very limited microbial colonization of SOM reserves. However, the positive effect of SOM content on decomposition rate indicates that the local availability of SOM may be limiting for microbial mineralization. The observed co-limitation of SOMdecomposition was accurately modeled with the Michaelis-Menten equation. Finally, incorporating this equation in a simple model of soil Cdynamics explained how carbon often continuously accumulates in undisturbed soils whereas it reaches steady state in cultivated soils. Moreover, I present the first parameterized PE embedding plant-soil model (SYMPHONY) which provides realistic predictions on forage production, soil C storage and N leaching for a permanent grassland. SYMPHONY also shows that plant persistence depends on a fine adjustment of microbial mineralization of SOM to plant nutrient uptake. This fine adjustment was modeled by considering the destruction of SOM through PE and the interactions between two microbial functional groups: SOM-decomposers and SOM-builders. Moreover, consistent with recent observations, SYMPHONY explains how elevated CO2 induce modification of soil microbial communities leading to an intensification of SOM mineralization and a decrease in the soil C stock. By incubating a large range of soils sampled at global scale, I show that PE is a common phenomenon which occurs virtually in all soil types and ecosystems (grasslands, croplands, forests, savannahs and orchard) justifying current efforts to incorporate it in ecosystem models. The intensity of PE in soils was significantly correlated to silt and clay SOM pools which contain the major part of SOM reserve. Nevertheless, most of the PE variability (79%) is not explained by soil properties and SOM pools. This result suggests that the main source of PE variability is the microbial component of soils, which should be characterized in further studies. In conclusion, PE is a common phenomenon that may be incorporated in ecosystem models with a few additional parameters, improving the accuracy of their predictions on ecosystem functioning and response to global change.; La prise en compte du priming effect * (PE) dans les modèles biogéochimiques est essentielle afin de mieux prévoir les conséquences du changement global sur le cycle du C (C) dans les écosystèmes et les interactions avec le climat. Au cours de la dernière décennie, de nombreux travaux ont été réalisés afin de modéliser le PE. Cependant, quelques connaissances de base nécessaires à cette modélisation du PE manquent tels que la relation entre le taux de décomposition des matières organique du sol (MOS) et la biomasse des décomposeurs (MB). En outre, le PE n'a jamais été inséré dans un modèle sol-plante afin de déterminer son rôle dans les interactions plante-sol. Dans ce contexte, les principaux objectifs de la thèse sont 1) d’intégrer l'activité, de la biomasse et de la diversité des microorganismes du sol dans les modèles de dynamique du C et de l’azote (N) des écosystèmes afin de simuler le PE, et 2) de déterminer les conséquences de cette intégration pour le fonctionnement des écosystèmes et la réponse au changement global.Ces objectifs ont été atteints grâce à la combinaison de la diverses approches telle que la modélisation, l’expérimentation et les analyses statistiques. Dans une expérience de laboratoire, je montre que le taux de décomposition des MOS augmente 1) linéairement avec la MB et 2) avec un effet de saturation avec la teneur en MOS. La réponse linéaire de décomposition des MOS à la MB s'explique par la colonisation très limitée du sol et des réserves des MOS par les microorganismes. Cependant, la limitation de la décomposition par la teneur en MOS montre que la disponibilité locale des MOS peut être un facteur limitatif pour la minéralisation microbienne. La co-limitation observée de la décomposition des MOS est correctement modélisée avec l'équation de Michaelis-Menten. L'intégration de cette équation dans un modèle simple de dynamique des MOS permet d’expliquer comment les MOS s'accumulent souvent continuellement dans les sols non perturbés alors qu'elles stagnent dans les sols cultivés. Cette présente également le premier modèle d’écosystème paramétré incorporant le PE (SYMPHONY). Ce modèle génère des prévisions réalistes sur la production de fourrage, stockage de C dans le sol et lessivage de l'azote pour des prairies permanentes. SYMPHONY montre également que la persistance des plantes dans les écosystèmes dépend d'un réglage fin de la minéralisation microbienne de MOS au besoin en nutriments des plantes. Ce réglage est modélisé par SYMPHONY en considérant la destruction de MOS par le PE et les interactions entre deux groupes fonctionnels microbiens: les décomposeurs des MOSet les stockeurs de MOS.Enfin, conformément aux récentes observations, SYMPHONY explique comment l’augmentation du CO2 atmosphérique induit une modification des communautés microbiennes du sol conduisant à une intensification de la minéralisation microbienne et à une diminution du stock des MOS dans le sol. En incubant une large gamme de sols prélevés à l'échelle mondiale, nous montrons que PE est un phénomène ubiquiste présent dans tous les types de sols et d’écosystèmes testés (les prairies, les sols cultivés, les forêts, les savanes et les fruitiers justifiant les efforts actuellement entrepris pour l’intégrer dans les modèles biogéochimiques. En outre, l'intensité du PE des sols est significativement corrélée aux pools de MOS associées aux argiles et limons lesquels renfermant la plus grande partie du réservoir en MOS des sols. Cependant, la plupart de la variabilité de PE (79%) ne peut être expliquée par les propriétés du sol et les pools de MOS. Ce résultat suggère que la principale source de variabilité du PE des sols réside dans la composante microbienne qui devrait faire l’objet d’une caractérisation dans une prochaine étude. En conclusion, le PE est un phénomène ubiquiste qui peut être incorporé dans les modèles de l'écosystème avec quelques paramètres supplémentaires, améliorant substantiellement la qualité des prédictions sur le fonctionnement des écosystèmes et leurs réponses au changement global.

Master; Les sols sont les principaux réservoirs de carbone et d’azote, atomes essentielles à la vie. Les cycles biogéochimiques formés sont en partie régulés par les microorganismes. Avec les changements climatiques prévus, l’augmentation de la température et le changement des précipitations, la désertification des terres va s’agrandir augmentant le biome aride et entrainant une mortalité chez les microorganismes. V. Maire et collaborateur proposent en 2013 l’existence d’un métabolisme oxydatif extracellulaire, l’EXOMET. Ils font l’hypothèse qu’à la mort des cellules, les enzymes adsorbées maintiennent leurs activités permettant la reconstitution des métabolismes par ce pool enzymatique. Afin de vérifier si l’aridité augmente la présence de l’EXOMET dans des sols naturellement soumis à un gradient d’aridité, 6 sols du Botswana, 12 sols d’Espagne et 5 sols de France sont incubés en microcosmes suite à une irradiation aux rayons gamma ou sans traitement préalable. Pour chaque sol, l’émission de CO2, la consommation d’O2, le carbone organique dissous et inorganique, l’azote organique dissous et l’azote minéral sont analysés. Seule la minéralisation du carbone peut être corrélée avec l’indice d’aridité, la contribution de l’EXOMET augmente dans les climats plus humides mais elle peut également être influencée à d’autres facteurs comme la composition des sols et le pH.

Master; Cette étude est le résultat d’analyses effectuées après deux campagnes de terrain en Guyane française, en 2011 et 2012, qui ont abouti à l’établissement d’une chronoséquence composée de 5 sites de forêts témoins et 23 sites de prairies âgées de 6 mois à 41 ans permettant de reconstituer la dynamique du carbone (C) du sol sous prairies issues de la déforestation de la forêt amazonienne. Des corrections effectuées sur la masse de terre fine des échantillons et sur leur teneur en argiles a permis de prendre en compte les spécificités pédologiques de chaque site et les particularités propres aux écosystèmes naturels et anthropisés afin de rendre tous les sites étudiés comparables entre eux. Par ailleurs, le C hérité des forêts d’origine a également été pris en compte. Les résultats montrent une tendance générale à la hausse du stock de C du sol sous prairies âgées, avec un décollage 25 ans après leur mise en place. Les prairies âgées de 20 ans et plus possèdent un stock moyen de C du sol corrigé pour une profondeur de 1 m supérieur aux forêts témoins et aux prairies de moins de 20 ans. Les analyses isotopiques montrent que le stock de C issu des espèces en C4 (graminées) ne cesse d’augmenter depuis la mise en place des prairies. A l’inverse, le C issu des espèces en C3 diminue jusqu’à 20 ans après la mise en place de ces prairies et s’accroit fortement après 25 ans. Ce décollage pourrait s’expliquer par le maintien d’une couverture dominante en graminées, et probablement par le développement important de certaines adventices sur les prairies âgées. Cette étude confirme le rôle que pourraient jouer les prairies dans la lutte contre le changement climatique et dans la recherche d’une agriculture plus durable en permettant le stockage de matière organique garantissant une fertilité accrue sur le long terme et une capacité de séquestration de C importante.