Analyse du fonctionnement biologique du sol par l'étude de la nématofaune : semis direct versus labour sur les hautes terres près d'Antsirabé (Madagascar)

International audienceLes nématodes du sol possèdent des qualités multiples (abondance, diversité taxonomique et fonctionnelle, existence d'indices liés à l'état du sol) qui en font des indicateurs pertinents du fonctionnement du sol. Ils intègrent un grand nombre d'informations sur l'état de la micro-chaîne trophique du sol (compartiment microbien, microfaune et mésofaune) qui est responsable de la décomposition et de la minéralisation des nutriments. La nématofaune a été analysée dans un essai comparant le semis direct avec restitution des résidus de récolte, au labour avec exportation des résidus de récolte dans une rotation soja/riz sur les hautes terres de Madagascar. Les résultats obtenus durant les deux années d'étude (année en soja & année en riz) ne sont pas significativement différents pour une grande majorité de paramètres nématologiques ce qui signifie qu'à moyen terme de cette expérience (essai mis en place depuis 10 ans), les différences interannuelles liées à la culture sont très inférieures aux différences liées au travail du sol et à la fertilisation. L'absence de travail du sol simultané à la restitution des résidus de récolte, ainsi que la fertilisation minérale et organique, induit une augmentation de l'abondance de la nématofaune tellurique. L'analyse de la composition de la nématofaune indique que les densités de nématodes phytophages d'une part et de nématodes omnivores et prédateurs d'autre part, sont significativement augmentées dans les systèmes en semis direct, alors que les fertilisations organiques et minérales induisent une augmentation de la plupart des groupes trophiques incluant également les microbivores (bactérivores et fongivores). L'analyse des indices nématofauniques montre que le travail du sol ainsi que l'absence de fertilisation sont aussi responsables de la simplification qualitative de la micro-chaîne trophique du sol. Les systèmes en semis direct présentent une nématofaune plus complexe et moins opportuniste incluant des taxons sensibles aux perturbations contrairement au système labouré

The characterization of the soil biological quality of organic viticulture can be achieved by analyzing soil nematofauna

Soil nematofauna is a bioindicator that can highlight changes in biological functioning when changing agricultural practices. In the present study, the effects of conversion of vineyards to organic agriculture on biological soil quality were evaluated. Twenty four conventional plots and organic plots in Cruscades (Aude) were studied: they were divided into four groups: (1) conventional, (2) converted for 7 years (Bio 7 years), (3) converted for 11 years (Bio 11) and (4) converted for 17 (Bio 17). The nematofauna was analyzed in 96 soil samples (four samples per plot) taken from the surface layer of soil (0-15 cm). The more longer the conversion of plots to organic viticulture, the higher the biological activity of the soil; it is the abundance of non-plant parasitic nematodes that gives this indication. The effects of conversion on soil biological functioning were significant relatively late (≥ 11 years) after the change of practices. Moreover, the study of the functional diversity of nematodes shows that the soil remained disturbed in the organic as in the conventional system (no increasing complexity of the soil food web measured by the structure index)

Effect of a legume cover crop on carbon storage and erosion in an ultisol under maize cultivation in southern Benin

Field experiment was conducted from 1988 to 1999 at an experimental farm at Agonkanmey, near Cotonou in southern Benin, to study the effect of relay-cropping maize through Mucuna pruriens (var. utilis). The relay-cropping system was compared with traditional maize cropping system without any input, and with a maize cropping system with mineral fertilizers (NPK). Special attention was given on the changes in soil C during the period of the experiment in relation to residue biomass C returned to the soil, runoff and soil erosion losses, and loss of C with erosion. The soils are classified as sandy loam Typic Kandiustult. The general properties of these soils are given. For this soil type, relay cropping of maize and mucuna was very effective in enhancing C sequestration: change in Ct (total C content) stock for 0 to 40 cm depth was 1.3 t C/ha per year over the 12-year period of the experiment, ranging among the highest rates recorded for the eco-region. This increase resulted first from the high amount of residue biomass provided by mucuna, which amounted to 10 t DM/ha per year (83% aboveground). Mucuna residues, supplying the soil with N, also favoured the production of maize biomass, and total mucuna plus maize residue biomass returned to the soil was approximately 20 t/ha per year. In contrast, non-fertilized and fertilized continuous maize cultivation resulted in -0.2 and 0.2 t C/ha per year change in Ct stock for 0 to 40 cm depth, respectively. Total residue biomass was 8 and 13 t/ha per year, including 77 and 29% by weeds, respectively. Thick mulch produced by mucuna decreased losses by runoff and erosion, which were 0.28, 0.12 and 0.08 mm/mm and 34.0, 9.0 and 3.0 t/ha per year in unfertilized, fertilized with NPK and mucuna treatments, respectively. Eroded C was estimated at 0.3, 0.1 and 1.0 t C/ha per year in unfertilized, fertilized with NPK and mucuna treatments, respectively. Through its benefits on soil organic matter management, weed suppression and erosion control, cropping systems including a legume crop may have an adverse impact from a global change standpoint

Effect of a legume cover crop (Mucuna pruriens var. utilis) on soil carbon in an ultisol under maize cultivation in southern Benin

Long term fallow is no longer possible in densely populated tropical areas, but legume cover crops can help maintain soil fertility. Our work aimed to study changes in soil carbon in a sandy loam Ultisol in Benin, which involved a 12-year experiment on three maize cropping systems under manual tillage: traditional no-input cultivation (T), mineral fertilized cultivation (NPK), and association with Mucuna pruriens (M). The origin of soil carbon was also determined through the natural abundance of soil and biomass C-13. In T, NPK and M changes in soil carbon at 0-40 cm were -0.2, +0.2 and +1.3 t C ha(-1) yr(-1), with residue carbon amounting to 3.5, 6.4 and 10.0 t C ha(-1) yr(-1), respectively. After 12 years of experimentation, carbon originating from maize in litter-plus-soil (0-40 cm) represented less than 4% of both total carbon and overall maize residue carbon. In contrast, carbon originating from mucuna in litter-plus-soil represented more than 50% of both total carbon and overall mucuna residue carbon in M, possibly due to accelerated mineralization of native soil carbon (priming effect) and slow mulch decomposition. Carbon originating from weeds in litter-plus-soil represented c. 10% of both total carbon and overall weed residue carbon in T and NPK. Thus mucuna mulch was very effective in promoting carbon sequestration in the soil studied

JWST imaging of edge-on protoplanetary disks II. Appearance of edge-on disks with a tilted inner region: case study of IRAS04302+2247

We present JWST imaging from 2$\mu$m to 21$\mu$m of the edge-on protoplanetary disk around the embedded young star IRAS04302+2247. The structure of the source shows two reflection nebulae separated by a dark lane. The source extent is dominated by the extended filamentary envelope at $\sim$4.4$\mu$m and shorter wavelengths, transitioning at 7$\mu$m and longer wavelengths to more compact lobes of scattered light from the disk itself. The dark lane thickness does not vary significantly with wavelength, which we interpret as an indication for intermediate-sized ($\sim10\mu$m) grains in the upper layers of the disk. Intriguingly, we find that the brightest nebula of IRAS40302 switches side between 12.8$\mu$m and 21$\mu$m. We explore the effect of a tilted inner region on the general appearance of edge-on disks. We find that radiative transfer models of a disk including a tilted inner region can reproduce an inversion in the brightest nebula. In addition, for specific orientations, the model also predicts strong lateral asymmetries, which can occur for more than half possible viewing azimuths. A large number of edge-on protoplanetary disks observed in scattered light show such lateral asymmetries (15/20), which suggests that a large fraction of protoplanetary disks might host a tilted inner region. Stellar spots may also induce lateral asymmetries, which are expected to vary over a significantly shorter timescale. Variability studies of edge-on disks would allow to test the dominant scenario for the origin of these asymmetries.Comment: Accepted for publication in Ap

JWST imaging of edge-on protoplanetary disks. I. Fully vertically mixed 10μ\mum grains in the outer regions of a 1000 au disk

Scattered light imaging of protoplanetary disks provides key insights on the geometry and dust properties in the disk surface. Here we present JWST 2--21\,$\mu$m images of a 1000\,au-radius edge-on protoplanetary disk surrounding an 0.4\,$M_\odot$ young star in Taurus, 2MASS\,J04202144+2813491. These observations represent the longest wavelengths at which a protoplanetary disk is spatially resolved in scattered light. We combine these observations with HST optical images and ALMA continuum and CO mapping. We find that the changes in the scattered light disk morphology are remarkably small across a factor of 30 in wavelength, indicating that dust in the disk surface layers is characterized by an almost gray opacity law. Using radiative transfer models, we conclude that grains up to $\gtrsim10\,\mu$m in size are fully coupled to the gas in this system, whereas grains $\gtrsim100\,\mu$m are strongly settled towards the midplane. Further analyses of these observations, and similar ones of other edge-on disks, will provide strong empirical constraints on disk dynamics and evolution and grain growth models. In addition, the 7.7 and 12.\,$\mu$m JWST images reveal an X-shaped feature located above the warm molecular layer traced by CO line emission. The highest elevations at which this feature is detectable roughly match the maximal extent of the disk in visible wavelength scattered light as well as of an unusual kinematic signature in CO. We propose that these phenomena could be related to a disk wind entraining small dust grains.Comment: Accepted for publication in the Astronomical Journa

Modest dust settling in the IRAS04302+2247 Class I protoplanetary disk

We present new VLA observations, between 6.8mm and 66mm, of the edge-on Class~I disk IRAS04302+2247. Observations at 6.8mm and 9.2mm lead to the detection of thermal emission from the disk, while shallow observations at the other wavelengths are used to correct for emission from other processes. The disk radial brightness profile transitions from broadly extended in previous ALMA 0.9mm and 2.1mm observations to much more centrally brightened at 6.8mm and 9.2mm, which can be explained by optical depth effects. The radiative transfer modeling of the 0.9mm, 2.1mm, and 9.2mm data suggests that the grains are smaller than 1cm in the outer regions of the disk and allows us to obtain the first lower limit for the scale height of grains emitting at millimeter wavelengths in a protoplanetary disk. We find that the millimeter dust scale height is between 1au and 6au at a radius 100au from the central star, while the gas scale height is estimated to be about 7au, indicating a modest level of settling. The estimated dust height is intermediate between less evolved Class 0 sources, that are found to be vertically thick, and more evolved Class II sources, which show a significant level of settling. This suggests that we are witnessing an intermediate stage of dust settling.Comment: Accepted for publication in the Astrophysical Journa

Gas- and particle-phase products from the photooxidation of acenaphthene and acenaphthylene by OH radicals

This work is focused on the gas-phase oxidation of acenaphthylene and acenaphthene by OH radicals and associated secondary organic aerosol (SOA) formation under low and high-NOx conditions. Experiments were carried out in an atmospheric simulation chamber using a proton transfer reaction time-of-flight-mass spectrometer (PTR-TOF-MS) and an aerosol time-of-flight-mass spectrometer (ATOFMS) to chemically characterize the gas- and particle-phase products, respectively. Due to the structures of these two aromatic compounds, the proposed chemical mechanisms exhibit some differences. In the case of acenaphthene, H-atom abstraction from the saturated cyclopenta-fused ring was found to be competitive with the OH-addition to the aromatic rings. During the photooxidation of acenaphthene using nitrous acid (HONO), aromatic ring-opening products such as indanone and indanone carbaldehyde, generated through OH addition to the aromatic ring, were formed in higher yields compared to low-NOx conditions. In the case of acenaphthylene, OH addition to the unsaturated cyclopenta-fused ring was strongly favored. Hence, ring-retaining species such as acenaphthenone and acenaphthenequinone, were identified as the main reaction products in both gas- and particle-phases, especially under high-NOx conditions. Subsequent SOA formation was observed in all experiments and SOA yields were determined under low/high-NOx conditions to be 0.61/0.46 and 0.68/0.55 from the OH-initiated oxidation of acenaphthylene and acenaphthene, respectively