Saïdou Sall

Quantification of Key Genes Steering the Microbial Nitrogen Cycle in the Rhizosphere of Sorghum Cultivars in Tropical Agroecosystems

ABSTRACT The effect of agricultural management practices on geochemical cycles in moderate ecosystems is by far better understood than in semiarid regions, where fertilizer availability and climatic conditions are less favorable. We studied the impact of different fertilizer regimens in an agricultural long-term observatory in Burkina Faso at three different plant development stages (early leaf development, flowering, and senescence) of sorghum cultivars. Using real-time PCR, we investigated functional microbial communities involved in key processes of the nitrogen cycle (nitrogen fixation, ammonia oxidation, and denitrification) in the rhizosphere. The results indicate that fertilizer treatments and plant development stages combined with environmental factors affected the abundance of the targeted functional genes in the rhizosphere. While nitrogen-fixing populations dominated the investigated communities when organic fertilizers (manure and straw) were applied, their numbers were comparatively reduced in urea-treated plots. In contrast, ammonia-oxidizing bacteria (AOB) increased not only in absolute numbers but also in relation to the other bacterial groups investigated in the urea-amended plots. Ammonia-oxidizing archaea exhibited higher numbers compared to AOB independent of fertilizer application. Similarly, denitrifiers were also more abundant in the urea-treated plots. Our data imply as well that, more than in moderate regions, water availability might shape microbial communities in the rhizosphere, since low gene abundance data were obtained for all tested genes at the flowering stage, when water availability was very limited.

The effects of organic amendments on the interactions between a nematophagous fungus Arthrobotrys oligospora and the root-knot nematode Meloidogyne mayaguensis parasitizing tomato plants.

Abstract. The aim of this study was to examine the effects of amendments with leaf biomass on the development of tomato plants in a soil where root-knot nematodes (Meloidogyne mayaguensis) and/or a nematophagous fungus (Arthrobotrys oligospora, strain ORS 18697) had been inoculated. Six origins of leaf biomass were chosen: Acacia mangium, Acacia holosericea, Eucalyptus camaldulensis, Casuarina equisetifolia, Azadirachta indica and Sorghum vulgare. These leaf biomass types inhibited the development of the aerial parts of the tomato plants. This negative effect was not observed when the fungus was inoculated. On the contrary, plant growth was stimulated. Moreover, the antagonistic activity of Arthrobotrys oligospora was strengthened in the presence of ground leaf powder, especially that from Acacia holosericea. The effects of phenolic compounds on fungal growth and predatory activity and on plant growth are discussed.

Cyclogenesis off the African Coast: The Case of Cindy in August 1999

Abstract Using radar data from Dakar (Sengal), National Centers for Environmental Prediction–National Center for Atmospheric Research (NCEP–NCAR) reanalyses, outgoing longwave radiation provided by the National Oceanic and Atmospheric Administration (NOAA) Television Infrared Observation Satellite (TIROS) satellite series as well as data from the National Hurricane Center (NHC), a cyclogenesis process leading to the birth of a tropical cyclone from a Sahelian mesoscale convective system (MCS) off the African coast of Senegal is described. The cause of this evolution seems to be the coincidence of the MCS with an easterly wave over a warm sea, the presence of a wide area of precipitable water vapor, strong convergence in the low and midtropospheric layers, and an easterly vertical shear of the zonal wind. As a result, a dynamically well organized convective system built up and the system rapidly strengthened. Before moving away from the African coast of Senegal, this perturbation, which became the tropical...

In Situ Effect of Some Tropical Litters on N Mineralization

ABSTRACT Soil nitrogen mineralization patterns were investigated under field conditions in the presence of five leaf litters of different qualities, Faidherbia albida, Azadirachta indica, Andropogon gayanus, Casuarina equisetifolia, and Eragrostis tremula. The experimentation was conducted in a tropical sandy soil in central Senegal over 12 months. No relationship could be drawn between litter quality (N content, cellulose, hemicellulose, lignin) and N mineralization during this field experimentation. By contrast, the presence of the litter modified the soil N cycle. At all sampling dates, the patterns of N mineralization were most high in litter amendment. The content of mineral N was highest in Casuarina equisetifolia amendment (17.03 μg N g−1 soil in November 2000, and 28.53 μg N g−1 soil in March 2001). Soil incubated in the absence of the different litters exhibited a different pattern of N mineralization. Net mineralization was observed when the soil had been previously submitted to the influence of Casuarina equisetifolia (4.87 μg N g−1 soil), while net immobilization was observed in the case of Azadirachta indica (8.13 μg N g−1 soil).

Analysis of Strengthening and Dissipating Mesoscale Convective Systems Propagating off the West African Coast

AbstractA large number of Atlantic tropical depressions are generated in the eastern basin in relation to the African easterly wave (AEW) and embedded mesoscale convective systems (MCSs) coming from the African continent. In this paper, the structures of strengthening and dissipating MCSs evolving near the West African coast are analyzed, including the role of the ocean surface conditions in their evolution.Satellite infrared brightness temperature and meteorological radar data over seven summer seasons between 1993 and 2006 are used to subjectively select 20 cases of strengthening and dissipating MCSs in the vicinity of the Senegal coast. With these observed MCSs, a lagged composite analysis is then performed using Interim ECMWF Re-Analysis (ERA-Interim) and Climate Forecast System Reanalysis (CFSR).It is shown that the strengthening MCS is generally preceded by prior passage of an AEW near the West African coast. This previous wave trough is associated with a convective cyclonic circulation in the low a...

Trains of African Easterly Waves and Their Relationship to Tropical Cyclone Genesis in the Eastern Atlantic

AbstractIn this study, the relationship between trains of African easterly waves (AEWs) and downstream tropical cyclogenesis is studied. Based on 19 summer seasons (July–September from 1990 to 2008) of ERA-Interim reanalysis fields and brightness temperature from the Cloud User Archive, the signature of AEW troughs and embedded convection are tracked from the West African coast to the central Atlantic. The tracked systems are separated into four groups: (i) systems originating from the north zone of the midtropospheric African easterly jet (AEJ), (ii) those coming from the south part of AEJ, (iii) systems that are associated with a downstream trough located around 2000 km westward (termed DUO systems), and (iv) those that are not associated with such a close downstream trough (termed SOLO systems).By monitoring the embedded 700-hPa-filtered relative vorticity and 850-hPa wind convergence anomaly associated with these families along their trajectories, it is shown that the DUO generally have stronger dynam...