Microbiotic soil crusts in the Sahel of Western Niger and their influence on Soil POROSITY and water dynamics

International audienceMicrobiotic soil crusts are common features of the surface of fallow land in Western Niger. We investigated the interaction between these microbial covers and the porosity and water dynamics of soils at the surface of a Sahelian landscape. The soil pore system was examined by microscopic observations and mercury porosimetry. The soil water retention capacity was measured using a Richard pressure membrane apparatus. Runoff measurements were performed in situ at a one m2 scale under natural rainfall. Samples with dense cyanobacterial cover had microbially-originated pores ranging from 0.04 and 50 μm in size. These samples also showed higher total soil porosity and retained two to four times more water than samples with thin microbial cover. These properties are closely related to the hydrophobic nature of cyanobacterial components. Final runoff values obtained on densely covered surfaces were significantly higher compared to those measured on surfaces with thin microbial cover due to the geometry of the microbially-originated pore system and its functioning

The AMMA mulid network for aerosol characterization in West Africa

Three ground based portable low power consumption microlidars (MULID) have been built and deployed at three remote sites in Banizoumbou (Niger), Cinzana (Mali) and M'Bour (Senegal) in the framework of the African Monsoon Multidisciplinary Analyses (AMMA) project for the characterization of aerosols optical properties. A description of the instrument and a discussion of the data inversion method, including a careful analysis of measurement uncertainties (systematic and statistical errors) are presented. Some case studies of typical lidar profiles observed over the Banizoumbou site during 2006 are shown and discussed with respect to the AERONET 7-day back-trajectories and the biomass burning emissions from the Combustion Emission database for the AMMA campaign

Apport de la matière organique dans l'étude de la dynamique sédimentaire lacustre en zone sahélienne (exemples de deux complexes limniques, SW Niger)

International audienceLe remplissage sédimentaire des mares de Bangou-Bi et de Tankalawal (SW Niger) a fait l'objet d'une étude de son contenu organique qui fournit des informations sur l'origine et les conditions de dépôt du matériel sédimentaire. Deux carottes mesurant respectivement 140 cm (carotte BB2) et 60 cm (carotte TK1) ont été étudiées. Les échantillons ont été soumis à une pyrolyse Rock-Eval 6 renseignant sur la géochimie globale de la Matière Organique (MO) et le signal S2 de la pyrolyse a été exploité afin de mieux préciser la nature de ces MO. Le niveau argilo-limoneux de la carotte BB2 contient une MO aquatique et terrestre dont la composition en composés hydrocarbonés se répartit entre les biopolymères (F1+F2), et les géopolymères matures (F3) et immatures (F4). Cette MO serait déposée dans un environnement calme. En revanche le niveau inférieur sableux renferme une MO terrestre très altérée et enrichie en géopolymères matures qui serait amenée par des pulses détritiques grossiers (colluvionnement) issus du bassin versant. Les valeurs des paramètres géochimiques et leur évolution avec la profondeur indiquent que la MO de la carotte TK1 est aquatique et a enregistré une dégradation en milieu anoxique. Cette MO contenant majoritairement des géopolymères matures (F4) riches en lipides se révèle étonnamment plus résistante face à l'altération que celle présente dans la carotte BB2

Biological macropores effect on runoff and infiltration: a combined experimental and modelling approach

International audienceThe influence of the large macropores made by the soil macrofauna on water infiltration has been well studied for about two decades. However, the results depend on the measurement methods, the variability is very high, and we do not yet really understand all the processes and their consequences. A more detailed description of the key processes should allow more formalization through modelling and a comparison between the various experimental results. We present here an example of such an approach, that goes from field study to modelling, about the influence of termites on infiltration. The effect of termites on runoff was studied during 4 years, under natural rainfall, in Niger. A spatialised model was developed to simulate the interception of runoff by the macropores. The results of the field experiment underline the great and durable impact of termites on infiltration, and allow to better understand the processes responsible of this effect at various time or space scales. Simulations provide a deeper insight on the observed processes and reinforce our interpretations

On-farm evaluation of field-scale soil losses by wind erosion under traditional management in the Sahel

In the West African Sahel, few direct estimates are currently available on the extent of soil losses by wind erosion in traditionally managed farmers fields. Field scale sediment balances in western Niger were derived from airborne sediment fluxes measured using BSNE sand traps. Results from a first study in 1997 indicated that sediment fluxes in a cultivated field increased linearly over distances = 80 m, irrespective of wind power. Sediment deposition in an adjacent fallow was well described by an exponential decay function with a near constant trapping efficiency coefficient of 0.1 m-1 for incoming sediment fluxes = 10 kg m-1. Mass balances up to -17.5 and + 10.5 Mg ha-1 were measured in a single storm in the field and fallow, respectively. Starting in 1998, a second study was setup in a newly cleared, 8 ha farmer's field equipped at 87 locations with BSNE sand traps. Whereas a net sediment balance of +5.4 Mg ha-1 was measured in 1998, the experimental field was subject to a net soil loss of -5.0 Mg ha-1 in 1999. This was attributed to changes in ground cover and differences in sediment influx from adjacent fields