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Le suivi des feux de brousse au Sénégal par télédétection
... Des différences importantes sont également à noter concernant les émissions liées aux feux de végétation. Dans le chapitre 1, les surfaces brûlées avaient été estimées selon une moyenne définie à partir d'observations satellitaires réalisées au Sénégal (Kane and Prevost, 1994;Nielsen et al., 2003). Ces surfaces se sont révélées être nulles l'année de mise en oeuvre de l'approche écosystémique. ...
This thesis aims to assess the impact of ruminant herds on the GHG balance of sylvo-pastoral ecosystems under semi-arid tropical climate. The study site is a circular area of 15 km radius centered on the Widou borehole (15°59’N, 15°19’W, 706 km²) representative of the sylvo-pastoral Ferlo Region in the Sahel belt of West Africa (North of Senegal). To achieve the GHG balance assessment, all GHG emissions related to ecosystem functioning (enteric fermentation from livestock and termites, microbial activity from soil and water, bush fire and borehole’s motor pump) and total carbon accumulation in the main sinks (soil organic matter, wood and root masses and livestock mass) were assessed at the monthly time scale and over the year cycle taking into account the spatial heterogeneity (6 landscape unit identified).
The main results of this thesis are:
• A strong spatial and temporal variability of GHG emissions from soil and surface water due to the deposition of livestock excretions , to biological activity of water and soil fauna and to root respiration. CO2 and N2O emissions from soil mainly relate to the excreta deposited while the CH4 emissions are strongly associated with the surface water and soil hydromorphic and anaerobic conditions. Stagnant water in the ponds and around the borehole are the main sources of CH4 in the study area due to the deposition of excreta by livestock in water when watering.
• A strong spatial heterogeneity of the greenhouse gas balance in relation to livestock activity. The landscape units that contribute more to the provision of forage to livestock (rangelands, forest plantations) have negative balances ranging between -0.08 and -0.92teq-C/ha/year. The landscape units that contribute less to livestock fodder provision and receive a large share of excreta deposition (pastoralist settlements, ponds and vicinity of borehole) have positive balances ranging between +0.01 and + 32.70teq-C/ha/year.
• A large seasonal variability of enteric methane emissions by livestock due to the regional mobility of the herds and the changing diets over time. In the context of a dry tropical climate with marked seasons in Sub-Saharan Africa the enteric CH4 emission of one Tropical livestock Unit over the year would be 27.9kgCH4/year, thus only 59% of the emission assessed by IPCC largely referred to in international literature.
• A large seasonal variability of GHG balance across the sylvo-pastoral landscape with a positive balance in the rainy season at + 0.18tC-eq/ha/month due to high GHG emissions caused by high soil moisture and to livestock excreta and vegetal organic matter deposited in the course of the preceding dry season. The GHG balance is negative during the cold dry season (-0.64teq-C/ha/year) and hot dry season (-0.13 teqC/ha/year) because some livestock area leaving the area in transhumance and because soil biological activity decreases as soil dry up.
• The GHG balance over the year and across the pastoral landscape is -0,02teq-C/ha/year setting the sylvo-pastoral ecosystem in an overall GHG equilibrium. Indeed GHG emissions would be offset by carbon storage over a year cycle.
This thesis highlights the benefits of an ecosystem approach of the GHG balance to better understand the drivers of the GHG balance and to suggest effective and appropriate mitigation options. Indeed this approach is built around the development of dynamic and spatialized visions of soil-plant-animal interactions in the ecosystem and the integration of these interaction effects within the GHG balance.
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