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Livestock induces strong spatial heterogeneity of soil CO2, N2O, CH4 emissions within a semi-arid sylvo-pastoral landscape in West Africa

Assouma M.H., Serça D., Guérin F., Blanfort V., Lecomte P., Touré I., Ickowicz A., Manlay R., Bernoux M., Vayssières J.. 2017. Journal of Arid Land, 9 (2) : p. 210-221.

Greenhouse gas (GHG) emissions from the surface soils and surface water receiving animal excreta may be important components of the GHG balance of terrestrial ecosystems, but the associated processes are poorly documented in tropical environments, especially in tropical arid and semi-arid areas. A typical sylvo-pastoral landscape in the semi-arid zone of Senegal, West Africa, was investigated in this study. The study area (706 km² of managed pastoral land) was a circular zone with a radius of 15 km centered on a borehole used to water livestock. The landscape supports a stocking rate ranging from 0.11 to 0.39 tropical livestock units per hectare depending on the seasonal movements of the livestock. Six landscape units were investigated (land in the vicinity of the borehole, natural ponds, natural rangelands, forest plantations, settlements, and enclosed plots). Carbon dioxide (CO2), nitrous oxide (N2O) and methane (CH4) fluxes were measured with static chambers set up at 13 sites covering the six landscape units, and the 13 sites are assumed to be representative of the spatial heterogeneity of the emissions. A total of 216 fluxes were measured during the one-year study period (May 2014 to April 2015). At the landscape level, soils and surface water emitted an average 19.8 t C-CO2 eq/(hm²¿a) (CO2: 82%, N2O: 15%, and CH4: 3%), but detailed results revealed notable spatial heterogeneity of GHG emissions. CO2 fluxes ranged from 1148.2 (±91.6) mg/(m²¿d) in rangelands to 97,980.2 (±14,861.7) mg/(m²¿d) in surface water in the vicinity of the borehole. N2O fluxes ranged from 0.6 (±0.1) mg/(m²¿d) in forest plantations to 22.6 (±10.8) mg/(m²¿d) in the vicinity of the borehole. CH4 fluxes ranged from¿3.2 (±0.3) mg/(m²¿d) in forest plantations to 8788.5 (±2295.9) mg/(m²¿d) from surface water in the vicinity of the borehole. This study identified GHG emission ¿hot spots¿ in the landscape. Emissions from the surface soils were significantly higher in the landscape units most frequently used by the animals, i.e., in the vicinity of the borehole and settlements; and emissions measured from surface water in the vicinity of the borehole and from natural ponds were on average about 10 times higher than soil emissions. (Résumé d'auteur)

Mots-clés : émission; agroforesterie; système agrosylvopastoral; paysage; eau superficielle; sol; émission atmosphérique; oxyde nitreux; méthane; dioxyde de carbone; Élevage; gaz à effet de serre; afrique occidentale; sénégal

Thématique : Elevage - Considérations générales; Pollution; Chimie et physique du sol; Conservation de la nature et ressources foncières

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