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Soil carbon stocks after conversion of Amazonian tropical forest to grazed pasture: Importance of deep soil layers

Stahl C., Freycon V., Fontaine S., Dezécache C., Ponchant L., Picon-Cochard C., Klumpp K., Soussana J.F., Blanfort V.. 2016. Regional Environmental Change, 16 (7) : p. 2059-2069.

DOI: 10.1007/s10113-016-0936-0

Recent studies suggest that carbon (C) is stored in the topsoil of pastures established after deforestation. However, little is known about the long-term capacity of tropical pastures to sequester C in different soil layers after deforestation. Deep soil layers are generally not taken into consideration or are underestimated when C storage is calculated. Here we show that in French Guiana, the C stored in the deep soil layers contributes significantly to C stocks down to a depth of 100 cm and that C is sequestered in recalcitrant soil organic matter in the soil below a depth of 20 cm. The contribution of the 50¿100 cm soil layer increased from 22 to 31 % with the age of the pasture. We show that long-term C sequestration in C4 tropical pastures is linked to the development of C3 species (legumes and shrubs), which increase both inputs of N into the ecosystem and the C:N ratio of soil organic matter. The deep soil under old pastures contained more C3 carbon than the native forest. If C sequestration in the deep soil is taken into account, our results suggest that the soil C stock in pastures in Amazonia would be higher with sustainable pasture management, in particular by promoting the development of legumes already in place and by introducing new species.

Mots-clés : forêt; forêt tropicale; séquestration du carbone; cycle du carbone; déboisement; pâturages; matière organique du sol; couverture végétale; Écosystème forestier; plante d'abroutissement; plante fourragère; utilisation des terres; guyane française; france

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