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Continuous soil carbon storage of old permanent pastures in Amazonia

Stahl C., Fontaine S., Klumpp K., Picon-Cochard C., Grise M.M., Dezécache C., Ponchant L., Freycon V., Blanc L., Bonal D., Burban B., Soussana J.F., Blanfort V.. 2017. Global Change Biology, 23 (8) : p. 3382-3392.

DOI: 10.1111/gcb.13573

Amazonian forests continuously accumulate carbon (C) in biomass and in soil, representing a carbon sink of 0.42¿0.65 GtC yr-1. In recent decades, more than 15% of Amazonian forests have been converted into pastures, resulting in net C emissions (~200 tC ha-1) due to biomass burning and litter mineralization in the first years after deforestation. However, little is known about the capacity of tropical pastures to restore a C sink. Our study shows in French Amazonia that the C storage observed in native forest can be partly restored in old (=24 year) tropical pastures managed with a low stocking rate (±1 LSU ha-1) and without the use of fire since their establishment. A unique combination of a large chronosequence study and eddy covariance measurements showed that pastures stored between -1.27 ± 0.37 and -5.31 ± 2.08 tC ha-1 yr-1 while the nearby native forest stored -3.31 ± 0.44 tC ha-1 yr-1. This carbon is mainly sequestered in the humus of deep soil layers (20¿100 cm), whereas no C storage was observed in the 0- to 20-cm layer. C storage in C4 tropical pasture is associated with the installation and development of C3 species, which increase either the input of N to the ecosystem or the C:N ratio of soil organic matter. Efforts to curb deforestation remain an obvious priority to preserve forest C stocks and biodiversity. However, our results show that if sustainable management is applied in tropical pastures coming from deforestation (avoiding fires and overgrazing, using a grazing rotation plan and a mixture of C3 and C4 species), they can ensure a continuous C storage, thereby adding to the current C sink of Amazonian forests.

Mots-clés : forêt; forêt tropicale; déboisement; pâturages; séquestration du carbone; cycle du carbone; émission atmosphérique; matière organique du sol; litière forestière; Écosystème forestier; plante fourragère; utilisation des terres; guyane française; france

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