Continuous soil carbon storage of old permanent pastures in Amazonia

Amazonian forests continuously accumulate carbon (C) in biomass and in soil, representing a carbon sink of 0.420.65 GtC yr(-1). In recent decades, more than 15% of Amazonian forests have been converted into pastures, resulting in net C emissions (similar to 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.