Soil carbon stocks after conversion of Amazonian tropical forest to grazed pasture: importance of deep soil layers
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.
KeywordsC3 contribution Deep soil C Native forest Old pasture Mixed-grass pasture
We would like to thank A. Etienne, M. Koese, F. Kwasie and O. Ngwete for their valuable help collecting data for this study. We are grateful to S. Revaillot, C. Bréchet and C. Hossann for chemical and isotopic analyses. We thank the guest editor Dr Frank O’Mara and tree anonymous reviewers for their comments on a previous version of this manuscript. This study was part of the Carpagg projects funded by the French Ministry of Research, CIRAD, INRA and CNES, these projects are co-funded by European regional development fund (ERDF/FEDER, 2007–2013). This work also benefited from an “Investissement d’Avenir” Grant managed by the Agence Nationale de la Recherche (CEBA, ref. ANR-10-LABX-0025) and some support from the European project Animal Change (FP7 KKBE 2010–2014) and European Research Council Synergy Grant ERC-2013-SyG-610028 IMBALANCE-P.
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