Soil Carbon Storage in Agroforestry Systems: Implications for Climate Change Mitigation and Food Security.

Soil organic matter contains more reactive organic carbon (C) than any other single terrestrial pool on Earth. Therefore, soils play a major role in storing C and thus reducing carbon dioxide concentration in the atmosphere.  Since C movement through soil can be a long-term process, the effect of a specific management practice or land-use system (LUS) may not be clear in a short time-span.  Given the relationship between soil aggregates and size-fractions on C retention, understanding the relationship between soil aggregates/fraction-sizes and soil C stock across LUS can be useful in assessing the C sequestration potential (CSP) of soils. Our recent studies on C storage in different soil fraction-size classes at varying soil depths up to 1 m under agroforestry systems (AFS) in six different ecological regions in five countries (Florida, USA; Mali, West Africa; Kerala, India; Central and Northwestern Spain; Bahia, Brazil; and Minas-Gerais, Brazil) showed that C stock measurements in different fraction sizes supplemented by information on the nature of soil aggregates can give insights into the long-term C sequestration potential (CSP) of the soil. Furthermore, relatively high soil C stocks were found in under- or little-exploited areas of the Cerrado (savanna) of Brazil suggesting the importance of land-use history on the current C stock. Opening up these lands for commercial use could lead to release of C to atmosphere and degradation of the ecosystem; on the other hand, adoption of tree-based integrated LUS will help retain these C stocks in soils. Apart from the proven environmental benefits of sustainable AFS, trading the C stored in them can contribute to financial security of the landowners, and help ensure food security for millions of smallholder agroforestry practitioners in the tropics.