Soil Carbon Sequestration Under Agroforestry: A Multi-Location, Multi-System Study.

The United Nations Framework Convention on Climate Change (UNFCCC) allows the use of carbon (C) sequestration through afforestation and reforestation (A & R) as greenhouse (GHG) offset activities. Consequently, the role of tree-based land-use systems such as agroforestry as a strategy for C sequestration has raised considerable expectations. During the past five years, we studied soil C sequestration under different agroforestry systems in five countries: silvopasture in Ultisols and Spodosols in Florida, USA; the traditional “dehesa” (silvopasture) in northern Spain;  multispecies homegardens in Kerala, India; traditional intercropping under scattered trees in Segou, Mali; shaded cacao system in Bahia, Brazil; and silvopasture under Eucalyptus sp. in Minas Gerais, Brazil. The studies involved some uniform procedures: soil fractionation into different particle-size classes, and use of stable isotope ratio to determine, in tree + grass systems, the contribution of trees and grasses to soil C in depth classes up to 1 m depth. In addition to the known facts about the extreme variability in C content of soils, the study pointed to several important observations. These included: i) tree-based agricultural systems, compared to treeless systems, can store significantly more C in deeper layers of soils under similar agroecological conditions; ii) higher SOC content is associated with higher species richness and tree density, especially in the upper 50 cm soil and in the <53 µm soil fraction; iii) soil near the tree, compared to away from the tree, had higher C storage iv) trees (C3 plants) contribute more C in the silt- + clay- sized (<53 µm) fractions than C4 plants in deeper soil profile; v) long-term presence of trees on landscape promoted storage of more stable C in soil; and vi) although traditional agroforestry systems had larger C stock than the improved systems, they have only limited potential for sequestering additional C.