Carbon Storage in Soil-Size Fractions under Cacao Agroforestry Systems in Bahia, Brazil.

Abstract
Compared with agricultural systems, shaded-perennial agroforestry systems contain relatively higher quantities of soil carbon (C) because of continuous deposition of plant residues; however, the amount of C sequestered in the soil will vary depending on the turnover time and the extent of physical protection against decomposition of different soil organic matter (SOM) fractions. The objective of this study was to characterize soil organic carbon pools in relation to different soil aggregate-size classes in different soil layers up to one meter depth in cacao (Theobroma cacao) agroforestry systems in Bahia, Brazil. Soil samples were collected from four depth classes (0 – 10, 10 – 30, 30 – 60, and 60 – 100 cm) under three land-use systems in reddish-yellow Oxisol, in Bahia, Brazil; the land-use systems were: (1) 30-year-old stands of cacao with Erythrina spp. (Erythrina poeppigiana) as shade trees, (2) cacao under natural forest (Cabruca), and (3) an adjacent natural forest. The soil sample from each layer was separated by wet sieving into three aggregate-size classes (>250 µm, 250 – 53 µm, and <53 µm). The organic C stock in the 0 – 100 cm soil layer did not vary among different systems (mean: 302 Mg ha^-1); however, in the 0 – 30 cm layer, it was in the order:  Cabruca > cacao with Erythrina spp. > natural forest (154, 118, and 84 Mg ha^-1, respectively). The C content was higher in the silt+clay sized (< 53 µm) fraction (50.2 g kg^-1) than in the other two fractions: 250 – 53 µm (44.7 g kg^-1) and >250 µm (45.3 g kg^-1).  The results show the overall high amounts of soil C stock in all the systems and the presence of more C in the silt + clay soil fraction.