Soil Carbon and Nutrient Contents under Cacao Agroforestry Systems in Bahia, Brazil.

In Brazil, cacao (Theobroma cacao) agroforestry systems are adopted and promoted mostly in highly weathered soils with acidic pH and low fertility. Therefore, it is important to optimize nutrient cycling in these systems to maintain sustainable production. The objectives of this study were to determine  the soil carbon (C) content; soil microbial biomass C, nitrogen (N), and phosphorus (P) contents; extent of soil C and N mineralization; and total and labile (fast mineralization) soil organic P in different cacao agroforestry systems in southern Bahia, Brazil. Three systems were selected: cacao under Erythrina poeppigiana, cacao under natural forest (‘Cabruca’), and a cacao germplasm collection area, all in reddish yellow Oxisols or Inceptisols. Soil samples were collected from four depth classes (0 – 5, 5 – 15, 15 – 30, and 30 – 50 cm) for soil C analysis. Microbial attributes and organic P were measured on only the 0 – 5 cm depth. Mean soil C ranged from 70 to 108 Mg ha^-1, microbial C from 173 to 356 kg ha^-1, microbial N from 117 to 212 kg ha^-1, and microbial P from 3 to 9 kg ha^-1. The amount of soil C mineralized in 111 days ranged from 23 to 61 kg ha^-1 and that of N from 61 to 81 kg ha^-1. Total organic P content ranged from 11 to 195 kg ha^-1 and labile organic P from 5 to 10 kg ha^-1. This study shows that cacao agroforestry systems store relatively high amounts of C in the soil and consequently have a potential to increase the soil quality and reduce the atmospheric CO₂ emission. Furthermore, the high amounts of soil microbial biomass and organic P could be relevant for cacao nutrition considering the low amount of N (24 kg ha^-1) and P (5 kg ha^-1) exported through cacao seed harvest.