Soil carbon stocks and origin under different cacao agroforestry systems in Southern Bahia, Brazil

Cacao agroforestry systems (AFS) are characterized by accumulating large amounts of soil organic carbon (SOC). However, information on the influence of shade trees on SOC stock up to 100 cm depth is scarce in the literature. The objectives of this study were to quantify the SOC storage under different cacao AFS, natural forest and pasture to a depth of 100 cm; and to evaluate the origin of SOC using the 13C isotopic ratio as an indicator of relative contribution of trees (C3 plants) and grass (C4 plants), after 4 years of pasture conversion into cacao and rubber AFS. SOC was determined by dry combustion in six layers (0-10, 10-20, 20-40, 40-60, 60-80 and 80-100 cm). The natural abundance of 13C technique was used only on cacao and rubber lines in contrast to the SOC accumulated by pasture before conversion into AFS. The SOC stock in a depth between 0-100 cm was significantly high in the pasture and in both younger cacao and rubber AFS (4 years old). Systems that were more than 20 years old 'cabruca', cacao and Erythrina and cacao and rubber system (20 years) had lower SOC stocks and did not differ significantly from the natural forest. In the surface layers of soil there were significant differences in the SOC stock. However, below 20 cm all the systems were similar to each other. The SOC stock in the first 20 cm accounted between 31 and 44% of total SOC stored in the 100 cm in the seven systems, followed by 19-23% at a depth of 20-40 cm, 15-18% at a depth of 40-60 cm and 11-14% in the last two depths. After four years of establishment cacao and rubber AFS were the most efficient systems in the accumulation of SOC in the first 20 cm of soil and consequently up to 100 cm deep. Cacao was more efficient than rubber tree to accumulate C3-derived C. While the rubber increased by almost 70% of C3, cacao increased 131%. After the 40 cm of depth the SOC is still from the original natural forest.