Submitted to: Agroforestry Systems
Publication Type: Peer reviewed journal
Publication Acceptance Date: 3/25/2010
Publication Date: 4/14/2010
Citation: Barreto, A.B., Gamma-Rodrigues, E.F., Gamma-Rodrigues, A.C., Polidoro, J.C., Moco, M.K., Machado, R.C., Baligar, V.C. 2010. Distribution of oxidizable organic c fractions in soils under cacao agroforestry systems in Southern Bahia, Brazil. Agroforestry Systems. 81:213-220. Interpretive Summary: In tropical and subtropical regions, organic C significantly affects the chemical, physical, and biological properties of soils. Agroforestry systems adapted for cacao cultivations can play a major role in the sequestration of carbon (C) because of their higher input of organic materials through litter deposit to the soil. Cacao agroforestry systems are effective in improving and conserving soil quality by the continuous deposition of plant materials above and below ground. In conventional sample methods total organic C (TOC) is determined and related to soil quality. This research shows that labile organic C fractions (more easily decomposable) are more reliable indicators of changes in soil quality and land management practices. In cacao agroforesty systems higher levels of labile organic fractions were observed at surface soil layers indicating that such systems of cacao management play an important role in efficient nutrient cycling and sequestration of C thereby assisting the mitigation of negative impact of greenhouse effect from CO2. Information generated from this research will be helpful to cacao farmers to manage tree component of agroforestry based cacao plantation to improve soil fertility and eventually increase the cacao sustainability and yield potentials.
Technical Abstract: Agroforestry systems can play a major role in the sequestration of carbon (C) because of their higher input of organic materials to the soil. The importance of organic carbon to the physical, chemical, and biological aspects of soil quality is well recognized. However, total organic carbon measurement might not be sensitive indicator of changes in the soil quality. Adoption of extraction methods that preferentially extract the more labile fractions might be a more useful approach for characterization of soil organic carbon from different soils. This study aimed to evaluate the distribution of organic carbon (C) fractions in different soil layers to a depth of 50 cm in two soil orders under cacao (Theobroma cacao) agroforestry systems (AFS) in Bahia, Brazil. Soil samples were collected from four depths (0-5, 5-10, 10-30 and 30-50 cm) under two cacao agroforestry systems (30-year-old stands of cacao under the shade of Erythrina spp. and - Erythrina glauca) in Oxisol and Cabisol. A modified Walkley-Black method was used to determine oxidizable carbon in four C fractions with different labile forms of C. These C fractions includes: 1, labile fraction; 2 moderate labile fraction; 3, low labile fraction and 4, recalcitrant fraction. Overall, in both agroforesty systems of cacao management the C fractions generally declined with increasing soil depth. The C fractions 1 and 2 were 50% higher in the upper soil layers (0-5 and 5-10 cm). In both soils, at all depths, more than 50% of the organic C was found in the more labile fraction (fraction 1). High ratio of C fraction 1 (more labile C)-to-total organic C was observed (around 54 and 59%, on Oxisol and Cambisol, respectively), indicating a large input of organic matter in these soils under the cacao agroforestry systems.