Depth-wise distribution of soil-carbon stock in aggregate-sized fractions under shaded-perennial agroforestry systems in the Western Ghats of Karnataka, India

Growing shade-tolerant perennial crops under trees is an economically attractive land-use activity in the tropics; but the importance of these systems in facilitating ecosystem services such as soil carbon sequestration is seldom recognized. We assessed soil carbon stock at various depths (0-10, 10-30, 30-60, and 60-100 cm) in shaded perennial agroforestry systems (AFS) in Koppa (12 degrees 54 ' N, 75 degrees 04 ' E), Karnataka, India. The systems were coffee (Coffea canephora) under Grevillea robusta trees (Coffee + Grevillea), coffee under a mixture of shade trees (Coffee + Mixed Shade), tea (Camellia sinensis) under Grevillea robusta (Tea + Grevillea), a traditional smallholder farm with several common crops grown together (Homegarden), and a native moist deciduous forest (Forest). Four replicated composite soil samples were collected from each system for each depth class. Soil organic carbon (SOC) stock in three soil aggregate fractions (2000-250 mu m, 250-53 mu m, and < 53 mu m) as well as in the whole soil was determined. The SOC stock to 1-m depth were 172.3 and 142.4 Mg C ha(-1) under Forest and Coffee + Grevillea, respectively, and the lowest (89.3 Mg C ha(-1)) under Homegarden. No significant differences were noted in SOC within the silt + clay fraction (< 53 mu m) beyond 60 cm depth under Forest and other shaded AFS. Fitting a fixed-effects model, we found out that all three factors considered for this study: depth, aggregate size and treatment (land use system) had significant interaction effects on SOC stocks. The results show that deep rooted, tree-based systems, have higher total soil C stocks and more C in the smallest (< 53 mu m) soil fractions indicating the recalcitrant (longer-term storage) nature of C and implying consequent ecosystem benefit of reduced chances for soil C release back to the atmosphere.