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

Abstract

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°54′N, 75°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 µm, 250–53 µm, and < 53 µm) 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 µm) 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 µm) 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.