Effects of landuse change and topography on the quantity and distribution of soil organic carbon stocks on Acrisol catenas in tropical small-scale shade cocoa systems of the Ashanti region of Ghana

Abstract

Landuse conversion is the highest contributor to atmospheric CO2 in the tropics. While forest conversion to agriculture results in the depletion of soil organic carbon (SOC), cocoa agroforestry may improve SOC stocks. The aim of our study was to explore the effects of topography and landuse change from forest to cocoa agroforestry on the quantity, quality, and distribution of SOC stocks. A total of 324 soil samples from the upper 0–145 cm were collected from catenas with 3 replicates along 3 hillslope positions: upper hillslope/summit position, middle hillslope/backslope position and lower hillslopes/foot- or toeslope positions. SOC stocks were highest in the upper hillslope positions at the 0–30 and 0–60 cm depths and varied from 42.85 ± 10.91 to 44.04 ± 13.70 Mg C ha−1 (0–30 cm depth) and 57.66 ± 18.80 to 60.32 ± 16.36 Mg C ha−1 (0–60 cm depth) for land use types. Hillslope position (upper, middle or lower) significantly affected humic substances with the topsoil having more humified organic materials in the upper compared to other hillslope positions. Forest conversion to shaded-cocoa had a strong impact on the quality of humic substances. Cocoa agroforestry systems had significantly lower E4:E6 ratios with more labile carbon at the lower slope positions compared to similar positions in forest systems. The most labile and readily available organic molecules (fulvic acids) are under forest systems whereas higher proportion of stabilized less soluble organic molecules (lower E4:E6 ratio) are found in cocoa soils in the form of humic acids. Slope position impacted E4:E6 ratios with lower degree of humification of organic materials in lower hillslope positions across the two landuses. A positive correlation was observed between humic substances and clay content. Our study contributes to the understanding of the influence of topography on SOC quantity and quality in forest and cocoa systems in Ghana.