Soil carbon stock and nutrient characteristics of forest-savanna transition: Estimates from four land use systems in Ghana

Land use change has led to drastic soil degradation and greenhouse gas emissions with implications for sustainable agriculture and climate change. Here, we report soil carbon stock and nutrient compositions of four land use types in the forest-savanna transition agro-ecological zone of Ghana. These were cashew, mango, oil palm and arable land. Soil samples were collected at 0-15 cm and 15-30 cm depths from each land use type. Soil nutrients and organic carbon content were greater in the tree-based land use types than in the arable land. Total soil organic carbon (SOC) ranged from 1.71% under cashew to 1.12% under the palms at the 0-15 cm depth. Soil carbon stock under the cashew was 12.5% greater than that of mango (56 Mg C/ha), and 40% more than that under oil palm. However, active carbon or permanganate oxidizable carbon was greatest (similar to 130 mg/kg) under mango and least (similar to 92.6 mg/kg) in arable land in the surface soil. Overall, active carbon was dependent on soil total carbon in the land use types (r = 0.81-0.91). Soil microbial biomass carbon was least in arable land (p < 0.05) and similar among the tree-based systems (p > 0.05) at 0-15 cm depth. No clear trend was observed in the exchangeable base compositions in the surface soils but cashew and mango systems appeared to show significantly greater levels of exchangeable calcium and magnesium, respectively. We conclude that strong nexus between soil microbial biomass carbon, nitrogen and phosphorus, and active soil carbon may drive soil carbon dynamics in land use systems of tropical forest-savanna ecotone.