Impact of forest tree species composition on carbon sequestration and soil health in Ahmadu Bello university’s botanical garden

Abstract

Tree species composition strongly influences soil fertility and carbon storage, two critical components of ecosystem functioning. This study assessed the impact of selected forest tree species on soil physical and chemical properties, as well as carbon sequestration potential, within the Ahmadu Bello University Botanical Garden, Zaria. Soil samples were collected at a depths 0-30 cm under eight tree species. Soil parameters analysed included organic carbon, total nitrogen, available phosphorus, exchangeable bases, bulk density, and pH. Results revealed significant (p < 0.05) differences among species, with Azadirachta indica showing the highest OC (30.74 g/kg) and CS (11,084.64 kg/ha), while Gmelina arborea had the lowest OC (15.80 g/kg) and CS (6,662.73 kg/ha). Leguminous species such as Albizia lebbeck and Bauhinia monandra improved soil nitrogen content (20.67–17.67 g/kg), supporting their role in biological nitrogen fixation. Cynometra megalophylla showed the highest available phosphorus (50.14 mg/kg), indicating efficient phosphorus recycling through litter input. Soil pH varied significantly across species, with Azadirachta indica maintaining a near-neutral pH (7.64), while Gmelina arborea was more acidic (6.59). The study recommends integrating leguminous trees (e.g., Acacia, Albizia) with non-leguminous hardwoods (e.g., Tectona, Azadirachta) to balance soil fertility improvement with long-term carbon storage.