Carbon sequestration potential and phosphorus fraction dynamics under urban forest species in a humid tropical derived savanna of Southwestern Nigeria

Abstract

This study evaluated carbon sequestration potential and phosphorus (P) fractions in tropical plantation species in a humid tropical derived savanna zone of southwestern Nigeria. The study aimed to determine total organic carbon (TOC), non-hydrolysable carbon (NHC), and P forms from Albizia lebbeck, Bambusa vulgaris, Gmelina arborea, Leucaena leucocephala, Tectona grandis, and Treculia africana plantation soils through sampling depths at 0–20, 20–40, 40–60, and 60–80 cm randomly. The results indicated that sampled tree species ranged from 11 to 25 years. Albizia lebbeck plantation had higher carbon sequestration (47.84 ± 2.3 g kg −1 TOC) within surface soils (0–20 cm), 249% higher than the control. The 25-year-old Leucaena leucocephala plantations exhibited very high NHC content (0.73 ± 0.05 g kg⁻ 1 ) across the soil profile, and 49.2 ± 3.1% surface NHC content was retained at depth 60–80 cm. Bambusa vulgaris exhibited the steepest vertical carbon gradient, with a 56.3 ± 3.8% decrease in TOC from the surface to 60–80 cm. Gmelina arborea had higher carbon at 20–40 cm (2.58 ± 0.15 g kg⁻ 1 ) and was 18.9% higher than at the surface; depth-integrated carbon stock approximation showed that 0–20 cm sampling decreased the carbon sequestration potential by 32.7 ± 2.8%. Total OC showed a strong positive correlation with NHC (r = 0.894, P ≤ 0.01) and a significant positive relationship with labile P (r = 0.501, P ≤ 0.05) and negative correlations with stable Al-P (r = − 0.53, P ≤ 0.05) and Ca-P (r = − 0.613, P ≤ 0.05). The study concluded that indigenous species of urban forest trees enhanced soil carbon stabilization, regulated P availability, and improved soil health in humid tropical derived savanna environments.