Carbon stock in the solum of some coarse-textured soils under secondary forest, grassland fallow, and bare footpath in the derived savanna of south-eastern Nigeria

Abstract

Quantitative data on carbon stock (C stock) in and beyond the topsoil (0–30 cm) under natural terrestrial ecosystems in West African savanna could provide information about their relative potential, and management options, for C sequestration, but these data are still scanty in the region. In selected locations (Nsukka, Obimo, and Ibagwa-aka) in the derived savanna zone of south-eastern Nigeria, secondary forest (SFT), grassland fallow (GLF), and bare footpath (BFP) were sampled from the topsoils (0–30 cm) and subsoils (30–60 cm) in triplicate. The soils are generally sandy, with low (1.4–13.8%) mean silt content. Mean bulk density ranged from 1.30 to 1.83 Mg/m3. The soils were acidic (pHwater 4.0–4.8) and low in organic C (0.10–1.14%). There was a consistent trend in C stock (SFT > GLF > BFP) in the topsoil, whereas only higher values in SFT than BFP were consistent in the subsoil. In both soil layers, the scale of the differences among the land-cover types was location-specific. Values of C stock were higher in the topsoil than subsoil, except for GLF and BFP at Obimo due to recent bush burning. Irrespective of location, the mean topsoil–subsoil values under SFT, GLF, and BFP were 45.7–30.6, 27.7–25.8, and 19.0–18.8 Mg/ha, respectively. Soil structural stability, indexed as the ratio of organic matter to silt + clay, explained roughly 61 and 89% of the variability in C stock of topsoils and subsoils, respectively. These results should benefit the planning of C sequestration projects in savanna agroecosystems of West Africa.