Abstract
This study was conducted to evaluate the variation in soil carbon stock along a toposequence in Isiala Mbano, Southeastern Nigeria. A profile pit was dug on each of the three physiographic positions, top slope, mid slope and foot slope. The profile pits were delineated according to horizons. Samples were collected from different horizons, together with undisturbed core samples using core samplers. Samples were subjected to routine analysis. Soil organic carbon sequestration was calculated and relationships between soil organic carbon sequestration and soil properties were obtained. Organic carbon decreased with depth and increased from top to bottom slopes. From the physicochemical properties, the mean sand content was highest at the top slope (714 g kg-1), followed by the foot slope (694 g kg-1) and finally the mid slope (682 g kg-1). The mean silt and clay contents ranged from 106 to 120.0 and 168 to 200 g kg-1. Sand content decreased while clay increased with soil depth. The average bulk density recorded 1.20-1.56 mgm-3 with the foot slope having the highest value (1.56 mgm-3) the foot slope recorded the highest carbon sequestration (745.8 g.cm2), while the mid slope sequestered the least carbon (40.8 g.cm2). The mean total nitrogen was highest at the foot slope (0.12 %), while the least value was recorded at the top slope (0.04%). Also values of organic carbon were highest at a depth of 124-160cm at the foot slope (1.33%). Moisture content, total porosity, organic carbon, calcium, base saturation showed significant difference (p≤0.05) along the toposequence. The results obtained showed that different slopes on the toposequence sequester carbon differently. Therefore, farmers may periodically choose to adopt some farming practices such as mulching and cover cropping to build up organic matter, which will increase the organic carbon content.