Soil structure stability and organic carbon distribution along the toposequence of Orumba North in derived savannah of the South-East, Nigeria

In high-rainfall zones with severe water erosion problems, soil structure stability as promoted by soil organic carbon (SOC) should be prioritized. Landscape positions and land use influence soil properties and nutrient distribution. This study assessed soil structure stability and SOC content at top-slope, mid-slope and low-slope positions of a toposequence at Orumba North in derived savannah zone of the South-East, Nigeria. The top/mid-slope positions are situated at upland plains and the low-slope position at a lowland. A pedon was excavated and described in each topo-position, and the identified horizons tested for topo-position influence on soil texture, structure and SOC. The top/mid-slope positions show sandy, light-coloured, well-drained, friable, moist soils, whereas the low-slope soils are loamy sand but poorly drained with high water table, leading to the presence of mottles. Soil texture varied among the pedons. Clay content was higher at the top- than mid/low-slope positions; silt content was higher at the top/mid- than low-slope position. Soil aggregation indicated no regular trend, but water-stable aggregates, state of aggregation, aggregates’ mean-weight diameter (MWD) were all lowest and structure stability index highest at the mid-slope position. However, the generally less compacted surface soils showed higher values of these aggregation indices at the top- than mid/low-slope positions, except MWD that tended to decrease along the toposequence. Saturated hydraulic conductivity showed the least permeable soil at the poorly drained low-slope position. The SOC content, though generally low, increased along the toposequence (2.90, 5.00 and 6.70 g kg–1, respectively), but with top-slope < mid-slope position for surface soils only. Overall, the toposequence showed poorly developed and/or fairly unstable soil structure. The data presented highlight the somewhat inverse aggregation-SOC trends along humid tropical toposequences, serving as an indication of their possible relationship under contrasting drainage status.