Vertical Changes of Organic Carbon Storage, Structural and Hydraulic Properties of a Humid Tropical Soil under Vegetation Cover in Southeast Nigeria

Changes in organic carbon storage (OCS), structural and hydraulic properties with depth of soil are important phenomena that need to be understood per soil for effective soil management in crop production. A study was conducted in a Randomized Complete Block Design (RCBD) to assess the vertical changes in OCS, structural and hydraulic properties of soil under vegetation cover. The study involved 5 levels of sampling depth replicated nine (9) times to give forty five (45) observational units. The nine points for sampling were randomly selected and the five sampling depths were 0 – 20, 20 – 40, 40 – 60, 60 – 80 and 80 – 100 cm. Auger and core soil samples were collected at the designated depths. The samples were processed and analysed in the laboratory. The results of data analyses showed that depths varied significantly (P≤0.05) in the parameters measured. The highest OCS of 38.28 ton / ha was obtained at the 0 – 20 cm depth and the lowest (10.38 ton / ha) was obtained at 80 – 100 cm depth. The highest mean weight diameter of 1.00 mm and lowest of 0.72 mm were obtained at 0 – 20 and 80 – 100 cm depths, respectively. The lowest clay dispersion index of 25.43 % and the highest clay flocculation index of 74.53 % were obtained at 80 – 100 cm depth. Clay content and bulk density (BD) increased with depth and, the highest clay content of 212.4 g / kg and highest BD of 1.65 mg / m3 were obtained at the 80 – 100 cm depth. Saturated hydraulic conductivity (Ksat), total porosity (Pt) and macroporosity (Pm) decreased with depth. The highest Ksat of 3.11 cm / hr, Pt of 47.41 % and Pm of 24.13 % were obtained at 0 – 20 cm depth. The water retention characteristics increased with depth. The highest field capacity of 24.46 %, permanent wilting point of 11.16 % and available water capacity of 13.32 % were obtained at 80 – 100 cm depth. There were consistent changes in the soil properties across the depths. The reduced drainage at the higher depths should be considered in cultivating deep rooted crops which are sensitive to poor drainage. The top soils should be managed with good agronomic practices and minimum tillage to avert damages to the microaggregates.