Interactive Activities of Location, Slope, and Depth on The Physicochemical Properties of Soils of Imo River Watershed, Southeastern Nigeria

Abstract Understanding how slope and depth interact with soils in a watershed is crucial for nutrient cycling, agricultural productivity, and environmental sustainability. The interplay of environmental factors, topography, and soil management practices significantly influences the physicochemical characteristics of the soil ecosystem. This study investigates how location, slope, and depth interact to affect the physicochemical properties of soils along the Imo River watershed in Southeast Nigeria. A stratified random sampling technique was employed for the research. The experimental design was a three-factorial randomized complete block design (RCBD), replicated three times and including three land units (upper, middle, and lower course) as factor A, three slopes (upslope, midslope, and foot slope) as factor B, and six soil sample depths as factor C. Mean values were separated using the standard error of difference (SED) at a 5% probability level. Profile pits were established in the designated land units. A total of nine pits were excavated in each section of the river, with three pits each in the upslope, midslope, and foot slope areas. These pits were excavated, described, and sampled according to FAO (2006) guidelines, resulting in a total of 27 profile pits utilized for the study. Routine analyses were conducted on the sampled soil. Multiple regression analysis was applied to evaluate the variance associated with the optimal linear combination. The findings indicated that location, slope, depth, and their interactions significantly influenced the physicochemical properties of the soils at p<0.05 and p<0.0001. Soils from Isuochi exhibited the highest clay content, Mc (p<0.0001), while those from Amuzu had the lowest clay content (p<0.0001). The highest bulk density (Bd) and the lowest total porosity (Tp) were found in Obigbo soils, whereas Amuzu soils had the lowest Bd and the highest Tp (p<0.0001). Exchangeable bases (Ca, Mg, K) were significantly higher in soils from Isuochi (p<0.0001) and lower in those from Amuzu (p<0.0001). Organic matter (Om) levels were higher in soils from Obigbo and lower in those from Amuzu (p<0.0001). In terms of slope, footslope areas demonstrated the highest values for silt and clay compared to upslope and midslope regions. Moisture content was greatest at the footslope and least at the midslope (p<0.05). Organic matter was significantly greater at the footslope and significantly lower at the midslope (p<0.05). Effective cation exchange capacity (ECEC) was higher at the upslope and lowest at the midslope (p<0.05). In examining soil depth, the subsurface horizons showed the highest values for sand, clay, Mc, and Bd, while the surface horizons had the lowest values for sand, clay, Mc, and Bd (p<0.0001). The surface horizon contained more Om, total nitrogen (TN), and available phosphorus (Av.P) compared to the subsurface horizons. The interaction of watershed course (location), slope, and depth significantly impacted clay content, Mc, Bd, Tp, and pH (p<0.0001). Consequently, effective management practices such as crop rotation, organic manure application, agroforestry, and soil conservation techniques should be implemented, particularly in the upper landscape, to reduce erosion and enhance soil quality.