FORMS AND DISTRIBUTION OF SOIL PHOSPHORUS AS INFLUENCED BY SLOPE POSITION AND LOCATION IN THE SOUTHERN GUINEA SAVANNA OF NIGERIA

This study assessed the effect of slope position and location on the forms and distribution of soil phosphorus in part of Niger State within the southern Guinea savanna of Nigeria. The study was conducted at six locations (Gidan Mangoro, Gidan Kwanu, Maikukele, Kutigi, Lanle and Batati) in Niger State. Soil samples were collected at 0-30 cm depth along toposequences delineated into upper, middle and lower slope positions, using a 3 × 6 factorial arrangement fitted into a Randomized Complete Block Design with three replications. Samples were analysed for particle size, pH, organic carbon, exchangeable potassium, cation exchange capacity (CEC), total nitrogen and available phosphorus (Bray-1). Soil P was sequentially fractionated into ammonium chloride-P (NH4Cl-P), aluminium-P (Al-P), iron-P (Fe-P), occluded-P (Occl-P), calcium-P (Ca-P) and total P following standard laboratory analytical procedures. Data were subjected to analysis of variance (ANOVA) and means separated using the Duncan Multiple Range Test (DMRT) and at 5% probability. Slope position significantly (P<0.05) influenced available P and all P fractions except NH4Cl-P, with values generally increasing from the upper to the lower slope; Location had a significant effect on most parameters, with Gidan Mangoro and Maikukele generally recording the highest values for clay, Occl-P, Al-P and Ca-P, while Lanle recorded the highest Fe-P and total P. The relative abundance of inorganic P fractions followed the order available P > Al-P > NH4Cl-P > Ca-P > Fe-P > Occl-P. Total P ranged from 710.75 to 758.84 mg kg-1 across slope positions and from 724.00 to 769.57 mg/kg across locations. The distribution of phosphorus forms along the studied toposequences was not uniform, and lower slope positions consistently contained more available and total P, higher organic carbon and better overall soil quality than upper slope positions.