TOPOSEQUENCE-DEPTH DYNAMICS OF SOIL MICROBIAL BIOMASS, ENZYME ACTIVITIES AND FERTILITY INDICES UNDER DECOMPOSED OIL PALM LEAF LITTER IN IBIAE PLANTATION, NIGERIA

This study quantified the toposequence- and depth-mediated response of soil microbial properties to decomposed oil palm leaf litter in Ibiae Oil Palm Plantation, Cross River State, Nigeria. Soil samples were collected from lower, middle and upper slope positions at 0-15 cm and 15-30 cm depths, with adjacent non-oil-palm soil used as a control. Physico-chemical properties, microbial population, microbial biomass carbon and nitrogen, and selected enzyme activities were determined using standard soil analytical and microbiological protocols. Results showed that all plantation soils were sandy loam, but residue-influenced surface soils had stronger fertility and biological signals. Soil pH ranged from 5.5 to 6.2, while organic carbon and available phosphorus were highest at the lower slope surface horizon. Bacterial counts dominated the microbial community, and microbial biomass carbon and nitrogen declined with depth. Dehydrogenase, urease, lipase, amylase and phosphatase activities indicated active carbon, nitrogen and phosphorus turnover. The study demonstrates that decomposed oil palm leaf litter functions as a biologically active amendment for soil fertility restoration, residue recycling and sustainable plantation management.