Quantifying Soil Organic Carbon Fractions and Their Stability Under Different Land Use Systems in Kogi State, North Central Nigeria

This study quantifies soil organic carbon fractions and their chemical stability under contrasting land use systems in Kogi State, located in Nigeria's North Central geopolitical zone, addressing the research gap in fraction-specific organic carbon data needed for accurate carbon sequestration assessment and soil quality monitoring in the forest-savanna transition zone. Existing studies in the North Central zone have predominantly measured total organic carbon without partitioning into labile and stable fractions, preventing accurate assessment of carbon dynamics and sequestration permanence. Using a comparative analytical design, surface and subsurface soils were sampled from four land uses: secondary forest, bush fallow, continuous cropping, and agroforestry in Lokoja, Okene, and Kabba-Bunu local government areas. Organic carbon fractions were determined using acid hydrolysis fractionation (labile, cellulosic, acid-insoluble), permanganate oxidisable carbon (POXC), and physical fractionation (light fraction, coarse and fine mineral-associated fractions). Total organic carbon was measured by the Walkley-Black method. Thermogravimetric analysis was used to assess thermal stability of organic carbon. Findings revealed that continuous cropping soils had significantly lower labile POXC and light fraction carbon but retained proportionally more stable acid-insoluble fractions, indicating preferential depletion of biologically active fractions. Agroforestry soils showed the highest light fraction and POXC contents among managed systems, reflecting greater fresh organic matter inputs and biological activity. The study demonstrates that total organic carbon alone underestimates functional differences between management systems and recommends POXC as a sensitive management-sensitive carbon indicator for Kogi State monitoring programmes. Keywords: soil organic carbon fractions, lability, land use, Kogi State, carbon stability.