Mechanistic Drivers of Soil Carbon Sequestration Under Reforestation Scenarios in the Guinea-Sudan Savanna Transition Zone, Nigeria

Reforestation in the Guinea-Sudan savanna transition zone of Nigeria is promoted as a soil carbon sequestration strategy, but the mechanistic drivers governing the rate, depth, and stability of carbon accumulation under different reforestation approaches remain unresolved. This study elucidates the mechanistic drivers of soil carbon sequestration using a chronosequence of reforestation plots spanning 0 to 25 years of age, established with native species mixtures, exotic monocultures, and assisted natural regeneration across sites in Kogi and Nassarawa States. Soil organic carbon fractions were separated into labile, mineral-associated, and particulate pools using physical and chemical fractionation methods. Carbon-14 radiocarbon dating of bulk fractions and soil carbon stabilisation mechanisms were investigated using nuclear magnetic resonance spectroscopy and X-ray diffraction of mineral-organic associations. Soil microbial community structure and enzyme activities were characterised to assess the biological drivers of carbon transformation and stabilisation. Root biomass production and below-ground carbon inputs were quantified using in-growth core methods. Results show that native species mixtures generated 34 percent more stable mineral-associated soil organic carbon over 25 years than exotic monocultures, linked to more diverse root chemistry and enhanced microbial community complexity. Assisted natural regeneration achieved comparable mineral-associated carbon accumulation rates to native species plantations at substantially lower cost. Clay mineral type and cation exchange capacity were the dominant abiotic controllers of long-term carbon stabilisation. The study provides mechanistic insights that optimise carbon sequestration returns from reforestation investment and advances a novel carbon stability framework applicable across the Guinea-Sudan transition zone. Keywords: soil carbon sequestration, reforestation, carbon fractions, Guinea-Sudan savanna, mechanistic drivers.