Soil organic carbon (SOC) dynamics after slash-burn agriculture are poorly understood in African tropical forest, though recent studies have revealed C4 grass invasion as a forest understory influences SOC dynamics after deforestation. This study aimed to quantify the relative SOC contribution of C4 and C3 plants separately through the sequential fallow periods of forest (cropland, or 4-7, 20-30, or >50 years of fallow forest) in the tropical forest of eastern Cameroon. We evaluated the SOC stock and natural 13C abundance for each layer. The SOC stock was largest in 4-7 years fallow forest (136.6 ± 8.8 Mg C ha-1; 100 cm depth, and C4:C3 = 58:42), and decreased with increasing fallow period. SOC from C4 plants was larger in the 4-7 and 20-30 years fallow forests (57.2-60.4 ± 5.8 Mg C ha-1; 100 cm depth), while it clearly decreased in >50 years fallow forest (35.0 ± 4.1 Mg C ha-1; 100 cm depth), resulting in the smallest SOC in this mature forest (106.4 ± 12.9 Mg C ha-1; 100 cm depth). These findings indicate that C4 grass understories contributed to the SOC restoration during early fallow succession in the tropical forest of eastern Cameroon.