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In semiarid Sahelian region, the dynamics of soil organic carbon (SOC) and water are key to sustainable land management. This work focuses on the behaviour of carbon. A total of 33 soil profiles in four polders, ranging from 10 to 65 years in age, were sampled, analysed (0–1 m), and matched with marsh soil profiles in recent sediments considered as reference (t0) for carbon stocks determination. SOC and soil inorganic carbon (SIC) stocks show a spatial variability between polders. SOC stocks were t0 200 ± 0.8; t60 183 ± 34; and t65 189 ± 1.1 MgC·ha−1, whereas the SIC stocks were negligible. These results show the highest stocks of soil carbon observed for this climatic region. The SOC stocks were also calculated for the equivalent soil mass at a defined depth (0–0.3 m); the corrected calculation of SOC stocks (Scorr) for 2450 Mg·ha−1 of equivalent soil mass is t0 64 ± 1.9, t60 59 ± 9.8, and t65 53 ± 2.2 MgC·ha−1; the stocks decrease by −7.8% and −17.2% from t0 to t60 and t65. Carbon was inherited from the pre-existing·marsh and the polders have conserved high stock values.