The soil C capture capacity and organic matter turnover rate vary according to photosynthetic pathways; therefore the evaluation of C at sites suffering changes from C3 to C4 vegetation and vice versa, is important to identify impacts of land use change on C cycle.This study aims to evaluate C storage under different land uses, and soil C dynamics using the 13 C technique to identify the origin of soil C. In the Municipality of Alcalá, Department of Valle del Cauca, Colombia, the natural abundance of δ 13 C was estimated, and data on land use history were gathered to calculate the organic matter turnover rate.The contribution of each type of vegetation to total percentage organic C and to storage at 0.30 m was estimated at sites suffering changes from C3 to C4 vegetation and vice versa.Average δ 13 C ranged between -25.79 and -20.72‰ at the three depths evaluated.Over a period of 13 yr, mature fallow lands replaced more than 70% of the C fixed by pastures over a period of 60 yr, whereas paddocks, over a period of 17 yr, only managed to replace 37.9% of the C fixed by associated coffee plantations during a period of 50-100 years.We conclude that the use of 13 C avoided that C storage would have been attributed to current land uses when they are actually fixed by previous vegetation; and that C deposit from C3 vegetation is recalcitrant, while that corresponding to C4 vegetation has a relatively fast turnover rate.