The relationship between changes in land use and soil carbon and nitrogen was assessed in Alcalá (colombian coffee growing region), between 900 and 1600 m. Soils are Argiudolls and Melanudands and sampling units were defined according to the altitudinal gradient. Carbon and nitrogen content, C:N ratio, bulk density and 13C and 15 N abundances were measured. Significant differences (p < 0.05) were found due to altitude effect, differentiating the high zone by high C (37.06 -42.88 t ha -1 ), N (3.12-3.86 t ha -1 ) and C: N (11.17-11.97), low bulk density (0.56-0.65 g cm -3 ) and 15 N abundance (4.59-5.15 ‰). Fallow and monocultures stored more carbon in deeper layers of the soil, while most preserved sites showed low C, N contents. The 13 C in land uses ranged between -25.18‰ and -21.17 ‰, indicating that carbon was fixed mainly by C 3 plants. It was concluded that land use changes affect C stocks, due to practices like chemical fertilization; and Alcalá soils have high potential for ecosystem services related with edaphic carbon capture.Keywords: carbon sequestration in the Andes, land use changes, stable isotopes RESUMEN Se estudió la relación de cambios en el uso del terreno con el C y N edáficos, en el municipio de Alcalá (región cafetera colombiana), entre 900 y 1600 m de elevación. Los suelos son Argiudolls y Melanudands y las unidades de muestreo fueron definidas según el gradiente altitudinal. Se midieron contenidos de C y N, relación C:N, densidad aparente y abundancias de 13 C y 15 N. Se encontraron diferencias significativas (p < 0,05) por el efecto de la altura. La zona alta mostró mayores contenidos de C (37,06-42,88 t ha -1 ) y N (3,12 -3,86 t ha -1 ), mayor relación C: N (11,17 -11,97), menor densidad aparente (0.56 -0.65 g cm -3 ) y abundancia de 15N (4,59 -5,15‰). Barbechos y monocultivos almacenaron más carbono en la parte subterránea del suelo; mientras que en sitios más conservados los contenidos de C y N fueron bajos. El 13 C según los usos, fluctuó entre -25,18 y -21,27‰ indicando que la mayoría del C fue fijado por plantas C 3 . Se concluyó que los cambios en el uso afectan el almacenamiento de C, 222

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he soil C capture capacity and organic matter turnover rate vary according to photosynthetic pathways; therefore the evaluation of C at sites suffering changes from Cj 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 "C technique to identify the origin of soil C. In the Municipality of Alcalá, Department of Valle del Cauca, Colombia, the natural abundance of ô"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 C, to C4 vegetation and vice versa. Average ô''C ranged between -25.79 and -20.72%o 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 ' -' ' 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 C.i vegetation is recalcitrant, while that corresponding to C4 vegetation has a relatively fast turnover rate.