In weathered tropical soils, the inherent pedoclimatic characteristics strengthen the role of soil organic matter (SOM) pools within the carbon cycle for both forest and agricultural ecosystems. The fast SOM turnover and the humid climate hasten the impact of land use and emphasize the importance of water extractable organic matter (WEOM) on soil organic carbon (SOC) dynamics. The goal of this study was to determine the short-term dynamics of bulk SOM and WEOM pool in weathered Brazilian soil, as conditioned by both land use and type of plant inputs. To this aim we used the appraisal of the natural delta C-13-OC signature to follow the variation of SOC pools and to overcome the analytical issues related to low OC concentration of WEOM fractions. In a one-year laboratory incubation, soil samples from a natural forest and an adjacent site under sugarcane monoculture were amended with either forest litter or sugarcane straw to evaluate SOM and WEOM dynamics. The bulk SOM was mainly affected by inherited conditions rather than on the organic amendments. The incorporation of sugarcane residues in the forest site produced a partial replacement of the original SOC, while the amendments to the cultivated soil promoted a stable increase in SOC. The analysis of WEOM highlighted its influence as a reactive pool in the short-term SOM dynamics. The delta C-13 variation of WEOM from cultivated soil corroborated the significant ponderal rate of OC input from forest litter that accounted for 22\% of total dissolved carbon. Unexpected evidence on the shift of isotopic dilution between SOC pools indicates that the evaluation of delta C-13 to determine the origin of the SOC fraction should be conservatively adopted. The results outline that the steady state of SOM in forest systems may undergo a rapid decline with limited counteracting effects of exogenous OM inputs. Conversely, OC depleted agricultural soils may benefit from organic amendments, thus acting as an effective sink for SOC accumulation.