There is scant information of above-ground vegetative carbon (C) and belowground soil organic C and nitrogen (N) stocks at the watershed scale in the Caribbean. The Century Ecosystems model (ver. 4) was applied to the Río Grande de Arecibo watershed, a 45,000 ha watershed in north-central Puerto Rico, having land areas of 8% agriculture, 8.4% pasture, and 71% secondary forest. The objective of the study was to predict the storage of aboveground biomass C and N and belowground soil C and N as a function of climate, vegetation and soils. The RGA watershed has 21 mapping units consisting primarily of coarse-textured Inceptisols and fine-textured Ultisols. For each soil type, the equilibrium period was run to approximate soil C, soil N and forest C, to conditions were there was minimal anthropogenic disturbance. The soil C and N components of the Century model were calibrated using present state empirical data of soils under mature secondary forest areas throughout the watershed. Hurricane frequency and intensities, and historical landuse patterns during the nineteenth and mid-twentieth centuries were input to the model. By the end of 1940's the increase of the industrialization promoted abandonment of agriculture lands and re-growth of secondary forest. The Century model adequately predicted soil C and N in secondary forest but weakly in pasture and shaded coffee. The results suggest that the Century model can be used to predict SOC, SON and secondary forest C in other watersheds of the Caribbean having similar soil types.