Nutrients influence key processes of the tropical carbon cycle, such as primary production and soil microbial activity. Long-term, large-scale field experimental nutrient addition trials are the most direct way to test the role of nutrients on ecosystem processes. The EARTH Forest Fertilization Experiment (EFFEX) is the second oldest running tropical fertilization experiment in the world. Here we present results on tree growth, decomposition, soil carbon dioxide (CO2) fluxes, as well as soil properties after 12 years of continuous fertilization with nitrogen (N) and phosphorus (P). At the community level, there was no effect of fertilization on stem basal area, although there were species-specific effects. Several understory plants increased foliar N concentration with N addition, but this effect disappeared when N and P were added together. Leaf litter decay rate (k) and P immobilization were significantly greater in plots that received P (+P, +NP) than in plots that did not (+ N, controls). Nitrogen addition did not significantly affect litter decomposition, but caused a transient increase in soil CO2 flux that disappeared after three days A laboratory incubation using different forms of N and P fertilizer produced the same result. Nitrogen addition also lowered the activity of sulfatase enzymes and increased the activity of enzymes involved in carbon (C) decomposition. In terms of soil properties, N addition decreased pH by 0.2 units, while P addition doubled Ca concentration (the fertilizer contains Ca). Phosphorus addition increased labile inorganic phosphate concentration by 15-fold, doubled soil P saturation, and increased microbial P. However, P addition had little impact on organic P fractions, suggesting that P mineralization rates have not changed due to stoichiometric imbalances. Total C and N were unchanged by fertilization. Base cations at this site are some of the lowest reported for the Neotropics, and (other than Ca) were unchanged by fertilization. Overall, observations of the tree community at EFFEX suggest that, at this site, stochastic events, such as wind blowdowns and lightning may be more important drivers of aboveground biomass stocks and cycling than changes in nutrients, at least at the decadal timescale. In soils, there was a modest biogeochemical response to fertilization despite large changes in soil nutrient concentrations, perhaps related to base cation limitation or the duration of the experiment. Given the apparently context-dependent nature of fertilizer effects in the tropics and elsewhere, collaboration across regions and biomes is crucial to unravel the role of nutrients in carbon cycling.