Litterfall represents a major flux of carbon from the vegetation to the soil, but the effects of increasing aboveground litter production, a possible consequence of elevated atmospheric CO2 concentrations, on belowground carbon dynamics are poorly understood. We used large-scale monthly litter removal and addition treatments in a lowland tropical forest to assess the consequences of increased litterfall on belowground CO2 production. Over the second to the fourth year of treatments, litter addition increased soil respiration more than litter removal decreased it; soil respiration was on average 21% lower in the litter removal (L-) and 35% higher in the litter addition (L+) treatment compared to the controls (CT). There was no change in microbial biomass in the L+ plots, thus we predicted a 10% increase in soil respiration in the L+ plots, based on the 21% decrease in the L- plots and an 11% reduction due to lower fine root biomass in the L+ plots. The 35% measured increase in soil respiration was therefore 25% higher than predicted. It is likely that this ‘extra' CO2 was a result of priming effects, i.e. increased decomposition of older soil organic carbon by the addition of fresh organic matter. Our results suggest that increases in aboveground litter production as a result of global change could cause considerable losses of soil carbon to the atmosphere.