Differences in forest productivity due to climate change may result in permanently altered levels of litterfall and litter on the forest floor. Using experimental litter removal and litter addition treatments, we investigated the effects of increased and decreased litterfall on early-stage litter decomposition and the abundance of meso-arthropods in a moist tropical forest. Litterbags containing freshly fallen leaves of Cecropia insignis (above and below the litter on the forest floor, and with and without fungicide) and Simarouba amara, or untreated birch wood (Betula sp.) were placed in either (1) plots where all litterfall was removed monthly (L-); (2) plots where litterfall was doubled monthly (L+), or (3) control plots (CT). Litter removal significantly slowed decomposition of both species and reduced the abundance of meso-arthropods on Simarouba litter. The fungicide treatment did not reduce apparent mass loss of Cecropia leaves. The litter addition treatment accelerated the decay of birch wood, probably because of increased nutrient availability from the extra litter; but there was no change in leaf-litter decomposition or meso-arthropod abundance in the L+ treatment. After 68 days, the concentrations of nitrogen, phosphorus, potassium, and magnesium in partially decomposed Cecropia litter were higher in the L+ treatment and lower in the L- treatment. The accumulation of phosphorus and nitrogen was greater in the litter in L+ plots and lower in the L- plots while the release of potassium and magnesium from decomposing litter was lower in the L+ treatment and greater in the L- plots. Thus, differences in the quantity of litterfall affect decomposition with consequences for carbon and nutrient storage and cycling.