Controls over leaf litter decomposition in wet tropical forests

T ropical forests play a substantial role in the global carbon (C) cycle and are projected to experience significant changes in climate, highlighting the im portance of understanding the factors th a t contro l organic m atte r decom position in this biome. In the tropics, high tem perature and rainfall lead to some o f the highest rates o f litter decom position on earth , and given the near-op tim al ab io tic conditions, litter quality likely exerts disproportionate contro l over litter decom position. Y et interactions between litter quality and abiotic variables, m ost notably precipitation, rem ain poorly resolved, especially for the w etter end o f the tropical forest biom e. W e assessed the im portance o f variation in litter chem istry and precipitation in a low land tropical rain forest in southw est C osta Rica th a t receives >5000 m m o f precipitation per year, using litter from 11 different canopy tree species in conjunction w ith a th roughfa ll m an ipu la tion experim ent. In general, despite the exceptionally high rainfall in this forest, sim ulated throughfall reductions consistently suppressed rates o f litter decom position. Overall, variation between species was greater than th a t induced by m anipulating throughfall and was best explained by initial litter solubility and lignin:P ratios. Collectively, these results support a m odel o f litter decom position in which m ass loss rates are positively correlated w ith rainfall up to very high rates o f m ean annual precipitation and highlight the im portance o f phosphorus availability in controlling m icrobial processes in m any low land tropical forests.