Exploring relationships between enzyme activities and leaf litter decomposition in a wet tropical forest

The theory of ecological stoichiometry predicts that the microbial biomass should regulate production of extracellular enzymes to target the resource in shortest supply. Therefore, microbial communities on decomposing leaf litter should optimize allocation to C-, N-, and P-degrading enzymes according to the stoichiometry of the foliar substrate. Because extracellular enzymes are the proximate agents of leaf litter decay, shifts in microbial enzyme allocation may influence overall rates of litter mass loss. To test these hypotheses, I measured fungal growth and the activities of acid phosphatase (AP), beta-glucosidase (BG), cellobiohydrolase (CB) and glycine aminopeptidase (GAP) on decaying leaf litter of five plant species over the course of a 394-day decomposition experiment. I used regression and correlation analyses to link to interspecific variation in mass loss rates with enzyme activities and foliar nutrient content. Enzymes explained 35% of the variance in foliar decay rates across plant species, yet fungal abundance and enzyme activities were unrelated to foliar concentrations of N, P, K, or 9 other nutrients. Furthermore, relative activities of C-, N-, and P-acquiring enzymes did not vary across litter types despite wide variance in foliar C:N and C:P ratios. This weak relationship between litter stoichiometry and decomposition rates suggests that nutrients are not the primary control on microbial growth or enzyme allocation in this tropical forest. However, substantial interspecific differences in fungal abundance and enzyme activities imply that differences in litter composition strongly influence microbial communities and the ecosystem processes they mediate.