Print PageIn nutrient poor tropical rainforests, leaf litter decomposition is of key importance for nutrient availability to plants and soil microorganisms. In addition, microbial decomposers are supposed to be sensitive to substrate C to nutrient ratios because they display a narrower C:nutrient ratio than the plant litter they consume. In an Amazonian rainforest we tested resource limitation and stoichiometric constraints on leaf litter decomposition and substrate induced respiration (SIR) as a proxy for microbial biomass in leaf litter from six tropical tree species displaying a wide range of C:N:P stoichiometry. With an additional external supply of mineral N and P, and organic C (in the form of cellulose), we evaluated whether non-limiting external resources can compensate for non-optimal litter elemental ratios and whether the initial litter stoichiometry would influence the magnitude of the response of microbial processes to external resource supplies. Leaf litterbags with small and coarse mesh size (to allow or not access of fauna) were placed in a fully factorial CNP fertilization experiment with 5 independent blocks in a natural tropical rainforest at Paracou, French Guiana. After 4 months, litter mass loss and SIR in remaining litter and associated underlying soil were analyzed.
Results/Conclusions
Litter mass loss and microbial biomass were influenced by litter species identity, fauna presence, and resource addition, and their interactions. Overall, litter mass loss strongly increased in the presence of fauna and responded positively to NP and CNP additions. Interestingly, the NP and CNP effect on litter mass loss correlated positively with increasing initial leaf litter N:P ratio, but only in presence of fauna. Similarly, microbial biomass in litter also increased in the NP and CNP treatments and was higher in litterbags excluding fauna. Microbial biomass in soil underneath litterbags only responded to resource addition, and substantially increased with the addition of P and even more so with the combined addition of P and C. Our results show that (i) initial litter stoichiometry had a relatively weak impact on the decomposition and associated microbial biomass of that litter, (ii) litter decomposition and associated microbial biomass were accelerated with combined addition of N and P: this effect was mediated by fauna and especially as initial litter stoichiometry was unfavorable. Our data suggest that litter stoichiometry is an important determinant for how decomposers respond to increased external nutrient supply and that soil microorganisms are co-limited by P and C in this Amazonian rainforest.
