Mycorrhizal fungi associate with roots, providing essential nutrients to their host in return for carbon. Variation in form and function of the two dominant mycorrhizal lineages, ectomycorrhizae (ECM) and arbuscular mycorrhizae (AM), lead to accumulation of organic carbon in ECM-dominated surface soils in temperate and tropical forests. In temperate systems, this is partially explained by ECM-associated trees producing leaf litter that is more chemically resistant to decomposition than AM-associated trees. However, tropical forests support high species diversity, resulting in overlapping ECM and AM litter chemistry. To disentangle the effects of litter chemistry from mycorrhizal status, we measured leaf litter decomposition rates in adjacent ECM and AM dominated tropical forest stands in the Fortuna Forest Reserve, Panama. In each forest stand, we decomposed litter from two ECM-associated species and four AM-associated species that varied in initial litter chemistry. Leaves sealed in 2 mm mesh bags were placed under the litter layer in the field and collected at six time points over 24 months to estimate decomposition rates from mass loss. We calculated decay rates (k) using the negative exponential decomposition equation and tested the effects of soil fertility, mycorrhizal status and leaf species on calculated k-values using a linear mixed model.
Results/Conclusions
We found that leaf species significantly affected leaf litter decomposition rates (P = < 0.0001), but mycorrhizal status of the forest stand had no effect (P = 0.62). Litter from two AM species (Citharexylum macradenium and Sapium spp.) decomposed faster than litter from the ECM species (Oreomunnea mexicana and Quercus insignis), but litter from the other two AM species (Micropholis melinoniana and Cecropia spp.) decomposed more slowly than ECM litter. Our results demonstrate that AM-associated trees do not inherently produce leaf litter that is more chemically vulnerable to decomposition than leaf litter from EM-associated trees. Our results also suggest that leaf litter decomposition is not affected by the dominant mycorrhizal association in a forest stand. We conclude that the observed accumulation of organic matter in ECM surface soils of tropical forests cannot be attributed to slower EM leaf litter decomposition or direct interactions between ECM fungi and leaf litter decomposition. Instead, we hypothesize that greater root production from ECM-associated tree species or ECM fungal biomass inputs rather than slower decomposition rates lead to the ECM surface soil organic matter accumulation.