Neotropical forests are significantly influenced by leaf cutter ants (LCA) which are the most important herbivore in these systems. LCA cut fresh leaves and bring large amounts of plant biomass into their nests to grow their fungus gardens. The excavation and continual maintenance of their large nests modifies soil characteristics and biogeochemistry with direct and indirect impacts on soil organic carbon (SOC) dynamics. The aim of this study was to quantify the effects of LCA (Atta cephalotes) on soil carbon, nitrogen, phosphorus, C, N, and P degrading enzymes, and microbial fractions across a 1 m soil depth profile and comparing between two different soils (residual and alluvial) and forest types (primary and secondary) in a wet tropical rainforest in Costa Rica. We hypothesized that LCA modifications of soil structure and continual input of fresh organic matter will influence biogeochemical processes, microbial biomass and degrading enzymes.
All soil C parameters were highly variable among sites and between nests and controls. Contrary to our expectations, C mineralisation rates were higher in control soils, where C degrading enzymes were in higher concentrations (around 250 µmol). Soil N and P increased with soil depth and were generally higher in nests than controls. Our results indicate greater heterogeneity inside the nests than previously expected. We explain our findings in terms of the removal of leaf and organic matter from the nest surface by LCA, which may have led to a reduction in available carbon substrate for microbial decomposition.