Tropical forests are extremely diverse and have the highest productivity per area of any biome. However, we have less understanding of belowground carbon (C) balance of tropical ecosystems. Leaf cutter ants (LCA) are ecosystem engineers that alter soil structure, soil nutrient pools, and soil gas fluxes, which may create spatial heterogeneity in dynamics of roots and mycorrhizal fungi and C cycling.
We examined how soil conditions and root and hyphal dynamics differ between LCA nest and non-nest areas in a lowland tropical forest at the La Selva Biological Station, Costa Rica, hypothesizing there would be greater root and hyphal growth and turnover in LCA nests relating to differences in abiotic soil conditions. Sensor arrays were installed in February 2015 and March 2016 at nest and non-nest sites, each consisting of one automated minirhizotron (AMR) and a series of soil moisture, CO2, O2, and temperature sensors placed at four depths. Images were collected daily and sensors collected data every fifteen minutes to generate daily averages. AMR images were analyzed using Rootfly to measure roots and hyphae for lifespan, biomass C, and turnover rate calculations. These data were regressed with sensor data to determine differences between nest and non-nest sites.
Soil moisture, CO2, and O2 concentrations differed between nest and non-nest soils. Nest soils generally had lower soil moisture and CO2 along with higher O2 when compared with non-nest soils. Soil moisture dropped significantly in nest soils during dryer periods while remaining more stable and consistent in non-nest soils. There was no difference in temperature between nest and non-nest soils. Soil moisture had a significant relationship with root and hyphal growth such that drying of soils yielded greater root and hyphal activity and soil saturation increased die-off.
The sensor data suggest that LCA alter their soil environment to reduce CO2 concentrations and soil moisture while raising O2 concentrations above surrounding non-nest soils. In turn, the changes in soil physical properties influence root and hyphal dynamics such that growth occurs at different rates between nest and non-nest soils. Differences in soil structure and root and hyphal dynamics lead to greater CO2 release from LCA nests than to non-nest soils. As LCA contribute to spatial heterogeneity in CO2 release, their distribution may be important when scaling up and generating C balance estimates across lowland tropical forest landscapes.