This project will provide information on the contribution of an ecologically important ant guild to the carbon cycle and will help improve predictions of future carbon dynamics in tropical forests. One of the most conspicuous features of a tropical forest is the abundance of leaf cutter ants. Despite their prominence, little is known about the overall contribution of leaf cutter ants to carbon and nutrient cycles in tropical ecosystems. These tropical and subtropical forested ecosystems cover 17% of the Earth's land mass and store approximately 40% of all carbon and are an important determinant of the global carbon cycle. Leaf cutters are the dominant herbivores in tropical forest ecosystems, bringing 10-50 % of all surrounding vegetation into the nest, fertilizing nest soils, and promoting the release of carbon dioxide and methane into the atmosphere. The network of leafcutter ant trails extend throughout the forest and workers walk single-file, carrying pieces of leaves into their nests, where special fungi break down the plant material and produce hyphae, on which the ants feed. Leaf cutter ant nests are massive in size and during nest construction and maintenance ants mix leaf particles into the soil and alter soil chemistry. Leaf cutter ant activities have the possibility of controlling aspects of ecosystem dynamics, though the degree of their influence has not been quantified. The project will focus on the role of leafcutter ant nests as hotspots of carbon dioxide and methane emissions and the role of nests in altering soil chemistry and creating heterogeneity in carbon (C), nitrogen (N), and phosphorus (P) in the soil across the landscape. The work will be conducted at La Selva Biological Research Station, Costa Rica, which is an epicenter of tropical scientific research, hosting a large number of student groups, researchers from all over the world. The project will characterize C dynamics on leafcutter ant nests and differentiate the sources of carbon dioxide and methane emissions between the activities of ants, fungi, the nest microbial community, and roots and hyphae. The N, and P biogeochemistry of active leafcutter ant nests will be quantified as well as the soil legacy effects after nests are abandoned. The project uses a multi-faceted approach, including coupling continuous measurements of soil carbon dioxide emissions with discrete measures of methane efflux, stable C isotopes of carbon dioxide to determine sources, soil C and N pools and fluxes, estimates of root and fungal biomass, and microbial community and functional indices. Measurements will be performed on ant nests, paired non-nest sites, and along a sequence of nests abandoned at different times in the past to quantify how long the impact of altered biogeochemistry persists in areas occupied by nests. These biogeochemical fluxes will be incorporated into an ecosystem model to estimate C flux at local-to-regional scales. The project will strengthen international collaborations by working with researchers and students from the US, Costa Rica, and New Zealand. In addition to training a postdoctoral researcher and mentoring several REU students, project scientists will conduct an annual workshop to introduce students and researchers to cutting edge field instrumentation and analysis techniques and provide opportunities to use these new approaches at La Selva. La Selva Biological Research Station is an epicenter of tropical scientific research, hosting a large number of student groups, researchers from all over the world, and serving as a center for ecotourism, thus the proposed research will be widely disseminated.