Collaborative Research: The Role of Iron Redox Dynamics in Carbon Losses from Tropical Forest Soils

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An understanding of how carbon and other elements are transformed in forests is critical to sustaining and nurturing this important global natural resource. In tropical forests, these transformations are likely to respond to variations in climate, and in turn, cause feedbacks in the global climate system. In particular, humid tropical forests cycle carbon more rapidly than any other ecosystem on Earth. A consistently warm and wet climate promotes rapid plant growth, as well as rapid decomposition of dead plant material. However, in their wet, clay-rich soils, biological activity frequently consumes oxygen faster than it is replaced by diffusion, temporarily creating regions of low oxygen availability in the surface soils. Periods of low oxygen availability are generally thought to enhance soil carbon storage by slowing the activity of micro-organisms that consume dead plant matter. If oxygen availability fluctuates, this may lead to more complex interactions. The proposed work will lead to a better understanding of these linkages, help explain why tropical forests have the highest global rates of soil carbon dioxide emissions, and inform global models and predictions used by public policy makers. In addition, the project will educate and train K-12 students and communicate scientific findings to the public. Project leader Dr. Silver will develop and run annual workshops on communicating climate change science together with local collaborators in Puerto Rico. Both PIs will provide direct student instruction including workshops with elementary students in California and internships for students from Georgia high schools with lower college attendance.This research will contribute to a better understanding of the controls on carbon cycling in tropical forests by: (1) determining the role of iron-redox cycling in organic matter decomposition; (2) quantifying the importance of two novel biogeochemical processes in tropical forests - abiotic iron oxidation coupled to carbon oxidation, and increased dissolved organic carbon production from iron oxidation. Both of these processes have the potential to contribute significantly to ecosystem carbon fluxes and to alter our understanding of carbon dynamics in tropical ecosystems. Studies will use a combination of laboratory, field experiments and modeling. Field experiments will be conducted in the Luquillo Experimental Forest, Puerto Rico, and will take advantage of an automated sensor network installed as part of the NSF-funded Critical Zone Observatory, as well as related research that will form the template from which to conduct experiments on the interactions of iron and carbon biogeochemistry. Radiocarbon measurements will determine the relative age of oxidized carbon, and 13C labeled substrates will be used to explore the potential for iron oxidation to stimulate the decomposition of complex carbon compounds. Structural equations and kinetic models will allow us to scale the work to the ecosystem level. The conceptual and numerical models developed from this research will address missing linkages between microscale redox processes to ecosystem level carbon cycling across a range of forest soil conditions and landscape positions. This work will help determine the potential sensitivity of tropical forest biogeochemistry to changes in climate and associated redox conditions, as well as feedbacks from soil carbon dynamics to atmospheric CO2.