Microbial Carbon Transformations in Wet Tropical Soils: The Effects of Warming, Drying, and Redox Fluctuation

Tropical forest soils store more carbon (C) --as plant litter and decomposed organic matter-- than any other terrestrial ecosystem and play a critical role in the production of greenhouse gases (methane, nitrous oxide, carbon dioxide) that affect both atmospheric chemistry and climate. Humid tropical forests also exchange vast amounts of carbon, water, and energy with the atmosphere and can lose large amounts of dissolved carbon via runoff and leaching. The rapid carbon cycling characteristic of wet tropical ecosystems is driven in part by high rainfall and warm temperatures. This combination of environmental conditions causes tropical soils to alternate between oxygenated and anaerobic conditions, affecting the fate of soil carbon by changing mineral reactivity and the metabolic pathways used by tropical soil microorganisms that regulate many aspects of the belowground carbon cycle. In the coming half century, tropical forests are predicted to see a 2 - 5 degree (Celsius) temperature increase, plus substantial differences in the amount and timing of rainfall. Although tropical soils are clearly important to the global C cycle, we have a surprisingly poor understanding of how soil carbon cycling in wet tropical forests will respond to climate change. This makes predicting future climate impacts extremely difficult. Our ability to forecast how new moisture and temperature patterns will shape tropical microbial activity is a gap in knowledge because so little is known about the fundamental abilities and chemical preferences of tropical soil microorganisms. If wet tropical forests experience shifts in rainfall patterns, becoming generally drier and more aerated, microbially-mediated processes that produce greenhouse gases or help store soil carbon will likely be affected. However, few studies of microbial diversity have been conducted in wet tropical soils and less than a handful have evaluated microbial function with modern DNA sequencing technologies. This project will examine the genomic content and potential of tropical soil microorganisms as they experience shifts in soil temperature, moisture, and oxygen availability. Samples will be collected from lab manipulated drying/redox incubations and a field soil warming that will begin in summer of 2014. This experiment will be the first of its kind in any tropical forest, and our research will be part of a larger project aimed at understanding warming responses to tropical processes experimentally from coordinated above- and belowground perspectives. We will partner with JGI to examine the genomic content and potential of tropical soil microorganisms as they experience shifts in soil temperature, moisture, and redox. At EMSL, we will collect bulk and spatially resolved chemical information on C species and Fe oxidation state from soils and extracted DOC. By also tracking the degradation and fate of organic carbon compounds as they move onto mineral surfaces (e.g. Fe-OOH) and into the dissolved organic carbon pool (DOC), our work will help increase the accuracy of predictions about how microbial processes affect whether organic carbon is retained or lost from tropical systems. The mechanistic understanding produced by this research will directly benefit attempts to improve the predictive capacity of mathematical models that forecast future tropical soil carbon balance.