Experimental litterfall manipulation effects on soil bacterial community structure and soil carbon cycling in a wet tropical forest

Global changes such as increasing atmospheric carbon dioxide (CO2) concentrations or climate change are likely to drive shifts in plant-derived carbon (C) inputs to terrestrial ecosystems via changes in litterfall and plant net primary production (NPP). However, the effects of shifting detrital C inputs on belowground microbial community function, C cycling and fluxes remain largely unknown, especially in tropical forest ecosystems. To investigate how shifts in bacterial community composition resulting from differences in C availability affect organic matter decomposition and how soil C pools and fluxes respond to shifts in C inputs, I utilized an in situ litter manipulation experiment in a tropical rain forest in Costa Rica. In one study, I assessed whether changes in bacterial community composition and diversity were related to changes in microbial community function. To do this I used bar-coded pyrosequencing and a series of laboratory incubations to test the potential functional significance of community shifts on organic matter decomposition. In another study, I assessed the effects of the litterfall manipulation on in situ dissolved organic matter (DOM) fluxes, internal C and nutrient cycling, and soil CO2 fluxes. The manipulation had clear effects on soil bacterial community composition but mixed effects on microbial community function. These results show that while resource-driven shifts in soil bacterial community composition have the potential to influence decomposition of specific C substrates, those differences may not translate to differences in mixed DOM decomposition rates in situ. In the second study, results showed that increasing and decreasing litterfall inputs drove rapid and significant shifts in belowground C cycling, suggesting that shifts in litterfall inputs in response to global environmental change could have important consequences for belowground C storage and fluxes in tropical rain forests. Furthermore, the observed responses highlight the potential for marked differences between tropical ecosystems and temperate ecosystems, where the effects of forest litter on belowground C cycling are typically much more subtle. Taken together, these studies demonstrate the strong potential impacts of shifts in plant-derived C inputs on C cycling and bacterial community structure while having complex effects on microbial community function