Microbial responses to recurring wet and dry cycles in a tropical Ultisol

Microbes contribute to the breakdown of soil organic matter (SOM) and also to SOM formation through microbial necromass and excreted biochemicals. Since SOM is one of the largest terrestrial pools of carbon, understanding the factors controlling microbial feedback to SOM is critical. Our current research is centered on how perturbations to the hydrological cycle will impact microbial trait distributions, biogeochemical cycling, and feedback to SOM composition. Model simulations project the tropics will experience increased drought-precipitation cycles. Microbial responses to such drying and rewetting events have commonly been studied in Mediterranean soils, with more limited experiments focusing on tropical soils.We want to understand the array of microbial traits involved in microbial response to perturbation, and how this might feedback onto the composition of SOM. To investigate this, we c ollected intact soil cores from clay-rich Ultisol on Buena Vista Peninsula within the Barro Colorado Nature Monument, Panama. Intact soil cores underwent recurring drying and wetting cycles to simulate fluctuating wet-dry periods under constant relative humidity and temperature. Another set of cores only underwent rewetting at the beginning and end of the incubation, simulating a long drought while a control set of cores were kept at constant moisture. Over the course of a five and half month incubation, we continuously measured CO 2 , N 2 O, CH 4 fluxes during the wet-dry cycles. Alongside gas flux sampling, intact cores were incubated and destructively sampled at discrete periods for total organic, dissolved organic carbon, nutrient concentrations, aggregate stability, microbial community composition and functional potential, and SOM composition. This contribution will discuss the outcome of this experiment, within the context of tropical soil carbon stability under future scenarios of more frequent drought and precipitation events.