COS 5-1 - Dispersal and wildfire effects on leaf litter decomposer microbial communities under drought

Abstract: Environmental disturbances can affect biogeochemical cycling by altering the composition and activity of soil decomposer microbial communities. However, the impact of dispersal on community dynamics and function in response to global changes like drought and their effect on carbon cycling is not well understood. Additionally, disturbances such as fire may remove the surface leaf litter layer of soil, which in turn may affect the assembly and functional responses of decomposers dispersing in new leaf litter. To investigate the effects of dispersal as well as a wildfire on decomposer communities, we conducted a 16-month field experiment on litter decomposition as part of the Loma Ridge Global Change Experiment (LRGCE). The LRGCE has experienced experimental drought since 2007 in two adjacent semi-arid vegetation communities, grassland, and coastal sage scrub, and in 2020 experienced a wildfire. We sterilized leaf litter from both vegetation types collected before the fire. Then, we inoculated the litter with microbiomes from unsterilized leaf litter collected before or after the wildfire from ambient precipitation conditions in each respective ecosystem. We used microbial cages with 0.2 μm pores to restrict microbial dispersal and a microbial cage with 2 mm pores to allow dispersal under both control and drought conditions. We measured litter decomposition rates, fungal and bacterial biomass, metabolomics, and metagenomics at three-time points during the experiment. We found that scrub litter decomposed faster than grass litter across all treatments. We observed a significant 3-way interaction between precipitation, dispersal, and vegetation treatment on litter decomposition. However, fire did not affect the leaf litter communities’ rate of litter decomposition. Despite not affecting decomposition rates, fire reduced fungal biomass for most treatments. Under limited dispersal, drought resulted in slower decomposition rates in grass communities compared to ambient conditions, but when litter was open to dispersal, drought led to faster decomposition rates for the shrub communities. These results suggest that microbial dispersal plays a crucial role in shaping decomposition responses to environmental perturbations such as drought in an ecosystem-specific manner. Additionally, while fire may affect surface soil microbial communities, in fire-adapted ecosystems, there may not be a strong effect on decomposer community function.