The size and diversity of microbes determines carbon use efficiency in soil

Soil harbors the most biodiverse habitat on earth, containing microorganisms spanning all three domains of life. These organisms and their interactions drive the cycling of soil carbon. Microbial carbon use efficiency (CUE) is an indicator of how microbial communities influence soil organic carbon storage through biomass production. As a physiological trait, CUE varies among microorganisms. Despite this, few studies have directly examined how biotic factors drive CUE. For instance, a microorganism's body size can influence its growth rates and interactions within the soil environment with potential consequences for CUE. Additionally, although biodiversity is posited as a driver of ecosystem processes, evidence connecting variation in microbial community composition and CUE remains scarce. To address these knowledge gaps, we manipulated microbial body size and biodiversity through size-selective filtering. Here we show that microbial community structure is a strong driver of CUE. As the maximum body size of the bacteria community decreased, we found a reduction in bacterial diversity and functional potential that lowered the community CUE. Additionally, the inclusion of micro-eukaryotic organisms shifted soil carbon cycling and resulted in a higher CUE, microbial biomass, and soil carbon to nitrogen ratio. These results highlight the importance of microbial traits, community structure, and trophic interactions in mediating soil carbon cycling.