Microbial trait multifunctionality drives soil organic matter formation potential

Abstract Soil microbes are a major source of organic residues that accumulate as soil organic matter (SOM), the largest terrestrial reservoir of carbon on Earth. As such, there is growing interest in determining the microbial traits that drive SOM formation and stabilization; however, whether certain microbial traits consistently predict SOM accumulation across different functional pools (e.g., total vs. stable SOM) is unresolved. To address these uncertainties, we incubated individual species of fungi in SOM-free model soils, allowing us to directly relate the physiological, morphological, and biochemical traits of fungi to their SOM formation potentials. We find that the formation of different SOM functional pools is associated with distinct fungal traits, and that ‘multifunctional’ species with intermediate investment across this key grouping of traits (namely, carbon use efficiency, growth rate, turnover rate, and biomass protein and phenol contents) promote SOM formation, functional complexity, and stability. Our results highlight the limitations of categorical trait-based frameworks that describe binary (high/low) trade-offs between microbial traits, instead emphasizing the importance of synergies among microbial traits for the formation of functionally complex SOM.