Modeling Emergent Life History Strategies and Niches of Bacteria in the Rhizosphere

Nearly half of assimilated plant carbon is exuded into the soil habitat surrounding growing roots, where it may be transformed into microbial biomass and subsequently stabilized through mineral associations. Yet, the part of soil volume considered as the rhizosphere is subject to great uncertainty, and is rarely explicitly considered in soil biogeochemical models. In order to better understand the mechanistic underpinning for the apparent efficiency of the microbial route to mineral stabilization in the rhizosphere, we combined a hierarchical trait integration framework (microTrait) with a trait-based dynamic energy budget model (DEBmicroTrait). DEBmicroTrait was parameterized with genomic data of bacteria isolated from the rhizosphere of an annual grass in Hopland, California, USA, and benchmarked quantitatively and qualitatively against measured growth rates in batch culture, as well as substrate uptake preferences in mixed medium. Our modeling results suggest that soil microbial communities in the rhizosphere undergo succession to optimize the trade-off between power and efficiency in a predictable manner. Selection for efficiency is important later in the plant growing season, where DEBmicroTrait successfully predicts microbial substrate preferences for specific root exudate compounds (aromatic organic acids and plant hormones), with carbon-use efficiencies that are much higher than typical values observed in soil. The underlying multivariate trait strategies map to generalizable rules for how microbes acquire complex resources and regulate soil organic matter formation. These emergent trait-based microbial strategies will be tested dynamically by coupling to a plant model in order to simulate boundary conditions and plant-microbial interactions in different soil habitats (rhizosphere, detritusphere, bulk soil) at microcosm scale.