Traits of soil bacteria predict plant responses to soil moisture

Microbes can promote beneficial plant and animal responses to abiotic environments, but the ecological drivers of this benefit remain elusive. Here we investigated byproduct benefits, which occur when traits that increase the fitness of one species provide incidental benefits to another species with no direct cost to the provider species. In experimental mesocosms, microbial traits predicted plant responses to soil moisture such that bacteria with self-beneficial traits in drought increased plant early growth, size at reproduction, and chlorophyll concentration under drought, while bacteria with self-beneficial traits in well-watered environments increased these same plant traits in well-watered environments. Thus, microbial traits that promote microbial success in different soil moisture environments also promote plant success in these same environments. Our results show that the concept of byproduct benefits, originally conceived to explain the evolution of cooperation in pairwise mutualisms, also applies to interactions between plants and non-symbiotic soil microbes.