Heterosis, or hybrid vigor, refers to the superior phenotypes of hybrids relative to their parental inbred lines. While quantitative genetic and molecular mechanisms have explained some heterotic patterns, the mechanism is still largely unknown. Recently, soil microbes have been identified as drivers of maize heterosis. While manipulation of the soil microbial community consistently altered heterosis, the direction of this effect appeared to be dependent on the microbiome composition, environment, or both. Here we demonstrate that both the abiotic and biotic soil environment contribute to the strength of heterosis in maize. To determine the contribution of abiotic and biotic soil factors, two inbred maize lines (B73 and Mo17) and their hybrid (B73xMo17) were inoculated with four “live” and one “killed” soil microbial communities. The live soil microbial communities were collected from two agricultural and two prairie field sites, whereas the killed soil microbial community consisted of all four soils homogenized and autoclaved. In a second experiment, one of agricultural and prairie soils were used as inoculum for the same genotypes under high and low nutrient conditions. The microbial community composition, soil properties, and plant phenotypes were assessed for both experiments. After one month of growth, the reduction of soil microbes via soil sterilization resulted in increased heterosis relative to the live soil treatments. Additionally, heterosis decreased in the presence of high nutrients independent of soil inoculum. These results suggest that the reduction of soil microbes and nutrients impairs the growth of the inbred lines but not the hybrid.
