Serpentine soils are drought-prone and rich in heavy metals, and plants growing on serpentine soils host distinct microbial communities that may affect plant survival and phenotype. However, whether the rhizosphere communities of plants from different soil chemistries are initially distinct or diverge over time may help us understand drivers of microbial community structure and function in stressful soils. Here, we test the hypothesis that rhizosphere microbial communities will converge over time (plant development), independent of soil chemistry and microbial source. We grew Plantago erecta in serpentine or nonserpentine soil, with serpentine or nonserpentine microbes and tracked plant growth and root phenotypes. We used 16S rRNA gene barcoding to compare bacterial species composition at seedling, vegetative, early-, and late-flowering phases. Plant phenotype and rhizosphere bacterial communities were mainly structured by soil type, with minor contributions by plant development, microbe source and their interactions. Serpentine microorganisms promoted early flowering in plants on non-serpentine soils. Despite strong effects of soil chemistry, the convergence in bacterial community composition across development demonstrates the importance of the plant-microbe interactions in shaping microbial assembly processes across soil types.

Serpentine soils are drought-prone and rich in heavy metals, and plants growing on serpentine soils host distinct microbial communities that may affect plant survival and phenotype. However, whether the rhizosphere communities of plants from different soil chemistries are initially distinct or diverge over time may help us understand drivers of microbial community structure and function in stressful soils. Here, we test the hypothesis that rhizosphere microbial communities will converge over time (plant development), independent of soil chemistry and microbial source. We grew Plantago erecta in serpentine or nonserpentine soil, with serpentine or nonserpentine microbes and tracked plant growth and root phenotypes. We used 16S rRNA barcoding to compare bacterial species composition at seedling, vegetative, early-, and late-flowering phases. Plant phenotype and rhizosphere bacterial communities were mainly structured by soil type, with minor contributions by plant development, microbe source and their interactions. Serpentine microorganisms promoted early flowering in plants on non-serpentine soils. Despite strong effects of soil chemistry, the convergence in bacterial community composition across development demonstrates the importance of the plant-microbe interactions in shaping microbial assembly processes across soil types.