Contribution of conspecific soil microorganisms to tree seedling light responses: Insights from two tropical species with contrasting shade tolerance

Light intensity drives whole-plant carbon gain, with consequences for biomass production and plant community dynamics in forest systems. Recent studies suggest that soil microbial communities may mediate the impacts of resource availability on plant performance, yet little is known about the net effect of conspecific soil microorganisms for tree seedling light responses. Here we examined the interactive effects of light availability and presence of conspecific soil microorganisms on tree seedling growth, morphology and nutrient content for two congeneric tropical tree species. The two Bauhinia tree species with contrasting shade tolerance were grown in sterilized or unsterilized soil medium, under either high (50%) or low (10%) light conditions in a greenhouse experiment. Plant light responses and soil feedback effects were determined after 12 weeks. Results showed that the light-demanding tree species was generally more responsive to both light and soil microbes compared with its shade-tolerant congener. Presence of soil microbes enhanced plant growth and biomass responses to increased light availability for the light-demanding species alone, driven by positive soil feedback effects in high light. Six plant traits (leaf mass fraction, stem mass fraction, specific stem length, leaf phosphorus concentration, leaf nitrogen: phosphorus ratio and root nitrogen: phosphorus ratio) showed significant interactive effects between light and soil treatment. Observed changes to leaf biomass allocation in response to light in the presence of conspecific soil microorganisms were consistent with optimality theory and adjustments to maximize resource acquisition under different light conditions. In addition, presence of soil microbes decreased the average plasticity of plant nutrient content and stoichiometry in response to light for the light-demanding Bauhinia species. Together these results highlight the importance of conspecific soil microbes for plant-light relations, with implications for plant-plant interactions and species coexistence.