PS 29-157 - Plant-soil feedback effects on oak regeneration under red pine plantations

Abstract: Oaks (Quercus spp.) are an important component of many forests in the Lake States region of North America, but their dominance has declined partly due to poor regeneration under seed trees. Interestingly, oak regeneration success has been observed in adjacent red pine plantations. One potential mechanism is plant-soil feedback (PSF) which occurs when plants affect soil characteristics that influence seedling performance: Soil conditioned by red pine may be more favorable for oak seedling performance than soil under seed trees. Two primary components of PSF are microbial and nutrient feedbacks. In this study, we evaluated the two components of PSF on northern red oak (Q. rubra) seedlings in a greenhouse by using soils collected from four pairs of adjacent oak/red pine stands across the lower Peninsula of Michigan. To separate the effects of these two components of PSF on seedling performance, oak seedlings were grown in “microbial soil” (sterilized mixed soil inoculated with live oak or pine soil) and “nutrient soil” (sterilized oak or pine soil inoculated with live mixed soil) for 10 weeks. Biomass was measured for the harvested seedlings from both experiments. Soil pH was measured, and fungal microbiomes were sequenced (Illumina) from the initial soil inoculum and the roots of the seedlings harvested from the microbial soil experiment. We hypothesize that Q. rubra seedlings grow better in pine than oak soils due to beneficial PSF effects from both microbes and nutrients. Seedling biomass was greater in pine than oak soils in the microbial soil experiment, but only at low pH (< 5). Overall, fungal community compositions from the initial pine and oak soil inocula were distinct across the four sites, but this difference was not clear in the roots of the oak seedlings between the oak and pine inoculated treatments in the microbial feedback experiment. However, there were greater relative abundances of fungal taxa that were classified as root endophytes in the roots of oak seedlings grown in pine than oak soils, but only at low pH (< 5). In addition, relative abundances of fungal plant pathogens were greater in seedlings grown in pine than oak soils with high pH ( > 5) in the microbial feedback experiment. We found no significant effect of nutrient feedbacks on seedling biomass. Our findings suggest that soil fungal communities in pine stands can provide a better growth condition for oak seedlings than adjacent oak stands in regions with low soil pH.