Plant-soil feedbacks, or plant-mediated changes to soil communities that impact subsequent plant growth, influence coexistence and invasion in plant communities. Plants may influence soil microbes through active root exudation or passive leaching from plant tissues. Plant-soil feedbacks exist between invasive Lespedeza cuneata and its native competitors, and L. cuneata has been shown to influence soil microbes in greenhouse and field experiments. Understanding the mechanisms governing this influence on soil communities is necessary to determine the spatiotemporal scales that L. cuneata feedbacks operate on. Because plants continuously release root exudates in response to changing environmental conditions and needs, exudate-driven feedbacks may operate continuously and on smaller timescales than leachates, which are passively lost during rain events or plant senescence. The goal of this study was to examine multiple L. cuneata solutions that may play a role in facilitating feedbacks. We collected root exudates, root leachates, and shoot leachates from 400 L. cuneata plants, characterized these solutions using GC/MS and DOC/DON/DIN measurements, and used them to condition microbial community assembly in mostly sterile soil. Resulting microbial communities were examined by sequencing. We grew L. cuneata, native L. virginica, and native Panicum virgatum in conditioned soil and compared biomass across conditioning treatments.
We identified differences in the chemical makeup of L. cuneata solutions: Of the 72 total metabolites examined, 12 were found in root exudates only, 16 in shoot leachates only, and 2 in root leachates only. Root exudates had more DIN than either leachate, and all solutions had low amounts of DON. Shoot leachates had more DOC than root exudates and root leachates, which had similar amounts. Conditioning soil with these solutions lead to the development of unique microbial communities. Key differences in taxa enrichment were identified. L. cuneata root exudate conditioned soil negatively impacted the growth of all plant species. Root leachates benefitted the growth of L. virginica and P. virgatum, and shoot leachates benefitted the growth of P. virgatum. The unique effects of each of these solutions on subsequent plant growth indicate that they may be simultaneously influencing in situ plant-soil feedbacks in different ways.