Foliar pathogens and drought alter the strength and direction of plant-soil feedbacks in two herbaceous plants

Background/Question/Methods: A central challenge in ecology is to understand mechanisms maintaining species coexistence. An even more urgent challenge is to understand how a given mechanism may be altered by additional factors including disease or abiotic stress. Negative feedback between plants and their root-associate symbionts is thought to be one dominant coexistence mechanism. These negative plant-soil feedbacks occur when plant growth is suppressed due to the buildup of species-specific belowground natural enemies associated with conspecific adults. Here we test whether the strength and direction of plant-soil feedback changes when faced with stress from both foliar pathogens and water limitation. We performed a fully-factorial greenhouse experiment with two naturally co-occurring herbs Plantago lanceolata and Monarda fistulosa. We grew plants (n=485) singly in pots containing autoclaved soil inoculated with live or sterilized soil biota, previously conditioned by the conspecific or heterospecific plant species. Plants of each species x soil source (conspecific vs. heterospecific) x soil status (live vs. sterilized) combination were exposed to a species-specific foliar powdery mildew pathogen treatment (vs. uninfected) and drought treatment (vs. well watered). We measured plant growth, and collected soil and root samples to analyze bacterial and fungal communities via 16S and ITS sequencing.

Results/Conclusions: We found strong evidence that plant-soil feedbacks in this system significantly change under differing abiotic and biotic conditions. In the presence of live soil biota, each plant species grew worse with their own soil biota compared to heterospecific soil biota. However, this negative plant soil-feedback occurred only when both biotic (foliar powdery mildew pathogen) and abiotic (drought) stressors were present. In the absence of soil biota, we found no evidence for negative plant soil-feedback and neither foliar pathogens nor drought affected the strength of the plant-soil feedback. We are currently analyzing 16S and ITS sequences from soil and root samples, using a DADA2 workflow, to identify and describe the microbial communities and natural enemies present in the soils of different treatments. Whereas previous studies have demonstrated the existence of plant-soil feedbacks, our study is one of the first to demonstrate that the strengths of these feedbacks are altered by foliar pathogens and drought. Understanding of how above- and belowground plant-pathogen interactions are influenced by changes in climate variability will be crucial to maintain species coexistence and diversity under global change.