COS 131-1 - The impact of invasive herbivores and nitrogen deposition on the developing soil microbial community of Mount St. Helens

Abstract: Primary succession is often viewed through the lens of plant succession, however, plant-soil-microbial interactions influence plant assembly and soil development. In 1980 the Mount St. Helens, WA (MSH) volcano erupted creating a primary successional landscape devoid of organic matter called the Pumice Plain. Soon after the eruption the foundational woody species Sitka willow (Salix sitchensis) established. At MSH, an invasive stem-boring weevil greatly reduces Sitka willow size and increases mortality. Nitrogen deposition is a significant global change factor and is known to have significant effects on microbial community composition. How N-deposition and herbivory interact in a successional context is unknown. Here we seek to determine the effect of herbivory and N-addition on the developing soil microbial community associated with willows at MSH. In 2013 we established two sites on the Pumice Plain. These sites contained 18 plots each with three replicates of N addition at 8 and 16 kg/ha/yr crossed with weevil exclusion. In 2020 we sampled under each willow, combined samples by plot, and measured ASV richness and composition of soil bacteria, ectomycorrhizae, and general saprotrophic fungi via amplicon sequencing in response to eight years of invertebrate herbivore exclusion and N-addition.

We found that soil bacterial composition on the Pumice Plain varied between sites (P=0.03). Bacterial richness was generally higher where weevils were excluded, however, there was no effect of N-addition. There were also differences in fungal composition between the two sites on the Pumice Plain suggesting underlying environmental differences driving composition and/or dispersal limitation . The most dominant fungal class was Agaricomycetes followed by Leotiomycetes and Sordariomycetes. Shifts in the soil microbial community with N-deposition, like we found, can alter soil pedogenisis and affect plant-microbial interactions. Taken together, we found that invasive species and nitrogen deposition can shift the microbial community in developing soils and in association with keystone plants roots which may alter successional trajectories.