Abstract: Soil microbes play a critical role in supporting plant growth and ecosystem health, plant performance, as they have a key role in plant productivity, nutrient uptake, defense response, environmental stress alleviation, and plant community composition. By facilitating processes such as leaf litter decomposition and nutrient cycling, soil microbial communities can support the growth and proliferation of invasive species, further disrupting plant-soil interactions. We investigated how soil microbial communities may influence the success of an invasive grass in the Southern California mediterranean ecosystem, Bromus madritensis. We hypothesized that the presence of leaf litter and nitrogen will have a positive relationship on the growth of B. madritensis. To test this relationship, we collected soil from ambient and nitrogen addition grassland plots at the Loma Ridge Global Change Experiment in Irvine, California. We sieved half of our soil collections to remove litter and any excess seeds. We then grew B. madritensis in a greenhouse from seeds using collected soil inoculum and established 5 treatments: (1) sterile soil, (2) inoculated soil, (3) inoculated soil with nitrogen added, (4) inoculated soil with nitrogen and leaf litter, and (5) inoculated soil with leaf litter. Upon germination, we harvested grasses every 3 days over a 12-week period and measured the above- and belowground biomass to quantify relative growth rates across treatments. We found that B. madritensis grew the most in the presence of both nitrogen and leaf litter, with an aboveground growth rate that was 15% greater than B. madritensis seedlings grown without leaf litter. While B. madritensis grew greatest in soil that contained added nitrogen, the inclusion of leaf litter also increased growth. Additionally, treatments with the addition of leaf litter resulted in a 22% increase in aboveground growth rates compared to treatments without leaf litter. Our findings suggest that soil microbial environments that contain nitrogen and leaf litter are important for optimal growth and productivity of B. madritensis. Understanding the relationship between soil microbial communities, nutrient cycling, and plant growth is essential for predicting and managing invasive plant species such as B. madritensis under changing climate conditions.
