Nitrogen deposition facilitates nonnative plant invasion through increased nitrogen availability and changes to plant-soil feedbacks

Background/Question/Methods

Increased resource availability and plant-soil feedbacks have both been invoked as potential mechanisms of exotic plant invasion. Anthropogenic nitrogen deposition has resulted in a loss of plant diversity and vegetation-type conversions in ecosystems worldwide, often involving invasive plant species. The driver of these drastic changes in plant communities may be the result of increased soil nitrogen availability, as well as shifts in soil microbial communities, both of which may favor more ruderal nonnatives. While nitrogen availability and soil biota are known to be critical determinants of plant-soil feedbacks, there are few studies examining these relationships in the context of nitrogen deposition and invasion. We sought to understand the influence of soil nitrogen availability and soil microbial communities impacted by simulated nitrogen deposition on the plant-soil feedbacks of three Mediterranean plant species which are invasive in southern California: Bromus diandrus, Centaurea melitensis, and Hirschfeldia incana. In a two-phase, full-factorial greenhouse experiment, we tested the effects of nitrogen availability and nitrogen-impacted soil communities on plant growth response and conspecific plant-soil feedbacks. We hypothesized that species would differ in their responses to nitrogen and soil biota from low and high N deposition field plots, but that overall increased nitrogen availability and nitrogen-impacted soil communities would increase plant performance.

Results/Conclusions

Plant species differed in their response to soil inoculum, relative to sterile controls, as well as soil nitrogen availability. B. diandrus and C. melitensis responded positively to soil biota, while H. incana had little response relative to sterile controls. In all species, increased nitrogen availability resulted in increased biomass and leaf area. In the first phase of the experiment, C. melitensis had the greatest growth response when inoculated with soil communities impacted by simulated nitrogen deposition, while B. diandrus and H. incana performed best in low deposition inoculum. However, in phase two, when plants were grown in soil previously conditioned by conspecifics, all three species exhibited the greatest growth response in high deposition inoculum and under high nitrogen availability. While species may differ in their response to soil biota and nutrient availability, conspecific plant-soil feedbacks appear to be most positive in all three species under high nitrogen availability and in soil communities previously impacted by simulated nitrogen deposition. Our results suggest that nitrogen deposition may facilitate exotic plant invasion through plant-soil feedbacks due to both the direct impact of soil nitrogen enrichment, as well as through shifts in the soil microbial community.