Nitrogen enrichment contributes to positive responses to soil microbial communities in three invasive plant species
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Increased resource availability and feedbacks with soil biota have both been invoked as potential mechanisms of plant invasion. Nitrogen (N) deposition can enhance invasion in some ecosystems, and this could be the result of increased soil N availability as well as shifts in soil biota. In a two-phase, full-factorial greenhouse experiment, we tested effects of N availability and N-impacted soil communities on growth responses of three Mediterranean plant species invasive in California: Bromus diandrus, Centaurea melitensis, and Hirschfeldia incana. In the first phase, plants were grown individually in pots and inoculated with sterile soil, soil from control field plots or soil from high N addition plots, and with or without supplemental N. In the second phase, we grew the same species in soils conditioned in the first phase. We hypothesized growth responses would differ across species due to species-specific relationships with soil biota, but overall increased N availability and N-impacted soil communities would enhance plant growth. In the first phase, Centaurea had the greatest growth response when inoculated with N-impacted soil, while Bromus and Hirschfeldia performed best in low N soil communities. However, in phase two all species exhibited positive growth responses in N-impacted soil communities under high N availability. While species may differ in responses to soil biota and N, growth responses to soils conditioned by conspecifics appear to be most positive in all species under high N availability and/or in soil communities previously impacted by simulated N deposition. Our results suggest N deposition could facilitate invasion due to direct impacts of soil N enrichment on plant growth, as well as through feedbacks with the soil microbial community.
KeywordsNitrogen deposition Plant–soil feedbacks Arbuscular mycorrhizal fungi Invasive species Coastal sage scrub
We would like to thank Dr. Irina Irvine for field and logistical support. Dr. Michael Allen, Dr. Jodie Holt and Dr. Louis Santiago provided valuable feedback regarding the design of this experiment and Erin Reilly and Amanda Haraksin assisted with greenhouse and laboratory work. We greatly appreciate all their contributions to this project. We are also grateful to two anonymous reviewers for feedback on an earlier draft of this paper. This research was supported in part by the National Park Service Air Resources Division (TASK AGREEMENT NO. J8C07110022).
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