Ecological restoration efforts can increase the diversity and function of degraded areas and inhibit establishment of non-native invasive plant species. However, current restoration practices cannot typically re-establish the full diversity and plant species composition of intact remnant plant communities. This project focused on the role of soil microbes in improving the establishment of native plants and in ameliorating the negative effects of non-native invasive plant species in grasslands. The researchers particularly focused on the beneficial effects of a group of soil fungi called arbuscular mycorrhizal (AM) fungi, which form symbiotic associations with most plant species. The researchers evaluated the potential beneficial effects of AM fungi on native grassland restoration and management and the potential to suppress non-native invasive plant species.

The research team evaluated the potential beneficial effects of AM fungi on native grassland restoration and management through a series of field surveys, field inoculation experiments, and greenhouse experiments. To better understand how AM fungal community composition is affected by anthropogenic disturbance across US grasslands, the research team sampled plant roots from pairs of remnant and disturbed sites spanning from western Oklahoma to eastern Illinois and assessed AM fungal community composition using environmental sequencing. To evaluate the sensitivity of non-native invasive plant species and native plant species to AM fungi researchers tested plant growth response with and without native AM fungi in a series of greenhouse assays. These assays were combined within a meta-analysis with data from previously published assays to generate robust conclusions on the relative responsiveness of native and non-native plant species.

The value of reintroduction of the native microbiome was tested in a series of field inoculation experiments. Five large field inoculation experiments were conducted across three Department of Defense properties (Fort Riley Army Base in Kansas, Tinker Air Force Base in Oklahoma, and the decommissioned Chanute Air Force Base in Illinois). In the grassland restoration experiments, native soil microbes were introduced in association with native plant species that are known to be good hosts for AM fungi. In each experiment, the same four host plant species, which were called nurse plants, were planted into each plot. Individual plot treatments varied with whole plots having 16 nurse plants inoculated with AM fungi isolated from nearby unplowed remnant grasslands, soil microbes freshly derived from native unplowed remnant grasslands, or the control non-inoculated plots. The generality of these field experiments was supplemented by three additional field inoculation assays featuring variations of the basic design. The context dependence in which inoculation of AM fungi would benefit native plant establishment was investigated in a series of greenhouse assays that manipulated different aspects of the environment.

We find that AM fungi are sensitive to anthropogenic disturbance, with weedy AM fungi dominating following mechanical disturbance and dominance of non-native invasive plant species. This degradation of the mycorrhizal community may facilitate the continued dominance of non-native invasive plant species because we find that non-native invasive and weedy native plant species are generally not responsive to mycorrhizal fungi and are not sensitive to different AM fungal taxa. In contrast, desirable, late successional native plant species are very responsive to mycorrhizal fungi and very sensitive to differences between AM fungal species, suggesting that the degradation of the soil community that occurs with non-native plant species invasion can inhibit native grassland restoration.

The research team found strong evidence that non-native invasive plant species can be inhibited and the quality of restorations can be improved by reintroducing native AM fungi into disturbed areas. Individual studies show that reintroduction of the native microbiome and native mycorrhizal fungi can improve plant diversity, accelerate succession, and increase the establishment of plants that are often missing from restored communities. Researchers did not find significant differences between the beneficial effects of inoculation with the local native soil microbiome compared to inoculation with cultured native AM fungal community, which suggests that the AM fungi were the major components of the soil microbiome that benefited native plant establishment and growth. Researchers also found that the benefits of inoculation can extend to non-inoculated neighboring native plants. Re-establishment of late successional plant species within a diverse native grassland can inhibit the dominance of non-native invasive plant species.

The benefits of inoculation with AM fungi will likely depend upon environmental context and on the source of the inocula. Researchers found that warming and drying environments may increase the competitive ability of the non-native invasive plant species, like old world bluestem, for example, by decreasing the importance of the interactions of soil microbes with native plants. Researchers also found that while using locally-adapted native AM fungal inocula was beneficial in general, the specific effects depended upon the ecological context. In particular, at early stages of succession, non-target native plant species may benefit most from well matched mycorrhizal fungal inocula, particularly during the first year of a restoration.

This work identifies AM fungi as keystone components of the plant microbiome and illustrates the potential value of reintroduction of native AM fungi as a native grassland management strategy to facilitate recovery and control of non-native invasive plant species. Researchers show that degradation of the AM fungal community was a major problem resulting from anthropogenic disturbance, such as due to military training, and dominance of non-native invasive plant species. Reintroduction of native AM fungi can accelerate grassland recovery by improving establishment and growth of highly desirable native plant species, and suppressing undesirable plant species including non-native invasive plant species. As the reintroduction of native AM fungi increases competitive ability of late successional long-lived native plant species, the implementation of this management approach could offer long-term solutions to recovery of grasslands from a history of disturbance and dominance by non-native invasive plant species.

The implementation of the inoculation approach on a large scale would require the development of native AM fungal cultures from native grasslands across the US. In addition, further research is required to test the conditions in which inoculation of native AM fungi is most advantageous and to document the long-term benefits of reintroduction of native AM fungi.