- Arbuscular mycorrhizal fungi (AMF) are plant symbionts that associate with the vast majority of terrestrial plants species. The AMF colonizes the plant roots by penetrating the root cortical cells, where the fungi exchanges mineral nutrients with the host plant for photosynthates. This association exists as a complex system in which both partners are expected to gain benefits. Such benefits eventually have cascading effects on the soil and plant communities. Furthermore, AMF can be inoculated as a management tool, and it has been used as a tool for ecological restoration on multiple ecosystems such as grasslands, forests and recently on rangelands. However, AMF inoculation of rangeland plant species is conducted without a clear understanding of their AMF interactions and effects. The goal of this dissertation is to enhance the understanding of the relationship between AMF and different important rangeland plant species. There is a lack of information for multiple valuable rangeland plant species regarding AMF interactions, this research is intended to be a starting point. We want to better understand how AMF can be used as a management tool for restoration programs in rangelands such as the sagebrush steppe.
The objectives of this study were: 1) to elucidate the effects of a commercial AMF inoculum on the growth of Artemisia tridentata ssp. wyomingensis, A. arbuscula, A. nova and Taeniatherum caput-medusae; 2) to evaluate the effect of a commercial AMF inoculum on biotic (interspecific competition from the invasive species T. caput-medusae) and abiotic (drought) stress amelioration on the sagebrush steppe plants A. tridentata ssp. wyomingensis, A. arbuscula, and A. nova; 3) to evaluate the mycorrhizal responsiveness and growth responses of T. caput-medusae, Ventenata dubia and Pseudoroegneria spicata to 3 different sources of AMF inoculum and biochar; and 4) to assess the bacterial diversity and abundance in a fire-disturbed sagebrush steppe ecosystem important for Purshia tridentata restoration.
We conducted a controlled greenhouse experiment to test if a commercial AMF inoculum could colonize and provide benefits to A. tridentata ssp. wyomingensis, A. arbuscula, A. nova and T. caput-medusae in sterile and live soil. We measured inoculum effects on root colonization means and biomass production. Inoculum viability was first confirmed using Trifolium incarnatum as a host. However, inoculum did not colonize the Artemisia or T. caput-medusae species. AMF colonization occurred in live soil only as it contained enough propagules. We observed a growth depression for all plants grown in live soil, compared to autoclaved soil, indicating either the presence of detrimental organisms (i.e pathogens/parasites) or other beneficial organisms in the live soil that were removed by sterilizing the soil. We also observed growth depressions in 2 of the 3 sage species when inoculum was added to autoclaved soil despite the fact that no roots were colonized by AMF, indicating that other factors in the inoculum itself (biotic or abiotic) influenced plant growth. The
differential effects of sterilizing soil and inoculating with a commercial mycorrhizal inoculum indicated the need to incorporate the soil microbial community in further assessments of using AMF inoculum.
We proceeded to test if a commercial AMF inoculum could colonize and mediate biotic (competition from T. caput-medusae) and abiotic (drought) stress on A. tridentata ssp. wyomingensis, A. arbuscula, and A. nova. Stress mediation was measured as an increase in biomass production and plant nutrient concentration. We found that the stressors (drought and competition) were the main drivers decreasing root colonization and plant biomass, while AMF inoculation could not provide evidence of stress amelioration. Inoculum promoted colonization of A. tridentata ssp. wyomingensis but did not colonize more than 5% on A. arbuscula and A. nova when grown on autoclaved soils. When grown on field soils alone, A. arbuscula and A. nova were colonized by 37%, and 33%. Competition and drought alone resulted in average 0.8% and 7.3% of colonization. On the other hand, T. caput-medusae averaged 18% colonization with inoculum and increased to 44% on field soils. We found data suggesting that T. caput-medusae might have gained benefits from AMF inoculum by increasing P and K compared to other treatments, indicating that this invasive species need to be further evaluated.
To further study the invasive species response to AMF, we evaluated T. caput-medusae along with V. dubia and the keystone rangeland species P. spicata to three sources of AMF (commercial AMF, early, and late seral soils) and biochar. We hypothesized that early seral plants would respond better to early seral AMF while the late seral plants would respond to commercial and late seral AMF; and mycorrhizal
responsiveness (growth difference between colonized and non-colonized plants) would be influenced by biochar. T. caput-medusae and V. dubia colonization was greatest when inoculated with early seral soils (38.83% & 19.61%), while P. spicata when inoculated with late seral soil and commercial AMF (32.08% & 32.19). However, biochar reduced the growth of the species and increased mycorrhizal responsiveness of T. caput-medusae from -1.22% to 8.88% and of V. dubia from -7.97% to 8.05% on late seral soils. Results indicated that both annual species might perform as facultative mycorrhizal plants, and more research is needed to understand such response. This suggests that both biochar and AMF need to be properly assessed before field applications to have a better understanding of possible outcomes.
Finally, another component required to further understand mycorrhizal interactions is the soil bacterial community. For this, we assessed the soil bacterial diversity and abundance that is important for Purshia tridentata in a sagebrush steppe ecosystem that was disturbed by fire by using molecular methods. Historical P. tridentata communities were removed by a wildfire in 2014 and subsequent restoration attempts have failed. We hypothesized that mycorrhizal and bacterial communities were negatively affected by the fire, and a valuable bacterial group (Frankia ssp.) will be restricted to the soil of the live plants canopies. We sampled soil under the canopy of remaining live P. tridentata individuals, soil from the canopy of burned P. tridentata individuals and in their interspaces among plants to account for site variability. Furthermore, we collected roots from live P. tridentata to assess the bacterial diversity and abundance directly associated with this keystone plant and its mycorrhizal colonization means. We found a larger diversity of bacterial families in soils from
under the canopy of live and burned P. tridentata individuals, but the abundance of families decreased under the burned canopies. However, bacterial abundance was greater on roots of live P. tridentata. Also, Frankia was mostly restricted to the roots of live P. tridentata. This study indicated that important bacterial groups might be altered by disturbance and restoration programs might need to incorporate not only traditional restoration tools but also AMF and bacterial cultures.
Our research is a starting point on AMF effects on important sagebrush steppe plant species. By conducting experimental methods under controlled conditions, we found that AMF inoculums are variable in their effects and require more research. Furthermore, other amendments such as biochar and components such as the bacterial communities need to be carefully addressed with in-deep research to further elucidate the best AMF practices in rangelands and the sagebrush steppe. This study is significant because it advises that AMF are not as straightforward to be used for restoration in rangelands, and much more research is needed before effectively using AMF inoculation as a tool. We also highlight that certain invasive species might benefit from AMF and care must be taken particularly on invaded rangelands.