Application of arbuscular mycorrizal fungal (AMF) inoculum is often recommended for revegetation sites in order to improve plant growth. At a roadside right-of-way site near Marysville, CA, we constructed plots on the fill slope embankment that were either inoculated with commercial AMF and seeded, or seeded without inoculum. After three seasons, we evaluated the sites to determine: 1) if mycorrhizal inoculation of the fill slope changed plant growth compared to no inoculation, and 2) if the fill slope AMF population was significantly different from the AMF population found in adjacent undisturbed soils. In addition, we set up a greenhouse experiment to determine: 1) if the two AMF populations (fill slope and ambient) differentially affected the growth of Lupinus nanus, Nassella pulchra, and Bromus hordeaceus, both in monoculture and when co-existing in pairs, and 2) if phosphorus is limiting to plant growth on the fill slope material. Our results indicate that the presence of either inoculated or ambient AMF population was always beneficial to the growth of L. nanus, but always a cost to the growth of B. hordeaceus. Replacing ambient AMF with fill slope AMF reduced the biomass and co-existence ratios of L. nanus and N. pulchra but was neutral or increased the biomass and co-existence ratios of B. hordeaceus. Based on the results with these soils and plant species, we conclude that the addition of ambient AMF and no additional phosphorus at this site has the potential to increase the growth of L. nanus and N. pulchra, especially when they co-exist together, and to reduce the growth of B. hordeaceus.
