Ranching operations in historical California have transformed much of the native coastal sage scrub (CSS) to nonnative grasslands. One nonnative invasive bunch-grass, Phalaris aquatica, was removed from 2006–2013 from a 25-acre field site in the Santa Monica Mountains National Recreation Area. In a greenhouse study, we demonstrated that soil legacy effects due to P. aquatica were significantly associated with less native plant growth. Our question now: is the inhibition of native plant growth in post-P. aquatica soil due to soil microbial composition, not just soil chemistry, and if so, can native plant inhibition be prevented by inoculation with native microbes? Three species of native plants (Artemisia californica, Salvia leucophylla, Baccharis pilularis) were grown in pots filled with either sterilized or unsterilized native CSS soil collected near the 25-acre post-P. aquatica site (N=30 pots per treatment). A total of 180 native plants were transplanted from these pots into the 25 acre post-P. aquatica site. During seven months of growth in the field, monthly soil cores were taken for nitrogen and microbial composition analysis, plant height and mortality was recorded, and (in the last month) rhizosphere soil was collected.
Native plants grew better with unsterilized native soil inoculum than with the addition of sterilized native soil inoculum. Artemisia californica had a 3:1 mortality ratio for plants grown with sterilized native soil, and the average plant heights of A. californica and B. pilularis were significantly higher (p= 0.012, 0.007) with unsterilized native soil. For the first two months of growth, the microbial composition between soil treatments were the same, but CSS and post-P. aquatica control soils were different from each other. In the last month of growth, rhizosphere soil from destructively sampled plants had different microbial composition between treatments. Concentrations of NH4+ and NO3- in treatment sites were similar between treatments, but slightly higher than CSS control soils and lower than post-P. aquatica control soils. The increased native plant growth and decreased native plant mortality in the post-P. aquatica site occurring after initial growth in unsterilized–but not sterilized–native soil, suggests that differences in the microbial composition of the soils are causing differences in plant growth. Additionally, differences in microbial communities observed between soil treatments in the last month of the study indicate that soil restoration via native soil inoculum will improve native plant restoration success in post-invasive-grass soils.