Remediating the microbial legacy effects of invasive grasses for restoration
Ranching operations in historical California have transformed much of the native coastal sage scrub (CSS) to nonnative grasslands. One of these grasses, Phalaris aquatica is an invasive bunchgrass that was removed from 2006–2013 from a 25-acre field site in the Santa Monica Mountains National Recreation Area. A preliminary greenhouse study we conducted last year 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 chemistry (abiotic) or soil microbial composition (biotic) causes? Three species of native plants (Artemisia californica, Salvia leucophylla, Baccharis pilularis) were grown in conetainers filled with either sterilized or unsterilized native CSS soil collected near the 25-acre post-P. aquatica site (N=30 conetainers per treatment). After three months, a total of 180 native plants were transplanted from these conetainers into the 25-acre post-P. aquatica site. Over the course of five months of growth in the field, monthly soil cores were taken for nitrogen and microbial composition analysis, plant height was measured, and plant mortality was recorded.
After seven months of growth in the field site, the native plants showed better growth in unsterilized native soil than in sterilized native soil. A. californica and S. leucophylla had 3:1 and 2:1 mortality ratios, respectively, for plants grown in sterilized vs. unsterilized native soil. The average plant heights of A. californica and B. pilularis were significantly higher (p= 0.012, 0.007) in unsterilized vs. sterilized native soil. The increased native plant growth and decreased native plant mortality in the post-P. aquatica site occurs after initial growth in unsterilized—but not sterilized—native soil, suggesting that differences in the microbial composition of the native and post-P. aquatica soils are causing differences in plant growth. The data also indicate that soil restoration, in the form of native soil addition, will improve the restoration success of native species in post-invasive-grass soil. Future research will focus on compositional analysis of microbial communities in the post-P. aquatica soil vs. the native soil to associate native plant growth with specific microbial groups.