Differential responses of native and exotic coastal sage scrub plant species to N additions and the soil microbial community

Native shrub species of southern California have a long history of displacement by exotic annual herbs and forbs. Such invasions may be mediated by interactions with the microbial community and changes in the N cycle as a result of N pollution. However, the simultaneous effects of the soil microbial community status and N fertilization on dominant native and exotic plant species growth have not been thoroughly explored in this ecosystem. Three species of native shrubs and of exotic annuals were grown in an orthogonal two-factor greenhouse experiment. To assess the importance of the soil microbial community pre-sterilized soils were inoculated with sterilized or non-sterilized field soil; to assess the importance of N type pots were fertilized with nitrate, ammonium or glycine solutions. Plant shoot and root biomass was measured after harvesting. The natives Artemisia californica and Eriogonum fasciculatum had lower growth in sterilized soil, suggesting microbial facilitation of these species, and E. fasciculatum higher growth with ammonia than either nitrate or glycine. Salvia apiana had equal growth under all conditions. The exotics Brassica nigra and Bromus madritensis grew equally in sterilized and unsterilized soil, and B. madritensis greater growth with ammonia fertilizer. Centaurea melitensis had greater growth in sterilized soil, and with either form of inorganic N. These results highlight the importance of the soil microbial community in contributing to relative success of native vs. exotic species, and could inform restoration approaches for these species.