XVI International Botanical Congess

We examined the fate of 15NO3 or 15NH4 in mycorrhizal Bromus madritensis, an invasive annual grass, and Artemisia californica, a coastal sage scrub (CSS) shrub. Inoculated Artemisia and Bromus seedlings were transplanted into field plots in pairs in monoculture or mixture, given 15N treatments after 5 weeks, then harvested after 72 hr. Shoot biomass and [15NO3] of Artemisia were significantly lower in mixture than in monoculture. Bromus had significantly higher shoot [15N] in mixed culture than in monoculture. Mycorrhizal influence on [15N] and N source was varied. These results suggest that Bromus may be a greater competitor for NO3 than Artemisia. In natural areas where NO3 deposition is increasing from auto exhaust, the observed encroachment of Bromus into CSS may be in part related to N nutrition.