Response to ammonium and nitrate by a mycorrhizal annual invasive grass and native shrub in southern California

The goal of this study was to determine the interaction of mycorrhizae and two N sources, ammonium (NH4+) and nitrate (NO3-), on the growth of a coast al sage scrub (CSS) species, Artemisia californica, and an exotic annual gr ass, Bromus madritensis ssp. rubens. Anthropogenic nitrogen deposition may be influencing the decline of CSS and replacement by exotic grasses, but th e extent to which mycorrhizae are involved in shrubland decline is unknown. NO3- is the dominant form of deposition in southern California, although t he native, uneutrophied soils have a greater concentration of NH4+. Seeds o f each species were germinated in pots of sterile soil, inoculated with nat ive soil containing mycorrhizal spores and infective root fragments, and fe rtilized with 50 mug/g of either NO3- or NH4+. NH4+ enhanced the growth of both mycorrhizal species, while NO3- did not. Control plants of B. madriten sis under low N had a significant response to mycorrhizae, but A. californi ca did not. Nitrate increased the growth of nonmycorrhizal A. californica a s much as the mycorrhizal NH4+ -treated plants. There is no evidence in thi s study to suggest that the decline of A. californica or increase in B. mad ritensis is due to a mycorrhizal response to NO3-, Other life history trait s of the two species must be used to explain the invasive behavior of the a nnual grass. Mycorrhizae may be more important in controlling plant growth in native uneutrophied soils dominated by NH4+ rather than NO3-.