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Response to ammonium and nitrate by a mycorrhizal annual invasive grass and native shrub in southern California¹

Department of Botany and Plant Sciences, University of California, Riverside, California 92521-0124 USA

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 coastal sage scrub (CSS) species, Artemisia californica, and an exotic annual grass, Bromus madritensis ssp. rubens. Anthropogenic nitrogen deposition may be influencing the decline of CSS and replacement by exotic grasses, but the extent to which mycorrhizae are involved in shrubland decline is unknown. NO₃^– is the dominant form of deposition in southern California, although the native, uneutrophied soils have a greater concentration of NH₄⁺. Seeds of each species were germinated in pots of sterile soil, inoculated with native soil containing mycorrhizal spores and infective root fragments, and fertilized with 50 µg/g of either NO₃^– or NH₄⁺. NH₄⁺ enhanced the growth of both mycorrhizal species, while NO₃^– did not. Control plants of B. madritensis under low N had a significant response to mycorrhizae, but A. californica did not. Nitrate increased the growth of nonmycorrhizal A. californica as much as the mycorrhizal NH₄⁺-treated plants. There is no evidence in this study to suggest that the decline of A. californica or increase in B. madritensis is due to a mycorrhizal response to NO₃^–. Other life history traits of the two species must be used to explain the invasive behavior of the annual grass. Mycorrhizae may be more important in controlling plant growth in native uneutrophied soils dominated by NH₄⁺ rather than NO₃^–.

Key Words: arbuscular mycorrhizae • Artemisia californica • Bromus madritensis ssp. rubens • coastal sage scrub • nitrogen deposition

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