ANNUAL AND PERENNIAL GRASS GROWTH ON NITROGEN-DEPLETED DECOMPOSED GRANITE

Decomposed granite (DG) is often difficult to revegetate because of it s low nitrogen (N) content and poor physical properties. Use of solubl e fertilizers on DG sites increases plant-available N but may encourag e invasive annuals to grow rapidly and exclude perennial species. This study evaluates the effect of N availability on two potential DG reve getation species: an invasive, exotic annual grass (Bromus mollis) and a native perennial grass (Elymus glaucus). Plants in 10-L pots filled with DG were irrigated with all essential elements except N, which wa s provided in treatments ranging from zero to 1000 mu M NO3-N. Shoot b iomass and root distribution were measured in monocultures and in mixt ures of annual and perennial plants, both when the two species were se eded simultaneously and when the perennials were seeded 50 days prior to the annuals. At the higher N treatments, growth of annuals exceeded that of perennials. At solution N concentrations lower then the 50-10 0 mu M treatments, however, growth of the perennial grasses equaled or exceeded that of the annuals. When seeded simultaneously, both specie s showed reduced biomass in mixtures to an extent similar to that when each species grew alone. When the perennials were already established , the biomass of annuals was reduced proportionately more than that of perennials, even at the highest N treatment. At low and medium N trea tments, root placement of the perennial was deeper than that of the an nual. At high N treatments, however, root distribution of both species was similar, and the deep rooting characteristic of the perennial was no longer observed.