Annual and Perennial Grass Growth on Nitrogen‐Depleted Decomposed Granite

Decomposed granite (DG) is often difficult to revegetate because of its low nitrogen (N) content and poor physical properties. Use of soluble fertilizers on DG sites increases plant‐available N but may encourage invasive annuals to grow rapidly and exclude perennial species. This study evaluates the effect of N availability on two potential DG revegetation 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 was provided in treatments ranging from zero to 1000 μM NO3‐N. Shoot biomass and root distribution were measured in monocultures and in mixtures of annual and perennial plants, both when the two species were seeded 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 50–100 μM treatments, however, growth of the perennial grasses equaled or exceeded that of the annuals. When seeded simultaneously, both species 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 treatments, root placement of the perennial was deeper than that of the annual. At high N treatments, however, root distribution of both species was similar, and the deep rooting characteristic of the perennial was no longer observed.