Contrasting responses of Intermountain West grasses to soil nitrogen.
The mechanisms responsible for soil-N-mediated species replacement of native perennial grasses by the invasive annual grasses cheatgrass (Bromus tectorum) and medusahead (Taeniatherum caput-medusae) on rangelands are not completely understood. In addition, the contributions of distinct forms of inorganic N (i.e., NH4+ and NO3-) to these shifts in species composition are currently unclear. Consequently, we conducted a greenhouse experiment to test 2 hypotheses: (1) that low N availability reduces growth (root and shoot) and N allocation of invasive annual seedlings more than native perennial species; and (2) that seedling growth and N allocation of invasive annual grasses is more responsive than native perennial grasses when supplied with NO3- relative to NH4+. We grew seedlings of 2 annual grasses and the native perennial grasses bluebunch wheatgrass (Pseudoroegneria spicata [Elymus spicatus]), and 4 populations of squirreltail (Elymus elymoides, E. multisetus) in separate pots and exposed them to treatments differing in N form and availability for 17 weeks. Unexpectedly, root and shoot growth of annual grasses were equal or greater than native perennial grasses under low N availability. Annual grasses took up more NO3- and allocated more growth and N to shoots than the perennial grasses (P<0.05). Perennial grasses had significantly greater root:shoot dry mass ratios than the invasive annual grasses across treatments (P<0.05). Invasive annual and native perennial grasses both had greater (P<0.05) shoot and root mass and allocated more N to these structures when supplied with NO3- relative to NH4+. The ecological implications of these growth and N allocation patterns in response to N availability and form provide important clues regarding the specific traits responsible for differences in competitive ability between invasive annual and native perennial grasses on semiarid rangelands.