Contrasting responses of Intermountain West grasses to soil nitrogen.

T.A. Monaco, D.A. Johnson, J.M. Norton, T.A. Jones, K.J. Connors, J.B. Norton, M.B. Redinbaugh

Abstract


The mechanisms responsible for soil-N-mediated species replacement of native perennial grasses by the invasive annual grasses cheatgrass (Bromus tectorum L.) and medusahead (Taeniatherum caput-medusae [L.] Nevski) 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 [Pursh] A. Love), and 4 populations of squirreltail (Elymus elymoides [Raf.] Swezey; E. multisetus [J.G. Smith] M.E. Jones) 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.

DOI:10.2458/azu_jrm_v56i3_monaco


Keywords


dry matter partitioning;Elymus multisetus;nutrient utilization;nutrient partitioning;nitrate fertilizers;soil nutrients;ammonium fertilizers;Taeniatherum caput-medusae;Elymus elymoides;Pseudoroegneria spicata;endemic species;shoots;Bromus tectorum;nitrogen;tillers;plant competition;invasive species;Utah;grasses

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