ARS | Publication request: Nitrogen uptake by perennial and invasive annual grass seedlings: Nitrogen form effects

Submitted to: Soil Science Society of America Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: March 28, 2009
Publication Date: September 11, 2009
Citation: Mackown, C.T., Jones, T.A., Johnson, D.A., Monaco, T.A., Redinbaugh, M.G. 2009. Nitrogen uptake by perennial and invasive annual grass seedlings: Nitrogen form effects. Soil Science Society of America Journal. 73:1864-1870.

Interpretive Summary: Incursion into perennial dominated rangelands of the Intermountain West by two winter exotic annual grasses, cheatgrass and medusahead, is one of the most serious plant invasion in North America. The invasions have decreased productivity and biological diversity and increased the frequency of rangeland wildfire. On disturbed sites, squirreltail, a short-lived native perennial, appears to compete against invasive non-native annuals when available soil N and nitrification are reduced. We tested the hypothesis that differences in N uptake activity could account for this observation. Four populations of squirreltail, bluebunch wheatgrass, a dominant native perennial, and the two invasive annuals were cultured on ammonium, nitrate, or ammonium nitrate nutrient solutions and N uptake activity measured. Regardless of N form, cheatgrass seedling growth was as much as 4.2-fold more than any of the other grasses and was greater than that of medusahead, which exceeded growth of all perennials except one. Cheatgrass had 1.5- to 2.2-fold greater nitrate uptake activity than the perennials, but the nitrate uptake activity of medusahead exceeded only two of the squirreltail populations. Ammonium uptake activities of perennials were not consistently more favorable than that of the annuals. The more vigorous growth of the invasive annual seedlings even with ammonium likely will be the primary factor affecting N capture and competition over these native perennial grasses. These results will be useful to rangeland scientists seeking solutions to use and restore disturbed landscapes of the Intermountain West.

Technical Abstract: Infestation by exotic annual cheatgrass (Bromus tectorum L.) and medusahead wildrye (Taeniatherum caput-medusae ssp. asperum [Simk.] Meldris) have decreased productivity and biological diversity and increased the frequency of rangeland wildfire in the Intermountain West. On disturbed sites, squirreltail (Elymus spp.), a short-lived native perennial, appears to compete against invasive non-native annuals when available soil N and nitrification are reduced. We tested the hypothesis that differences in N uptake activity could account for this observation. Seedlings of native perennial bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] A. Löve), four populations of squirreltail, and the two invasive annuals were cultured on ammonium, nitrate, or ammonium nitrate nutrient solutions and N uptake activity measured. Biomass of 4-wk-old seedlings cultured with nitrate and ammonium nitrate was 65% greater than those cultured with ammonium. Regardless of N form, cheatgrass biomass was as much as 4.2-fold greater than any of the other grasses and was greater than that of medusahead, which exceeded biomass of all perennials except one. Cheatgrass had 1.5- to 2.2-fold greater nitrate uptake activity than the perennials, but the nitrate uptake activity of medusahead exceeded only two of the squirreltail populations. Ammonium uptake activities of perennials were not consistently more favorable than those of the annuals. Even though differences in N uptake activities were found among the grasses, more vigorous seedling growth of the invasive annuals appears to be the primary factor affecting N capture and competition over these native perennial grasses.