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SOIL FERTILITY & PLANT NUTRITION

Nitrogen Uptake by Perennial and Invasive Annual Grass Seedlings: Nitrogen Form Effects

^a USDA-ARS, Grazinglands Research Lab., 7207 W. Cheyenne St., El Reno, OK 73036-2144
^b USDA-ARS, Forage and Range Research Lab., Utah State Univ., Logan, UT 84322-6300
^c USDA-ARS, Corn and Soybean Research, Wooster, OH 44691-4096

^* Corresponding author (Charles.MacKown{at}mac.com).

Invasive annual cheatgrass (Bromus tectorum L.) and medusahead wildrye [Taeniatherum caput-medusae (L.) Nevski ssp. asperum (Simonk.) Melderis] have decreased livestock productivity and biological diversity and increased the frequency of wildfire on rangelands in the western United States. On disturbed sites, squirreltail (Elymus sp.), a short-lived North American perennial, appears to compete against invasive exotic annuals when available soil N and nitrification are reduced. We tested the hypothesis that differences in N uptake activity could account for this phenomenon. North American seedlings of perennial bluebunch wheatgrass [Pseudoroegneria spicata (Pursh) Á. Löve], four populations of squirreltail, and two invasive exotic annuals were cultured in the laboratory on NH₄⁺, NO₃^–, or NH₄NO₃ nutrient solutions, and N uptake activity (mol kg^–1 root dry wt. h^–1) was measured. The overall biomass means of 4-wk-old seedlings cultured with NO₃^–, NH₄NO₃, and NH₄⁺ were 72, 67, and 42 mg seedling^–1, respectively. Regardless of N form, cheatgrass biomass was as much as 4.2-fold greater than any of the other grasses including medusahead, which exceeded the biomass of all perennials except one. Cheatgrass had 1.5- to 2.2-fold greater NO₃^– uptake activity than the perennials, but the NO₃^– 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. The vigorous seedling growth of the invasive annuals coupled with the greater NO₃^– uptake activity of cheatgrass appear to be primary traits of the invasive annuals, driving their superior N capture and competition compared with these North American perennial grasses.

Abbreviations: DAP, days after planting