Soil Nitrogen Controls on Grass Seedling Tiller Recruitment

Establishment of perennial grass seedlings is critical for repairing rangeland ecosystems in the Western United States. Mineral N in the soil may be one of the controlling factors for seedling establishment and may improve the competitive ability of perennial grasses compared to the annual grass, cheatgrass (Bromus tectorum L.). A greenhouse experiment was conducted to investigate the effects of immobilizing mineral soil N on tiller recruitment in grass seedlings. The perennial grasses big squirreltail (Elymus multisetus [J.G. Smith] M.E. Jones), “Goldar” bluebunch wheatgrass (Pseudoroegneria spicata [Pursh] A. Löve), and “CD II” crested wheatgrass (Agropyron cristatum [L.] Gaertner x A. desertorum [Fisch. ex Link] Schultes) were grown alone or with cheatgrass in pots for 7 weeks. Plants were grown in soil amended with ground barley straw to produce the following four treatments designed to immobilize mineral N: (1) no straw added (control), (2) 0.25 mg straw kg–1 soil, (3) 0.50 mg straw kg–1 soil, and (4) 1.00 mg straw kg–1 soil. Number of tillers for perennial grasses grown alone was not significantly different than perennial grasses grown with cheatgrass. Cheatgrass and crested wheatgrass plants grown alone produced significantly fewer (36 to 39 percent) new tillers per plant in the 1.00 mg kg–1 treatment compared to the control. Squirreltail had significantly fewer tillers per plant in the highest straw treatment compared to the control when grown with cheatgrass. Cheatgrass produced more than twice as many tillers as the perennial grasses both in the control (high mineral N) and in the high straw treatment (low mineral N). Soil in pots containing cheatgrass had 94 percent less mineral N than soil in pots containing bluebunch wheatgrass. These results suggest that cheatgrass will likely be an effective competitor with perennial grasses even if mineral soil N is drastically reduced by microbial immobilization. Introduction ___________________ Establishing perennial grasses in rangelands degraded by invasive annual grasses continues to be one of the greatest challenges to repairing ecosystems in the Intermountain West. Many characteristics of the invasive annual cheatgrass (Bromus tectorum L.) allow it to be a superior competitor for belowground soil resources (Harris 1967). A better understanding of how changes in mineral N availability alters species competitive interactions between cheatgrass and perennial grasses (Grime and others 1987; Tilman 1984) may lead to new weed management strategies for enhancing perennial grass establishment. Annual plants such as cheatgrass generally have a higher growth rate than longer lived perennial grasses (Arredondo and others 1998). In addition, annual grasses have higher NO3 uptake and greater N productivity (Garnier and Vancaeyzeele 1994; Poorter and others 1990), which may reduce the success of seeded perennial grasses on semiarid rangelands (Hironaka 1961). High growth rate and mineral N uptake allow cheatgrass seedlings to emerge earlier (for example, Pyke 1990) and exhibit greater early spring root growth and root proliferation in fertilized soil microsites (Caldwell and others 1991; Eissenstat and Caldwell 1988) than native perennial grasses. Recent evidence suggests that low concentrations of mineral N hinder potential shoot and root growth of both cheatgrass and native perennial grasses (Monaco and others 2003). However, it is uncertain whether reducing mineral N will improve the establishment of perennial grasses under competition with cheatgrass. We conducted a greenhouse experiment with potted plants to evaluate the hypothesis that low mineral N would improve the competitive ability of perennial grasses when grown with cheatgrass. Specifically, we anticipated that treatments designed to reduce soil mineral N would hinder juvenile tiller recruitment of cheatgrass relatively more than that of perennial grasses. Number of tillers were evaluated because tiller numbers directly affect numerous demographic and ecological processes including plant size, competitive ability, productivity, grazing resistance, and population persistence (Briske 1991). In seedlings of perennial and annual grasses, tiller number provides an effective measure of a plant’s potential growth and establishment. Materials and Methods __________ The effects of low mineral N on tiller recruitment in cheatgrass, squirreltail, bluebunch wheatgrass, and crested wheatgrass were evaluated in a greenhouse experiment in Logan, UT. A coarse-loamy soil characterized as a Xeric