Smooth Brome: An Unexpected Response to Straw-Amended Soil (Alberta)

Smooth brome (Bromus inermis), introduced from Europe and Eurasia in the late 1880s, is one of the most widely planted and agriculturally valuable forage grasses in western North America. This grass is a long-lived perennial that grows faster than many native grasses. Once established in native grasslands, smooth brome grass (brome) can spread rapidly, in some cases transforming diverse plant communities into monocultures by suppressing growth and abundance of native flora, in turn reducing wildlife habitat and natural diversity.

Many native species tolerate nitrogen-impoverished soil, whereas brome and other non-native grasses such as Kentucky bluegrass (Poa pratensis) require abundant available nitrogen. Researchers have had varying success improving the competitive ability of native grasses by depleting available nitrogen through combined applications of sugar, sawdust, and straw to the soil (Wilson and Gerry 1995, Reever-Morghan and Seastedt 1999).

Brome was accidentally seeded in an experiment to test the hypothesis that native grasses might dominate non-native grasses with straw amendments to reduce available nitrogen. We were restoring disturbed sites in rough fescue (Festuca hallii) grassland, and the research site was located near Byemoor in central Alberta, Canada, on a 72 m × 60 m nonproducing natural gas well site, in loam textured Dark Brown Chernozemic soil. The surrounding vegetation is native aspen parkland, comprising a mix of trembling aspen (Populus tremuloides) stands and rough fescue grassland. The preconstruction vegetation was grassland dominated by rough fescue, western porcupine grass (Hesperostipa curtiseta), western snowberry (Symphoricarpos occidentalis), and prickly rose (Rosa acicularis).

Figure 1.

Smooth brome grass samples collected from the Byemoor field site in Alberta, Canada: a) the control without straw, leaf length averaged 419 mm; and b) the high straw treatment (1 kg/m²), leaf length averaged 266 mm. Both length (F_2,15 = 27.01, p < 0.001) and biomass (F_2,15 = 30.93, p < 0.001) were significantly greater in the control treatment.

On July 5, 2007, the site was recontoured to its original relief and previously removed topsoil was replaced. On July 10, we applied chopped wheat straw (8 to 15 cm lengths) in 12 randomized strips (each 5 m long) and incorporated it with a rototiller, 15 to 25 cm into the soil. Straw was applied at high (1 kg/m²) and low (0.5 kg/m²) rates with a control of no straw. In 6 m randomly selected strips perpendicular to the straw treatments, we seeded monocultures of Kentucky bluegrass and rough fescue, a mix of native grasses, and, by mistake, a monoculture of brome in three of the strips. Upon discovering the brome the following summer (June 2008), we planned to eradicate it because we were concerned its strongly rhizomatous and prolific seeding properties could affect the experimental results. However, during the single growing season, we noticed a strong negative reaction of brome to the straw treatment, brome plants in the straw treatment being much smaller and sparser than those in the control.

Table 1.

Mean (± SD) smooth brome (Bromus inermis) measurements and soil chemical properties as affected by straw amendments. In both greenhouse and field experiments, leaf length and biomass were highest with no straw, while ammonium and potassium concentrations were lowest. Different letters indicate significant differences among the treatments.

Before applying glyphosate, we took plant and soil samples for analysis. Five plants, including roots, were randomly extracted from each treatment. Adjacent soil samples were taken to 15 cm, the depth of first-year root growth. We established an adjunct experiment at the University of Calgary greenhouses. Field soil was collected from an area within 20 km of the Byemoor site, with similar soil and vegetation composition. The soil was mixed with chopped wheat straw at the same rates as the field experiment. Five pots of each treatment were each seeded with five brome seeds, which were thinned after three weeks to one plant per pot. The grass grew for 20 weeks at ambient light and temperature, and regular watering with tap water...