Mineral Nitrogen in a Crested Wheatgrass Stand: Implications for Suppression of Cheatgrass

Abstract

Cheatgrass (Bromus tectorum L.) is an exotic annual grass causing ecosystem degradation in western US rangelands. We investigated potential mechanisms by which crested wheatgrass (Agropyron cristatum L. Gaertn., Agropyron desertorum [Fisch.{Ex Link} Scult.]) suppresses the growth and invasibility of cheatgrass. Research focused on monthly mineral soil N availability and the proportional concentration of NH4 +-N in a crested wheatgrass community by microsite (crested wheatgrass, unvegetated interspace, shrub subcanopy) and soil depth (0–15, 15–30 cm) over a 1-yr period. Mineral soil N in crested wheatgrass microsites ranged from 0.24 to 1.66 mmol ? kg-1 and was not appreciably lower than the other microsites or other ecosystems we have measured in the Great Basin. The molar proportion of NH4 +-N in the mineral N pool of crested wheatgrass averaged over 85%for the year and is significantly higher than the other microsites and far greater than other plant communities we have measured in the Great Basin. We conclude that crested wheatgrass does not suppress cheatgrass by controlling mineral N below a threshold level; rather, we hypothesize that it may limit nitrification and thereby reduce NO3 --N availability to the nitrophile cheatgrass.