Temporal Variation in Soil N Availability in An Agropyron Cristatum (crested wheatgrass) Planting.

Agropyron cristatum, a non-indigenous perennial grass, has been often planted to improve range condition. A. cristatum is one of few species to successfully exclude the exotic annual grass Bromus tectorum (cheatgrass). We hypothesize that A. cristatum resists B. tectorum invasion by reducing soil N availability below a threshold level. This hypothesis was tested in an A. cristatum stand northeast of Reno, NV seeded in 1960. For one-year, soil samples (4 replicates) were collected monthly, 0-15 cm by microsite (beneath A. cristatum, beneath a shrub, and in an un-vegetated interspace). Attributes measured included soil T, soil moisture content, and available N. Overall, available N was greatest beneath A. cristatum (0.60 mmol kg-1) and shrubs (0.61 mmol kg-1) than un-vegetated interspaces (0.38 mmol kg-1). Available N beneath A. cristatum ranged from 0.24 mmol kg-1 in April, 2009 to 1.66 mmol kg-1 in Jan., 2010 and did not significantly correlate with soil T or soil moisture content. The highest available N measured beneath A. cristatum is much less than could occur in surface mineral layers following wildfire – 3.28 and 2.14 mmol kg-1 for two sites in Nevada.