Effects of Bromus tectorum on nitrogen cycling and water balance in a Great Basin ecosytem: Implications for plant competition and ecosystem function

Invasion by exotic annual grasses is a significant problem in semiarid ecosystems of the western United States. To examine how annual grasses affect nitrogen (N) cycling relative to perennial vegetation, N dynamics were compared among soils from monodominant patches of the exotic annual grass Bromus tectorum, the perennial bunchgrass Elymus elymoides , and the shrub Artemisia tridentata, at Curlew Valley, a salt-desert shrub site in northern Utah. Because transect sampling determined that Elymus permits Artemisia recruitment in its stands, but appears to suppress Bromus where the two grasses occur, seasonal patterns of water and N uptake by the three species were investigated to better understand how resource competition may contribute to vegetation community composition at the Curlew Valley site. Finally, an overview of vegetation change at Curlew Valley was compiled to place current trends at the site in a historical context. Results of N cycling experiments indicated that greater lability of SOM and a soil environment more conducive to decomposition probably promote inorganic N production in Bromus-dominated soils relative to perennial-dominated soils. These factors appear to support soil faunal and microbial communities, which in turn contribute to distinct patterns of N cycling in the annual grass community. However, Bromus soils appear to receive lower N inputs from N-fixing crusts than perennial-dominated soils, and appear to be losing N. In the competition experiment, Artemisia growing with Bromus were suppressed, but those growing with Elymus showed little effect relative to Artemisia growing alone, an effect that appeared to be primarily mediated by spring moisture availability. Historical patterns of vegetation change pointed out that dominance by Bromus and Elymus and increased Artemisia cover are relatively recent phenomena at the Curlew Valley site. Invasion patterns of grasses are not clearly related to patterns of fire or grazing at the site, and appear to be influenced by edaphic factors and historical patterns of vegetation distribution.