Nitrogen dynamics in perennial- and annual-dominated arid rangeland

It is often assumed that displacement of native perennial vegetation by exotic annuals will alter nutrient cycling. Nitrogen dynamics of native bunchgrass vegetation and adjacent stands of the exotic annual grass Bromus tectorum were compared on three sites in south-eastern Washington, U.S.A. The stands of B. tectorum had dominated the sites for at least 40 years. It should be emphasized that these sites were not prone to frequent wildfires as can be the case in some B. tectorum -dominated ecosystems. Over a 2-year period very few consistent differences were found between the two vegetation types in above-ground standing crop, root mass, in situ N mineralization, extractable soil N, or total soil C or N. Bromus tectorum above-ground plant mass and litter tended to have lower C/N ratios than did the native vegetation, but the results were not consistent over time or site. It appears that the exotic annual adapted to the resources on the site and, at least at the fairly gross level measured, had little impact on soil nitrogen. These results suggest that caution must be taken when assuming that a change in vegetation type and growth form will necessarily alter soil N levels