NITROGEN-FERTILIZATION, BOTANICAL COMPOSITION AND BIOMASS PRODUCTION ON MIXED-GRASS RANGELAND

Many studies have reported nitrogen (N) fertilization of rangeland, bu t few have reported changes in botanical composition, which may be as important as changes in forage production, or were continued for as lo ng as 14 years. We determined frequency of occurrence of over 90 plant species in 1976-1988 under rates of 0, 22, or 34 kg N ha(-1) applied in spring or fall to mixed-grass rangeland in southeast Wyoming; frequ ency of 23 species will be reported. We also determined total biomass production and production of major species and species groups in 1982- 1988, Blue grama Bouteloua gracilis (H,B,K;,) Griffiths] frequency dec reased during years 5 through 7 because of the interaction of N and dr ought. The effects of long-term application of N decreased blue grama in year 12 and beyond Nitrogen fertilization increased frequency of we stern wheatgrass [Pascopyrum smithii (Rydb.) A. Level in all years exc ept the driest year of the study. Needleleaf sedge [Carex eleocharis B ailey] decreased because grazing had been removed from the study area; this occurred sooner and to a greater extent on fertilized than on un fertilized plots. Fourteen other perennial species were quite variable in response to the 3 rates and the 2 seasons of application. Frequenc y of 6 annual species fluctuated greatly among years and treatments, N itrogen fertilization did not increase average forage production enoug h to be profitable for cattle production.