| The production-oriented goals of agriculture have resulted in tremendous changes in land-use on the prairies with the conversion of native grassland to forage and crop land for food production and livestock fodder from 1930's. The relative benefits of introducing forage species to the Northern Great Plains has been examined in well publicized studies with contradictory conclusion because of the differences by time of establishment, treatments arrangment and experimental design. Consequently, in order to clear the issue with trustable data and address a relative correct conclusion we prompted this study to re-examine the issue. Two experimental sites which were totally undisturbed on Mixed Prairie were selected in southern Alberta. ADRI is located in the suburb of Lethbridge(49°07′N, 112°57′W), near Animal Disease Research Institution with plant community of Stipa– Bouteloua- Agropyron. Onefour site is close to the boundry of Alberta and America(49°03′N, 110°27′W), with the plant community of Stipa-Bouteloua. The design of the field study was a randomized complete block design with 7 treatments, 4 replicates. ANPP was investigated every year. Soil samples were taken in fall 2006 at 4 increments (0– 15, 15– 30, 30– 60, and 60– 90 cm depth) for analysis.The results showed:1. ANPP and Ha-ANPP among forage types was influenced (P<0.05) by community and its interaction with treatment and sampling period. ANPP of ADRI was significantly higher than native grassland in 1 to 4 years after sites established(P<0.05) while tended to be similar with native grassland in the following 5 to 13 years. ANPP of Onefour was always significantly higher than naitve grassland in 1 to 11 years. The study confirmed the relative ANPP advantage of Crested wheatgrass on the Onefour site but not on ADRI site. Wheat was the most productive forage on both sites and its ANPP advantage to the native harvest appeared sustainable.2. Converting native grassland to Russian wildrye and Crested wheatgrass 13/12 years had no effect on soil organic carbon and nitrogen concentration, soil organic carbon and nitrogen stock and light fraction qualities (P>0.05) while converting native grassland to Wheat led a lower soil organic carbon and nitrogen concentration, soil organic carbon and nitrogen stock and light fraction qualities less (P<0.05). All cropping treatments had no effect (P>0.05) on More stable carbon indicated the loss of soil carbon and nitrogen was mainly from labile organic matter pool.3. Crested wheatgrass led a higher carbon and nitrogen mineralization (P<0.05) while while Russian wild rye only led a higher carbon mineralization than native grassland (P<0.05). Converting native grassland to wheat significantly lower (P<0.05) carbon mineralization while had no effect on nitrogen mineralization (P>0.05).4. On the Mixed prairie, soil carbon and nitrogen stock, light fraction qualities, carbon mineralization and nitrogen mineralization were closely correlated to each other (P<0.05 or P<0.0001)while no relations between nitrogen mineralization and nitrificati was detected. ANPP positively liner related to soil organic carbon and nitrogen stock while negetive liner related to carbon nitrogen mineralization, nitrogen nitrification and their rates. No relations between ANPP and light fraction qualities were detected.5. Corse root biomass was significantly effected by site and treatment (P<0.05) while was not effected by their interactions (P>0.05). Russian wild rye led higher corse root biomass than native grassland while wheat significantly lower corse root biomass (P<0.05). Corse root biomass on ADRI site was higher than Onefour site.
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