Variation in prairie biomass production with species composition and age of prairie was examined in several restored prairies adjacent to the St. Olaf College campus in Northfield, Minnesota, USA. The prairies were established from 1980-2004 on land previously used for agriculture and have been maintained through periodic fire disturbances since restoration. We compared variation in soil nutrient levels, percent moisture, percent organic content and biomass content.

Results/Conclusions

Nitrogen levels showed little variation among prairies, but were significantly lower than an adjacent agricultural field. Orthophosphate phosphorus showed more variation and ranged from 26 ppm in the 1989 prairie to 83 ppm in the 1998 prairie. Percent soil organic content did not differ significantly among sites, but ranged from 2.988% in the 1989 prairie to 7.5% in the 2001 prairie. Biomass was harvested in 0.25m2 plots and analyzed for grass, forb and total biomass production. The most recently planted prairie, restored in 2004, had not been burned and showed the highest biomass production of 1673.37g/m2. The lowest biomass production of 573.12 g/m2 occurred in the 1989 prairie which was burned in spring 2007. Interestingly, the data suggest that total biomass decreases in older and more recently burned prairies. The prairie restored in 2004 produced the most biomass and also had the highest percentage of forbs. This indicates that grass-forb ratios may play an important role in the amount of biomass produced. Our results suggest that a mixed grass-forb prairie used as an energy source such as cellulosic ethanol will be more productive if harvested once every several years and not annually.