Nitrogen Fertilization Increases Productivity and Stability but Not Diversity in Prairies Managed for Bioenergy.

Tallgrass prairie is an endangered native ecosystem in much of the U.S. Corn Belt. Reconstructed prairies used as bioenergy feedstocks may provide an important opportunity to improve environmental quality and increase biodiversity in the region while producing a market-valued product. Managing prairies for bioenergy production differs from common prairie conservation management in ways that include the frequency and timing of disturbance events and nutrient management. Prairies grown for bioenergy production would be harvested annually in the fall or, less commonly, early in the spring, and may be fertilized to replace nutrients removed during biomass harvest.  We tested how spring nitrogen fertilization affected biomass yield, yield stability, and plant species diversity in prairies managed with an annual, fall harvest from 2009 to 2014 in a 9-hectare field experiment in Boone County, IA.  Over a 6-year period (years 2-7 after seeding), we measured machine-harvestable aboveground biomass after plant senescence and plant diversity in August for multispecies mixtures of prairie plants that received no fertilizer or 84 kg N ha^-1 yr^-1.  Over the study period, mean biomass yield was 52% greater (p<0.0001) in the fertilized prairie (8.04 ± 0.46 Mg ha^-1) than the unfertilized prairie (5.29 ± 0.38 Mg ha^-1).  Yield stability was also greater in the fertilized prairie (CV = 0.30 ± 0.01) than the unfertilized prairie (CV = 0.37 ± 0.03).  Although diversity differed among the treatments in the initial years of the study, by 2014, diversity, species richness, and species evenness did not differ between the fertilized and unfertilized prairie and the most dominant species in both treatments were warm-season (C₄) grasses.  In the context of managing prairies as a bioenergy feedstock, moderate nitrogen fertilization appears to be effective for increasing yield and yield stability without reducing plant species diversity compared with unfertilized prairie.