Perennial grasslands are being considered as a source of cellulosic biofuel; however, the long-term environmental sustainability of these bioenergy systems must be determined before they are widely grown. Agriculture is a major source of nitrous oxide (N2O), a potent greenhouse gas. N2O emissions may vary among plant communities due to differential soil nitrogen use. In south-central WI, we studied fertilized and unfertilized monoculture switchgrass, mixed native grasses, and native prairie bioenergy systems and an unfertilized restored prairie to address the questions: 1) How does plant species richness affect soil N2O emissions? 2) How does plant species composition affect soil N2O emissions? 3) Does nitrogen fertilizer have a direct effect on soil N2O emissions and/or an effect on the plant community? 4) What are the relationships among plant community, aboveground net primary productivity (ANPP), and soil N2O emissions? The plant community was characterized using the point-intercept method to give species percent cover in 63 1.5m x 1.5m quadrats. Within each quadrat a non-steady-state, non-through-flow trace gas flux chamber was used to measure soil N2O emissions. ANPP was estimated from a single, fall harvest of each quadrat.
We found a linear decrease in N2O emissions as plant species diversity increased in both fertilized and unfertilized grasslands (p<0.05). There was also, however, a linear decrease in aboveground productivity in more diverse grasslands whether fertilized (p<0.05) or unfertilized (p<0.0001). Fertilizer affected N2O emissions only in switchgrass monoculture (113.9 ± 16.2 mg N2O-N ha-1 hr-1 vs. 50.3 ± 5.0 mg N2O-N ha-1 hr-1 for fertilized and unfertilized, respectively, p<0.05) and fertilized switchgrass had significantly higher N2O emissions than any other grassland type (p<0.05). ANPP was sometimes dependent on grassland type with monoculture switchgrass having the highest (16.4 ± 1.1 Mg ha-1 yr-1 and 15.4 ± 1.1 Mg ha-1 yr-1 for fertilized and unfertilized, respectively) and restored prairie the lowest ANPP (6.3 ± 0.33 Mg ha-1 yr-1) (p<0.05). ANPP was not dependent on fertilizer treatment within each grassland type (p<0.05). Fertilizer did not significantly alter species composition, so fertilizer treatment effects were due directly to the additional nitrogen and not due to an altered plant community with changed nitrogen-use-efficiency. These data indicate there may be a trade-off between biomass yields and N2O emissions that should be considered as crop choice and management decisions are made regarding perennial grassland bioenergy systems.