The Effect of Nitrogen Fertilization and Species Composition on Greenhouse Gas Mitigation Potential in Perennial Grassland Bioenergy Cropping Systems.

The concentration of carbon dioxide (CO₂) and other greenhouse gases (GHG) in the atmosphere are increasing and altering global climate. Two pathways to mitigate atmospheric GHG emissions include 1) reducing fossil fuel consumption and CO₂ related emissions and 2) increasing terrestrial carbon (C) sinks. Perennial grassland bioenergy crops can mitigate GHG emissions via both pathways by displacing fossil fuel use with renewable biofuels and sequestering C in roots and soils. However, the relative contribution of biofuel and belowground C storage to net GHG mitigation potential is not well known, especially in response to management options such as plant diversity and N fertilization. We calculated the net GHG mitigation potential as the sum of fossil fuel offsets, carbon storage in soil and roots, and GHG emissions related to management, in five native perennial grassland bioenergy crops ranging from switchgrass monocultures to high-diversity polycultures. Each species mixture treatment was managed with and without 67 kg N ha^-1 for seven years at nine locations. Net GHG mitigation was similar for the switchgrass monoculture, a four-species grass mix, and an eight-species grass/legume mix, which were more effective than the 12- and 24-species high diversity mixes. N fertilizer increased fossil fuel offsets from biofuel production, but decreased root productivity, which resulted in an overall nil affect on net GHG mitigation for all species mixture treatments. There were no changes in soil organic C, but root C accounted for 43 % of the net GHG mitigation potential averaged across species mixture treatments. C storage in roots is substantial and should be considered when comparing the GHG mitigation potential of various perennial bioenergy cropping systems.

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