Annual life-cycle greenhouse gas impacts (as carbon dioxide equivalents). Source: University of MinnesotaAnnual life-cycle greenhouse gas impacts (as carbon dioxide equivalents). Source: University of Minnesota

The biofuel benefits of cultivating corn and other crops must be weighed in tandem with the environmental impacts of such land use. A 10-year field trial conducted by University of Minnesota researchers sought to determine the sustainability of growing native grasses on marginal prairie lands relative to that of more resource-intensive corn. Grassland species grown on abandoned agricultural lands have been proposed as enhancing climate mitigation potential, but can have low yields.

Moderate yield intensification treatments involving irrigation and 7 g/m2 of nitrogen fertilizer were applied to plots seeded with 32 species of prairie and savanna plants. Another series of plots were subjected to a more intensive treatment based on irrigation and 14 g/m2 of nitrogen.

Moderate intensification of grasslands had as great or greater biomass yields, soil carbon stores and root mass relative to the more intensive treatment, and had minimal effects on grassland stability, diversity and nitrate leaching. Compared with control plots, the moderate treatment of irrigation and fertilization resulted in 89% more yield, 61% more root carbon and 187% more soil carbon storage. The researchers found that use of the produced biomass for bioenergy would reap twice the greenhouse gas reductions.

The more intensive treatment resulted in 32% lower greenhouse gas benefits, 10 times greater nitrate leaching and 121% greater loss of plant diversity than the moderate treatment.

While yield was lower from the best performing grass plot relative to corn ethanol, this cultivation system offers greater scope for ecological restoration due to increased soil carbon storage and reduced fertilizer requirements.

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