Part of the reason Switchgrass is so efficient is the microorganisms that reside with the roots in a synergistic relationship. This USDOE study looks at biomass production from a profit potential perspective. This is important because the profit motive is still the most important consideration for farmers. They need to make a living. So we must take input expenses into account, such as fertilization.
Switchgrass Produces Biomass Efficiently - Renewable Energy World: "Biomass feedstock nitrogen study compares four grasses and finds that switchgrass is most efficient."