Alfalfa for Biomass: Multiple Bottom Line Benefits.

Legumes are known to provide numerous benefits to agro-ecosystems. For example, like deeply rooted perennial grasses used for cellulosic bioenergy, such as Miscanthus × giganteus and Panicum virgatum, alfalfa (Medicago sativa L.) greatly reduces nitrate leaching loss to tile drains or ground water. Unlike those grasses, alfalfa also fixes  nitrogen (N) from the atmosphere to fulfill its needs, improving the N status of the soil over time. As a bioenergy crop, it is expected that leaves would be separated to be used for feed or other purposes, while the stems would be the energy feedstock. Alfalfa is often managed as a short-lived perennial, providing considerable flexibility to the grower. Most growers in the upper Midwest rotate to a nonlegume after 3 to 4 years, but this ranges from 2 or fewer to at least 7 years, beyond which quantitative statistics are not available. Little fertilizer N is required for the following crop, which represents a considerable savings in energy input. Furthermore, a short rotation of alfalfa can replace the soil carbon lost during annual row crop production. Previous analyses of the net energy balance for alfalfa have relied on average yields reported in federal or state surveys, but a recent evaluation of that data showed that yields reported by the top 10 percent of growers often are 2- to 3-fold greater than the ‘average’ yield in a state. Here, the cumulative distribution of yields by state from the Census of Agriculture with recent literature reviews of actual fertilizer N requirements of first- and second-year corn (Zea mays L.) following alfalfa to provide insight into the feasible energy balances in this crop rotation.