In established perennial grass biomass production systems, application of nitrogen (N) fertilizer can be one of the greatest expenses, from both a financial and environmental perspective. The objective of this study was to determine N fertilizer replacement value of kura clover (Trifolium ambiguum Bieb.), a rhizomatous N-fixing legume, for biomass production of prairie cordgrass (Spartina pectinata Link.), a native warm-season grass, on marginal land.

Field plots were established at four diverse environments in Illinois (IL), Minnesota (MN), South Dakota (SD), and Wisconsin (WI) in 2010. Kura clover was transplanted on 30-cm centers in the field in all treatments in which it was a component. Prairie cordgrass seedlings were transplanted within the kura clover on 60, and 90-cm centers. Monoculture prairie cordgrass stands were established at the same population densities of mixed stands and fertilized with 0, 75, 150, and 225 kg N ha^-1to determine fertilizer N replacement value of the kura clover.

Biomass at each location was harvested in the fall in 2011 and 2012. Yields varied by location. Yield of prairie cordgrass and kura clover mixtures were lower than that of prairie cordgrass monocultures at all N fertilizer rates and all locations in 2011; however, in 2012 yield of prairie cordgrass and kura clover mixtures were higher than that of the prairie cordgrass monoculture that had received 0 kg N ha^-1 at all locations except for WI. Although yield tended to increase with nitrogen application, response of prairie cordgrass to nitrogen was not consistent across locations in 2011 and 2012. Prairie cordgrass planted on 60-cm spacing was always higher yielding than when planted on 90-cm spacing in 2011; however, in 2012 this yield difference was reduced. In 2012 yield of prairie cordgrass planted on 60- and 90-cm spacings was similar at all locations except for SD.