Biomass yield and nitrogen use of potential C4 and C3 dedicated energy crops in a Mediterranean climate

• Significant responses to N fertilization were observed for all crops in all years.
• Miscanthus and switchgrass achieved the highest fertilized and unfertilized yields.
• Miscanthus and switchgrass had the highest fertilizer responses and FUE.
• Miscanthus and switchgrass have most potential as dedicated energy crops.

Biomass-based fuels from bioenergy crops stand as viable alternatives to fossil fuels. Side-by-side information on yields and N requirements of potential bioenergy crops is, however, lacking. The objectives of our study were to evaluate yields, N removal, and response to N fertilization of perennial C4 and C3 species under different N availability conditions in an irrigated Mediterranean climate. Five C4 species – miscanthus (Miscanthus × giganteus Greef et Deu ex. Hodkinson et Renvoize), switchgrass (Panicum virgatum L.), big bluestem (Andropogon gerardii Vitman), bermudagrass (Cynodon dactylon L.), and elephantgrass (Cenchrus purpureus (Schumach.) Morrone)-, and two C3 species – tall wheatgrass (Thinopyrum ponticum Podp.) and tall fescue (Festuca arundinacea Schreb.) – were evaluated under three N fertilization rates (0, 100, and 200 kg N ha−1 yr−1) from 2009 to 2011 in Davis, CA, USA. Miscanthus, switchgrass, big bluestem, and tall wheatgrass were harvested once per year, tall fescue twice, and bermudagrass four times per year. Elephantgrass was eventually excluded from our study due to winter mortality. The highest N-fertilized and unfertilized dry matter yields were achieved by miscanthus and switchgrass, followed by tall wheatgrass, big bluestem, tall fescue, and bermudagrass, with highest dry matter yields of 33.9, 22.9, 17.2, 16.2, 15.6, and 12.0 Mg ha−1 yr−1, respectively. Significant responses to N were observed for all crops in all years. Our results suggest that miscanthus and switchgrass have greatest potential as bioenergy crops for several reasons: (1) these crops had the highest yields and required only a single-harvest per year, (2) greatest response to N with yields increasing by 10.9 and 8.4 Mg ha−1 in the highest N treatment and fertilizer use efficiency of 69 and 55 kg biomass kg−1 N applied, respectively, and (3) lowest biomass N mass fraction of 3.6 and 2.6 g kg−1, respectively.