Nitrogen and Water Use in Low and High Input Irrigated Switchgrass (Panicum virgatum) Systems in California.

Switchgrass, a promising bioenergy crop has not been evaluated for such uses in California, the nation’s highest consumer of ethanol.  The objectives of this study were to evaluate biomass productivity, water and nitrogen use under low and high input systems. The low input system consisted of a single fall harvest with irrigation cut-off at flowering (July). The high input system consisted of a 2-harvest system (July and fall) with irrigation throughout the growing season.  Sub-treatments were 0, 100 and 200 kg N ha^-1 yr^-1.  Miniplots of ¹⁵N were established in the 100 N treatment to evaluate fertilizer N dynamics.  In the first year, the low input system produced on average 29% less biomass than the high input system (17 Mg DM ha^-1 yr^-1 versus 23 Mg DM ha^-1 yr^-1 respectively), but required 35% less irrigation water, resulting in higher water use efficiency.  The 100 and 200 kg N ha^-1 yr^-1 N treatments produced significantly more biomass than the control, but were not different from each other.  Total N removal by harvest was 75 and 187 kg N ha^-1 yr^-1 in low and high input systems receiving 100 kg N ha^-1 yr^-1. Following the fall harvest, 92 and 80% of the fertilizer N was recovered (in the plant and soil) and 18 and 42% of the fertilizer N was removed by the crop in the low and high input systems, respectively.  At harvest 29% of the fertilizer N was stored in plant tissue (crown plus roots) and 46% was in the soil in the low input systems, versus 12% in plant tissue and 25% in the soil in the high input system. The results suggest that single-harvest systems were able to use resources more efficiently, achieving greater biomass production per unit of water and nitrogen applied while maintaining high biomass productivity.