Nitrogen conservation decreases with fertilizer addition in two perennial grass cropping systems for bioenergy

Warm-season prairie grasses are promising bioenergy crops that exhibit conservative nitrogen use and cycling, which promise to reduce expensive N amendments and contributions to environmental N pollution associated with annual cropping systems. However, efforts to maximize crop yields with fertilizer may reduce N conservation in these systems. We used two perennial grass systems of differing diversity levels – a restored tallgrass prairie and a Panicum virgatum (switchgrass) monoculture—to determine the effects of N fertilizer level and harvest timing on biomass yields, N concentrations and N removal at harvest. To address plant N conservation, we measured N resorption efficiency (proportion of N resorbed), resorption proficiency (minimum N level attained after senescence), and timing of resorption under different N fertilizer rates. Yield responses to N fertilizer were not consistent between cropping system, year, or harvest timing, and were generally weak, resulting in an average of only 1.27 times more biomass compared to unfertilized plots. In contrast, fertilized plots removed 1.67 times more N relative to unfertilized plots, as N removal was largely driven by increases in biomass N concentrations rather than increases in yield. N resorption was affected by fertilizer in switchgrass, but not in selected prairie species. Fertilized switchgrass plants took longer to reach their maximum resorption levels and had reduced resorption proficiency, despite higher resorption efficiencies. Our results suggest that striving to increase biomass yields with N fertilizer may be an imprudent approach to sustainable bioenergy production, because yield responses are highly variable and N conservation mechanisms are compromised.