Nitrogen Influences Biomass and Nutrient Partitioning by Perennial, Warm‐Season Grasses

Recent attention has focused on the use of perennial, warm‐season grasses as renewable energy crops. The objective of this study was to assess the effects of N fertilization on partitioning of biomass and nutrients between above‐ and belowground plant components by four warm‐season grass species in Iowa. In 2006–2007, established stands of big bluestem ( Andropogon geradii Vitman), switchgrass ( Panicum virgatum L.), indiangrass [ Sorghastrum nutans (L.) Nash], and eastern gamagrass [ Tripsacum dactyloides (L.) L.] were fertilized with 0, 65, 140, or 220 kg N ha −1 in the spring and harvested following frost in the fall. Dependent on grass species and year, yield response to N was linear or quadratic. Optimum yield after 2 yr was 13.5 Mg ha −1 at 140 kg N ha −1 for all grasses except eastern gamagrass, which demonstrated lower yield and a consistent linear N response. Nitrogen inputs had pronounced but grass‐specific effects on root biomass and nutrient partitioning. For big bluestem and switchgrass, 140 kg N ha −1 maximized root biomass and favored allocation of nutrients to roots over shoots. In contrast, for indiangrass and eastern gamagrass, root biomass and root nutrient allocation were adversely affected by N inputs. For all grasses, 220 kg N ha −1 shifted allocation of nutrients to shoots over roots. Selection of crops and management practices that optimize yield, and maintain a high level of resource partitioning to roots at low to intermediate N input rates will promote the development of productive and efficient bioenergy systems.